rucc_codegen/

frame.rs

//! The frame: what a function's stack looks like while it runs.
//!
//! Design: `spec/10-backend.md` section 10.7.
//!
//! This is worked out after register allocation and not before, because the largest area in most
//! frames is the spill slots and nothing knows how many of those there are until the allocator has
//! finished running out of registers. It is worked out from the rewritten function rather than
//! from the assignment alone, because the rewrite is what decides which scratch registers a reload
//! uses, and a scratch register a call preserves is one the prologue has to save.
//!
//! # What is in one
//!
//! Section 10.7 lists the areas and this is the order they are in, from the stack pointer upward,
//! which is the order of increasing address on every machine here.
//!
//! ```text
//!   incoming stack arguments      the caller wrote these and they are above everything
//!   return address                the call instruction pushed it, on a machine that does
//!   saved frame pointer           when the function keeps one
//!   saved general purpose regs    pushed, one word each
//!   saved vector registers        stored rather than pushed, since no machine here pushes one
//!   stack protector canary        when the function has one, above everything a local reaches
//!   locals                        what an alloca becomes, widest alignment first
//!   spill slots                   one for every value the allocator ran out of registers for
//!   outgoing argument area        at the bottom, because a call reads its stack arguments from
//!                                 the stack pointer upward
//! ```
//!
//! Every offset reported here is from the stack pointer as it stands in the body of the function,
//! which is after the prologue and before the epilogue. That is the one base register always
//! available. A frame pointer is a second way to reach the same bytes and the prologue is what
//! knows the distance between the two, so nothing here reports an offset from it. There are two
//! exceptions and [`Frame::incoming`] is one of them, because the bytes it reports are the caller's
//! rather than this function's, which is the one part of the picture a realigned frame loses sight
//! of. It says which register it counted from. The other is a frame that grows, which is the next
//! section and where the stack pointer stops being a base register at all.
//!
//! # Where the alignment comes from
//!
//! A call has to leave the stack pointer on a multiple of the convention's alignment, so a
//! function's own frame is what puts it back: the call that reached this function pushed a return
//! address and left the stack pointer one word off, and the prologue's pushes either fix that or
//! make it worse depending on how many there are. The size the prologue subtracts is therefore not
//! the size of the areas. It is whatever brings the stack pointer back to a multiple of the
//! alignment given the pushes in front of it, which is the arithmetic in [`Frame::of`].
//!
//! # The red zone
//!
//! A leaf function may use the bytes below the stack pointer without moving it, which is what
//! `red_zone` on a convention says and what makes a small leaf function's prologue and epilogue
//! empty. Then the offsets are negative, which is why they are signed, and the areas are in the
//! same order as ever, below the line rather than above it. Anything that calls, or is too big for
//! the zone, or wants more alignment than the stack pointer has for free, moves the stack pointer.
//!
//! # Realignment
//!
//! A local wanting more alignment than a call leaves the stack pointer with cannot be placed by
//! arithmetic, because nothing in the frame knows what the caller's stack pointer was a multiple
//! of. The prologue has to force it, and forcing it destroys the only record of where the caller's
//! stack was, so a realigned frame needs a frame pointer and the distance from the body's stack
//! pointer to the incoming arguments stops being a constant. [`Frame::realign`] is where that is
//! reported and it is why [`Frame::incoming`] answers from the frame pointer in such a frame and
//! from the stack pointer in every other one.
//!
//! # Growing
//!
//! A variable length array is bytes the function takes off the stack pointer where the declaration
//! stands, so in a function that has one the stack pointer is in a different place in the middle of
//! the body than it was at the top of it. Every other offset in the frame was a distance from the
//! stack pointer, and a distance from a register that moves is not a distance, so in a frame like
//! this they are all distances from the frame pointer instead. That is what [`Layout::grows`] says
//! and [`Frame::grows`] reports, and it is why such a frame keeps a frame pointer whatever the
//! flags asked for, the same way a realigned one does and for a version of the same reason.
//!
//! Three other things follow from it. The red zone is gone, because the zone is the bytes below the
//! stack pointer and the first thing an array like this does is move the stack pointer down over
//! them. The frame asks for the convention's alignment even when nothing in it wanted that much, so
//! that the stack pointer is on a multiple of it when the body starts and stays on one as each
//! array rounds its own size up. And the bytes the array hands out start above the outgoing
//! argument area rather than at the stack pointer, because that area stays at the bottom of the
//! frame wherever the bottom has moved to, which is what [`Frame::below`] is for.
//!
//! An array asking for more alignment than that is not a realignment of the frame, and nothing
//! here has to know about it. [`crate::expand::rounds`] asks for the alignment in extra bytes and
//! hands out an address inside them, so the stack pointer moves by a multiple of the convention's
//! alignment as it always did and the frame is an ordinary growing one.
//!
//! Realigning and growing together is the one combination that is not here. After the prologue has
//! forced an alignment the distance from the frame pointer to the body's stack pointer is already
//! not a constant, so there is no register left for the rest of the frame to be counted from, and
//! what fixes that is a second pointer held for the purpose. The lowering refuses that pair rather
//! than this guessing at it.
//!
//! # Late
//!
//! Where in the prologue the frame pointer is established is the platform's answer rather than this
//! file's, and [`rucc_target::CallRegs::late_frame_pointer`] is where the reason for it is written
//! down. On Windows it goes up after the frame has been taken rather than before, because the
//! unwind record there cannot describe the other order, and that moves it: it holds a copy of the
//! body's stack pointer rather than the address of the caller's copy of itself.
//!
//! Which is the easier of the two to lay out rather than the harder. Every offset here is from the
//! body's stack pointer already, so in a frame like this the frame pointer holds exactly what those
//! offsets are counted from, and a frame that grows needs no adjustment at all where the other
//! order needs the whole frame and every push taken off. [`Frame::late`] is what says which it is.
//!
//! The realigned frame is the one that cannot have it whatever the platform says. There the
//! prologue forces the alignment after the pushes, which leaves the pushes at a distance from the
//! body's stack pointer that is not a constant, so a pointer established after all that gives the
//! record nothing to count them from. Such a frame keeps the early order and is the one shape on
//! Windows that still has no record, which is `tamnd/rucc#1422`.

use rucc_mir::Func;
use rucc_regalloc::Allocation;
use rucc_regalloc::assign::Place;
use rucc_target::{CallRegs, PhysReg, RegClass, RegFile};

use crate::slots::{Cell, Slots};

/// One register the prologue puts away in the frame, and where in the frame it goes.
///
/// A pushed register does not need one of these, because where it goes is wherever the stack
/// pointer had reached, and the epilogue pops them back in the opposite order without having to
/// know. A register that is stored rather than pushed does need one.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Save {
    /// The register.
    pub reg: PhysReg,
    /// Where it goes, from the stack pointer in the body of the function.
    pub at: i32,
}

/// Where the arguments the caller passed on the stack are, and which register reaches them.
///
/// Two fields rather than one number because a realigned frame has no constant distance from its
/// stack pointer to the caller's. Forcing the alignment threw that distance away, and the frame
/// pointer is what still reaches the caller's stack afterwards, which is why a realigned frame is
/// made to keep one. So there is always an answer, and which register it is counted from is part of
/// it rather than something the reader is left to work out.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Incoming {
    /// How far above that register the first argument passed on the stack is.
    pub at: i32,
    /// Whether the register is the frame pointer rather than the stack pointer.
    pub through_frame_pointer: bool,
}

impl Incoming {
    /// That far above the stack pointer as it stands in the body of the function, which is where
    /// every other offset in a frame is from.
    #[must_use]
    pub fn from_stack(at: i32) -> Self {
        Self { at, through_frame_pointer: false }
    }

    /// That far above the frame pointer, which is the only way a realigned frame reaches back.
    #[must_use]
    pub fn from_frame(at: i32) -> Self {
        Self { at, through_frame_pointer: true }
    }
}

/// A piece of memory the function needs for its own use, which is what an `alloca` becomes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Local {
    /// How many bytes of it there are.
    pub size: u32,
    /// What its address has to be a multiple of.
    pub align: u32,
}

/// Everything about a function's frame that does not come out of its allocation.
#[derive(Debug, Clone, Copy)]
pub struct Layout<'a> {
    /// Where the convention this function is compiled for puts things.
    pub conv: &'a CallRegs,
    /// The registers the target has, which is what says how wide a spill slot of a class is.
    pub file: RegFile,
    /// The memory the function asked for itself, in the order it wants it reported back.
    pub locals: &'a [Local],
    /// How many bytes the widest call in the function needs for arguments it passes on the stack.
    pub outgoing: u32,
    /// Whether the function calls nothing, which is what the alignment and the red zone turn on.
    pub leaf: bool,
    /// Whether the function keeps a frame pointer, which `-fno-omit-frame-pointer` asks for and
    /// which a realigned or a dynamically grown frame requires whatever the flags say.
    pub frame_pointer: bool,
    /// Whether the function moves the stack pointer while it runs, which is what a variable length
    /// array does and what the rest of the frame then has to be reached around.
    ///
    /// See `Growing` in the module documentation. A frame like this keeps a frame pointer, takes
    /// its bytes rather than living in the red zone, and reports every offset in its body from the
    /// frame pointer, because the stack pointer stops being somewhere a constant reaches from.
    pub grows: bool,
    /// Whether the red zone may be used at all, which `-mno-red-zone` and every kernel turns off.
    pub red_zone: bool,
    /// Whether the frame holds a stack protector's canary, which `-fstack-protector` and the
    /// function's own attribute decide between them.
    ///
    /// A protected frame is never a leaf, whatever the function called, because the check at the
    /// end of it calls when it fails. The caller sets `leaf` accordingly rather than this working
    /// it out, so that there is one place a frame learns whether it owes an aligned stack pointer.
    pub protect: bool,
    /// Whether the function is written without a prologue or an epilogue, which
    /// `__attribute__((naked))` asks for.
    ///
    /// A frame like this is empty and nothing is written around the body. No register is put away,
    /// because the program said it would do that itself and the first thing one of these usually
    /// does is read something the saving would have moved. No bytes are taken, because taking them
    /// is the prologue's job and there is no prologue, which is why a naked function that wants any
    /// is refused rather than given a frame nothing sets up. See [`Frame::of`].
    pub naked: bool,
    /// Which locals and spill slots share their bytes with which, or `None` for a frame where
    /// every one of them gets a run of its own.
    ///
    /// Worked out in [`crate::slots`], because what may share is a question about liveness and this
    /// file is about arithmetic. `None` is the layout there was before that pass existed and is
    /// what `-fstack-reuse=none` asks for.
    pub share: Option<&'a Slots>,
}

impl<'a> Layout<'a> {
    /// A layout for a function with nothing in it but what its allocation says: a leaf with no
    /// locals and no calls, which is what every function is until the pieces that produce those
    /// exist.
    #[must_use]
    pub fn new(conv: &'a CallRegs, file: RegFile) -> Self {
        Self {
            conv,
            file,
            locals: &[],
            outgoing: 0,
            leaf: true,
            frame_pointer: false,
            grows: false,
            red_zone: true,
            protect: false,
            naked: false,
            share: None,
        }
    }
}

/// What a function's stack looks like while it runs.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Frame {
    saved_int: Vec<PhysReg>,
    saved_sse: Vec<Save>,
    slots: Vec<i32>,
    locals: Vec<i32>,
    canary: Option<i32>,
    outgoing: u32,
    below: u32,
    size: u32,
    realign: Option<u32>,
    incoming: Incoming,
    frame_pointer: bool,
    late: bool,
    grows: bool,
    naked: bool,
    usage: u32,
}

impl Frame {
    /// Works out the frame of a function the allocator has finished with.
    ///
    /// # Panics
    ///
    /// Panics on a frame of two gigabytes or more, which is a stack no machine here gives a
    /// thread, and on a local whose alignment is not a power of two.
    #[must_use]
    pub fn of(func: &Func, allocation: &Allocation, layout: &Layout<'_>) -> Self {
        let conv = layout.conv;
        let word = conv.word;
        // How far one push moves the stack pointer, which is the word on x86-64 and the whole
        // alignment on AArch64. A machine where it is the whole alignment is one whose stack
        // pointer is never allowed off it, and so a leaf there owes itself an aligned frame even
        // though it owes nobody else one.
        let push = conv.push;
        let aligned = push % conv.stack_align == 0;
        let (saved_int, vectors) = saved(func, allocation, layout);

        // The vector registers are saved in the frame rather than pushed, because no machine here
        // has an instruction that pushes one.
        let vector = width(layout, conv.sse_class);
        let mut top = 0;
        let mut align = word;
        let mut saved_sse = Vec::with_capacity(vectors.len());
        for reg in vectors {
            align = align.max(vector);
            saved_sse.push(Save { reg, at: offset(top) });
            top += vector;
        }

        // A frame that grows hands out the bytes above the outgoing area, and what makes that
        // address usable for anything is the stack pointer being on a multiple of the convention's
        // alignment when the body starts. Asking for that much here is what buys it: the area below
        // is padded to `align` and the frame is rounded to land the stack pointer back on it.
        if layout.grows {
            align = align.max(conv.stack_align);
        }

        // One list rather than two, because a local and a spill slot that are never both wanted can
        // be the same bytes and neither of them can share with something on the other list if the
        // two lists are placed one after the other. See [`crate::slots`]. A layout that was handed
        // no plan gets the one where nothing shares anything, which is the frame there was before
        // that pass existed.
        let apart;
        let plan = match layout.share {
            Some(plan) => plan,
            None => {
                apart = Slots::apart(layout.locals, &widths(layout, allocation));
                &apart
            }
        };
        let mut cells = Vec::with_capacity(plan.cells().len());
        let mut order: Vec<usize> = (0..plan.cells().len()).collect();
        // Widest alignment first, so that placing each one straight after the last never leaves a
        // hole bigger than the alignment the next one asked for. Within one alignment, the cells
        // that are a whole number of it go before the ones that are not, because a cell that ends
        // part way through leaves a hole in front of the next cell that asked for the same
        // alignment and none at all in front of a narrower one. A cell shared by a wide thing and
        // a strict one is exactly how a size that is not a multiple of its own alignment arises,
        // so without this a frame could come out larger for sharing than it was for not.
        order.sort_by_key(|&cell| {
            let Cell { size, align } = plan.cells()[cell];
            (std::cmp::Reverse(align), size % align != 0)
        });
        cells.resize(plan.cells().len(), 0);
        for cell in order {
            let Cell { size, align: want } = plan.cells()[cell];
            assert!(
                want.is_power_of_two(),
                "a local aligned to something that is not a power of 2"
            );
            align = align.max(want);
            top = top.next_multiple_of(want);
            cells[cell] = offset(top);
            top += size;
        }

        // Read back out to the two lists the rest of the compiler asks its questions in. A cell
        // several things share gives all of them the same offset, which is the whole point of it.
        let placed = |cell: Option<usize>| cells[cell.expect("a plan covering every slot")];
        let mut locals: Vec<i32> =
            (0..layout.locals.len()).map(|local| placed(plan.local(local))).collect();
        let mut slots: Vec<i32> = (0..allocation.assignment.slots().len())
            .map(|slot| placed(plan.slot(u32::try_from(slot).expect("a frame"))))
            .collect();

        // Above everything the function can reach through a local, which is the whole point of it.
        // A write that runs off the end of an array in this frame passes the canary before it
        // reaches the saved registers and the return address, so the check at the end of the
        // function sees a word that changed rather than a return that has already been taken.
        let mut canary = None;
        if layout.protect {
            top = top.next_multiple_of(word);
            canary = Some(offset(top));
            top += word;
        }

        // A call reads its stack arguments from the stack pointer upward, so the outgoing area is
        // at the bottom of the frame and its size is what shifts everything else.
        let outgoing = if layout.leaf { 0 } else { layout.outgoing.max(conv.shadow) };
        // Everything above it was placed as though it were not there, so moving it up by the size
        // of the area is what would break its alignment. The area is padded to the widest
        // alignment anything above it asked for, which costs at most that many bytes once and
        // costs nothing at all in the usual frame, where the area is a multiple of it already.
        // What the padding must not do is move the area itself: the callee reads its arguments
        // from the stack pointer, so the bottom of the area is the stack pointer whatever is
        // above it.
        let shifted = outgoing.next_multiple_of(align);
        let body = (top + shifted).next_multiple_of(word);

        let realign = (align > conv.stack_align).then_some(align);
        // Refused by [`crate::pipeline`] before anything gets here, because the two of them together
        // want one register twice. See `Growing` above.
        assert!(
            !(layout.grows && realign.is_some()),
            "a frame that grows and forces its alignment needs a second base register"
        );
        // Two frames keep one whatever the flags asked for, and each of them for its own version of
        // the same reason: the prologue is about to leave the stack pointer somewhere no constant
        // reaches the rest of the frame from, and the frame pointer is the register that still
        // does. Forcing an alignment is one of the two and growing while the function runs is the
        // other.
        //
        // A function that calls something on a machine whose call leaves the return address in a
        // register is a third. The call writes over that register, so the prologue has to put it
        // away, and it goes with the frame pointer as the one frame record the machine's unwinders
        // and `__builtin_frame_address` expect to find.
        let frame_pointer = layout.frame_pointer
            || realign.is_some()
            || layout.grows
            || (!layout.leaf && conv.link.is_some());
        // Where in the prologue the pointer is established, which is the platform's answer except
        // in the one frame that has an answer of its own. See `Late` above.
        let late = conv.late_frame_pointer && realign.is_none();

        // Where the stack pointer sits once the prologue has finished pushing: one return address
        // short of aligned when the function starts, and one push further off for every push. The
        // frame pointer is a push like any other here, which is why this is asked after the frames
        // that keep one without being asked to have said so.
        let pushed = u32::from(frame_pointer) + u32::try_from(saved_int.len()).expect("a frame");
        let entry = wrap(conv.stack_align, conv.return_address);
        let after = (entry + wrap(conv.stack_align, push * pushed)) % conv.stack_align;

        // A frame that grows cannot be one of the free ones. The red zone is the bytes below the
        // stack pointer, and the first thing a variable length array does is move the stack pointer
        // down over them, so what was in the zone would be handed out twice.
        let free = layout.leaf
            && layout.red_zone
            && realign.is_none()
            && !layout.grows
            && align <= word
            && body <= conv.red_zone;
        let size = match realign {
            _ if free => 0,
            // Once the prologue has forced the alignment, keeping the frame a multiple of it keeps
            // everything in the frame aligned too.
            Some(to) => body.next_multiple_of(to),
            // A leaf owes nobody an aligned stack pointer, so it takes exactly what it uses.
            None if layout.leaf && align <= word && !aligned => body,
            // The smallest frame that lands the stack pointer back on a multiple of the alignment
            // given where the pushes left it.
            None => body + (after + conv.stack_align - body % conv.stack_align) % conv.stack_align,
        };

        // With the stack pointer left where it was, the areas are the same areas in the same order
        // and they are below it rather than above it.
        //
        // A frame that grows is counted from the frame pointer instead, which is the same areas in
        // the same order with one more constant taken off: the prologue pushed the registers and
        // then took the frame, so the body's stack pointer is that far below where the frame
        // pointer was set. That distance is what a variable length array destroys and the frame
        // pointer is what is left, which is why a growing frame keeps one.
        //
        // Unless the pointer is established late, where there is nothing to take off: the prologue
        // points it at the stack pointer once the frame is whole, so the two hold the same address
        // when the body starts and every distance from one is a distance from the other.
        let mut shift = if free { -offset(body) } else { offset(shifted) };
        if layout.grows && !late {
            shift -= offset(size) + offset(push) * i32::try_from(saved_int.len()).expect("a frame");
        }
        for at in slots
            .iter_mut()
            .chain(locals.iter_mut())
            .chain(canary.iter_mut())
            .chain(saved_sse.iter_mut().map(|save| &mut save.at))
        {
            *at += shift;
        }

        // What `-fstack-usage` reports, worked out here because this is the one place every term
        // of it is in hand. See [`Frame::usage`] for what the number is. Every push comes before
        // the prologue forces an alignment, so the pushes are what gets rounded up.
        let pushes = conv.return_address + push * pushed;
        let usage = realign.map_or(pushes, |to| pushes.next_multiple_of(to)) + size;

        Self {
            saved_int,
            saved_sse,
            slots,
            locals,
            canary,
            outgoing,
            below: shifted,
            size,
            realign,
            incoming: match () {
                // A pointer established late holds what the body's stack pointer holds, so the
                // caller's stack is the whole frame and every push above it, which is the same
                // number a frame with no pointer counts from the stack pointer.
                () if late && layout.grows => {
                    Incoming::from_frame(offset(size + push * pushed + conv.return_address))
                }
                // The prologue saves the frame pointer before it does anything else and points it
                // at where it saved it, so the caller's stack is one push for that and one return
                // address above it, whatever the prologue did to the stack pointer afterwards.
                () if realign.is_some() || layout.grows => {
                    Incoming::from_frame(offset(push + conv.return_address))
                }
                () => Incoming::from_stack(offset(size + push * pushed + conv.return_address)),
            },
            frame_pointer,
            late,
            grows: layout.grows,
            naked: layout.naked,
            usage,
        }
    }

    /// The general purpose registers the prologue pushes, in the order it pushes them.
    ///
    /// The frame pointer is not among them even when the convention calls it a saved register,
    /// because a function that keeps one saves it as part of setting it up.
    #[must_use]
    pub fn saved_int(&self) -> &[PhysReg] {
        &self.saved_int
    }

    /// The vector registers the prologue stores into the frame, and where each of them goes.
    #[must_use]
    pub fn saved_sse(&self) -> &[Save] {
        &self.saved_sse
    }

    /// Where a spill slot is, from the stack pointer in the body of the function.
    #[must_use]
    pub fn slot(&self, slot: u32) -> Option<i32> {
        self.slots.get(usize::try_from(slot).ok()?).copied()
    }

    /// Where a local is, from the stack pointer in the body of the function.
    #[must_use]
    pub fn local(&self, local: usize) -> Option<i32> {
        self.locals.get(local).copied()
    }

    /// Where a local is, from the call frame address, which is what a debugger counts from.
    ///
    /// The call frame address is the stack pointer the caller held when it made the call, and
    /// [`Frame::incoming`] is already the distance up to it, since the first argument passed on the
    /// stack sits there. So the answer is one subtraction, and it is a negative number, because the
    /// frame is below the address the call was made from.
    ///
    /// `None` in a frame whose alignment the prologue had to force, where there is no answer to
    /// give. Rounding the stack pointer down throws away however far it was from where the caller
    /// left it, so the distance from the body's stack pointer up to the call frame address is not a
    /// constant in such a function, and the two numbers subtracted here are counted from different
    /// registers on top of that. What the locals of such a function want is a location counted from
    /// the frame pointer, which is a different expression from the one a frame base gives.
    ///
    /// `None` as well for a local this frame never placed.
    #[must_use]
    pub fn from_frame_base(&self, local: usize) -> Option<i32> {
        if self.realign.is_some() {
            return None;
        }
        Some(self.local(local)? - self.incoming.at)
    }

    /// Where a spill slot is, from the call frame address, which is what a debugger counts from.
    ///
    /// The same subtraction [`Frame::from_frame_base`] makes and `None` in the same function, for
    /// the same reasons. It is the other half of the same question: a local the program named is
    /// either in the part of the frame the front end asked for or in the part the allocator ran
    /// out of registers into, and a debugger wants both counted from the same place.
    #[must_use]
    pub fn slot_from_frame_base(&self, slot: u32) -> Option<i32> {
        if self.realign.is_some() {
            return None;
        }
        Some(self.slot(slot)? - self.incoming.at)
    }

    /// Where the stack protector's canary is, from the stack pointer in the body of the function,
    /// or `None` in a frame that has none.
    #[must_use]
    pub fn canary(&self) -> Option<i32> {
        self.canary
    }

    /// How many bytes the prologue takes off the stack pointer, which is nothing for a function
    /// small enough and quiet enough to live in the red zone.
    #[must_use]
    pub fn size(&self) -> u32 {
        self.size
    }

    /// How many bytes at the bottom of the frame belong to the arguments of calls this function
    /// makes, which is where the shadow space goes on Windows.
    #[must_use]
    pub fn outgoing(&self) -> u32 {
        self.outgoing
    }

    /// How many bytes at the bottom of the frame nothing else may be placed in, which is that area
    /// padded to the alignment everything above it asked for.
    ///
    /// What a variable length array has to step over. It takes its bytes off the stack pointer,
    /// which leaves them at the bottom of the frame where the next call is going to write its
    /// arguments, so the address it hands out is this far above the stack pointer rather than the
    /// stack pointer itself.
    #[must_use]
    pub fn below(&self) -> u32 {
        self.below
    }

    /// Whether the function moves the stack pointer while it runs.
    ///
    /// Every offset in the body of such a frame is from the frame pointer rather than from the
    /// stack pointer, because a variable length array leaves the stack pointer somewhere no
    /// constant reaches the rest of the frame from. See `Growing` in the module documentation.
    #[must_use]
    pub fn grows(&self) -> bool {
        self.grows
    }

    /// What the prologue has to force the stack pointer to be a multiple of, when a local wants
    /// more alignment than a call leaves it with.
    #[must_use]
    pub fn realign(&self) -> Option<u32> {
        self.realign
    }

    /// Where the first argument the caller passed on the stack is, and which register reaches it.
    ///
    /// The only offset here that is not always from the stack pointer. A realigned frame counts
    /// from the frame pointer instead, because forcing the alignment threw away however far the
    /// caller's stack pointer was from where the prologue wanted it, and the frame pointer is what
    /// reaches the caller's stack afterwards.
    #[must_use]
    pub fn incoming(&self) -> Incoming {
        self.incoming
    }

    /// Whether the function keeps a frame pointer.
    #[must_use]
    pub fn frame_pointer(&self) -> bool {
        self.frame_pointer
    }

    /// Whether nothing at all is to be written around the body, which `__attribute__((naked))`
    /// asks for. See [`Layout::naked`].
    ///
    /// Such a frame is empty, since [`crate::pipeline`] refuses a naked function that wanted any
    /// bytes rather than handing it a frame no prologue sets up. What is left for this to say is
    /// that the prologue and the epilogue are not to be written, and the epilogue is the point:
    /// the `ret` at the end of a function is written there, and a naked function ends where its
    /// own text ends.
    #[must_use]
    pub fn naked(&self) -> bool {
        self.naked
    }

    /// Whether the prologue points the frame pointer at the frame after taking it rather than
    /// before, which is [`rucc_target::CallRegs::late_frame_pointer`] and the one frame that cannot
    /// have it whatever the platform says. See `Late` in the module documentation.
    #[must_use]
    pub fn late(&self) -> bool {
        self.late
    }

    /// How many bytes of stack the function uses, not counting what a variable length array or an
    /// `alloca` takes while it runs, which is the number `-fstack-usage` reports.
    ///
    /// Counted the way gcc counts its own frames, so that the two compilers' reports can be
    /// compared function by function: the distance from the stack pointer the caller held just
    /// before its call instruction down to where the prologue leaves the stack pointer. That is
    /// the return address when the call pushes one, every register the prologue pushes, the frame
    /// pointer among them, and the size the prologue subtracts, which already holds the saved
    /// vector registers, the locals, the spill slots, the canary and the outgoing argument area.
    /// Bytes a leaf keeps in the red zone are not counted, and gcc does not count them either.
    ///
    /// A frame whose alignment the prologue forces has the pushes rounded up to that alignment
    /// before the rest is added, which is gcc's rule for the same frame. It is exact when the
    /// caller's stack pointer happened to be aligned that far already, and otherwise it is off by
    /// less than the alignment in one direction or the other, since how much the rounding throws
    /// away depends on where the caller's stack pointer was.
    ///
    /// The two numbers agree on what they count and not always on the bytes: a function whose
    /// locals rucc keeps in fewer or more slots than gcc does comes out smaller or larger by that
    /// much. A frame that [`Frame::grows`] is reported as `dynamic` beside this, the same as gcc.
    #[must_use]
    pub fn usage(&self) -> u32 {
        self.usage
    }
}

/// The registers a call preserves that this function writes anyway, so the prologue has to put
/// them back.
///
/// The rewritten function is what is read here rather than the assignment, because a spilled value
/// is reloaded into a scratch register that no assignment mentions, and a scratch register the
/// convention preserves is one this has to find.
///
/// Nothing at all in a naked function, which is the whole of what the attribute asks for. Such a
/// function names `%rbx` and `%rbp` in its own text and means the registers rather than places to
/// keep something, and putting them away in front of it would be the compiler answering a question
/// the program did not ask. See [`Layout::naked`].
fn saved(
    func: &Func,
    allocation: &Allocation,
    layout: &Layout<'_>,
) -> (Vec<PhysReg>, Vec<PhysReg>) {
    if layout.naked {
        return (Vec::new(), Vec::new());
    }
    let mut used: Vec<(RegClass, PhysReg)> = Vec::new();
    let mut note = |class: RegClass, at: PhysReg| {
        if !used.contains(&(class, at)) {
            used.push((class, at));
        }
    };
    for block in func.blocks() {
        for inst in func.insts(block) {
            for operand in &func[func[inst].operands] {
                if let Some(at) = operand.reg.phys() {
                    note(operand.class, at);
                }
            }
        }
    }
    for edit in &allocation.edits {
        for place in [edit.mov.from, edit.mov.to] {
            if let Place::Reg(at) = place {
                note(edit.class, at);
            }
        }
    }

    let conv = layout.conv;
    let wanted = |class: RegClass, at: PhysReg| used.contains(&(class, at));
    // In the convention's order rather than the order the function happened to reach for them, so
    // that two functions saving the same registers get the same prologue.
    let saved_int = conv
        .int_saved
        .iter()
        .copied()
        .filter(|&at| wanted(conv.int_class, at))
        .filter(|&at| !(layout.frame_pointer && at == conv.frame_pointer))
        .collect();
    let saved_sse =
        conv.sse_saved.iter().copied().filter(|&at| wanted(conv.sse_class, at)).collect();
    (saved_int, saved_sse)
}

/// How many bytes a value of a class takes on the stack.
///
/// A power of two at least a word wide, because a slot is addressed and an address that is not a
/// multiple of the size of the thing at it is a fault on some machines and slow on the rest. An
/// eighty bit `long double` takes sixteen bytes for that reason, which is what every compiler
/// does with one.
fn width(layout: &Layout<'_>, class: RegClass) -> u32 {
    let bits = layout.file.class(class).map_or(0, |info| info.bits);
    bits.div_ceil(8).max(layout.conv.word).next_power_of_two()
}

/// How many bytes each of an allocation's spill slots takes on the stack.
///
/// The same question the width of one register class is, asked of a whole allocation at once, and
/// public because [`crate::slots`] needs it to say how big a cell holding a spilled value has to
/// be, which it has to know before there is a frame to ask.
#[must_use]
pub fn widths(layout: &Layout<'_>, allocation: &Allocation) -> Vec<u32> {
    allocation.assignment.slots().iter().map(|&class| width(layout, class)).collect()
}

/// How far past a multiple of an alignment a number is, counted the other way: what has to be
/// added to it to reach the next one.
fn wrap(align: u32, value: u32) -> u32 {
    (align - value % align) % align
}

/// A distance in a frame, as the signed number every offset out of here is.
fn offset(bytes: u32) -> i32 {
    i32::try_from(bytes).expect("a frame under two gigabytes")
}

#[cfg(test)]
mod tests {
    use rucc_base::Interner;
    use rucc_mir::{Opcode, Operand, Reg};
    use rucc_regalloc::assign::Env;
    use rucc_target::x86_64::{GPR, RBP, REGS, SYSV, WIN64, XMM};

    use super::*;

    /// An environment offering that many of the convention's registers, with everything after
    /// them held back as scratch.
    fn env(conv: &CallRegs, count: usize) -> Env {
        Env::new().with(GPR, &conv.int_order[..count], &conv.int_order[count..])
    }

    /// A function of that many values, every one of them written before any is read, allocated
    /// with that many registers to hand out.
    ///
    /// Every value is live at the first read, so a count below the number of values is what puts
    /// the function under enough pressure to spill, and each read wants one value so a reload
    /// never needs more than one scratch register.
    fn pressure(conv: &CallRegs, values: usize, count: usize) -> (Func, Allocation) {
        let mut names = Interner::new();
        let mut func = Func::new(names.intern("f"));
        let opcode = Opcode::new(names.intern("x64.nop"));
        let block = func.create_block();
        let regs: Vec<Reg> = (0..values).map(|_| func.new_vreg(GPR)).collect();
        for &reg in &regs {
            func.build(block, opcode).def(reg, GPR).finish();
        }
        for &reg in &regs {
            func.build(block, opcode).uses(reg, GPR).finish();
        }
        let allocation = rucc_regalloc::run(&mut func, &env(conv, count), "test", true);
        (func, allocation)
    }

    /// What a list of registers is called, which is what an assertion reads.
    fn named(regs: &[PhysReg]) -> Vec<&'static str> {
        regs.iter().map(|&reg| REGS.name(GPR, reg).expect("a register")).collect()
    }

    #[test]
    fn a_function_that_needs_nothing_of_the_stack_has_no_frame_at_all() {
        let (func, allocation) = pressure(&SYSV, 2, 4);
        let frame = Frame::of(&func, &allocation, &Layout::new(&SYSV, REGS));

        assert_eq!(frame.size(), 0);
        assert_eq!(named(frame.saved_int()), Vec::<&str>::new());
        assert_eq!(frame.slot(0), None);
        // Nothing between the stack pointer and the return address the call pushed.
        assert_eq!(frame.incoming(), Incoming::from_stack(8));
    }

    #[test]
    fn a_small_leaf_function_puts_its_spills_in_the_red_zone_and_moves_nothing() {
        let (func, allocation) = pressure(&SYSV, 4, 2);
        let frame = Frame::of(&func, &allocation, &Layout::new(&SYSV, REGS));

        // Two registers for four values that are all live at once, so two are on the stack, and a
        // leaf function small enough is entitled to the bytes below the stack pointer.
        assert_eq!(frame.size(), 0);
        assert_eq!((frame.slot(0), frame.slot(1)), (Some(-16), Some(-8)));
        assert_eq!(frame.slot(2), None);
        assert_eq!(frame.incoming(), Incoming::from_stack(8));
    }

    #[test]
    fn a_leaf_function_told_it_has_no_red_zone_takes_the_bytes_instead() {
        let (func, allocation) = pressure(&SYSV, 4, 2);
        let base = Layout::new(&SYSV, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { red_zone: false, ..base });

        assert_eq!(frame.size(), 16);
        assert_eq!((frame.slot(0), frame.slot(1)), (Some(0), Some(8)));
        assert_eq!(frame.incoming(), Incoming::from_stack(24));
    }

    #[test]
    fn a_frame_too_big_for_the_red_zone_takes_the_bytes_whatever_else_is_true() {
        let (func, allocation) = pressure(&SYSV, 40, 2);
        let frame = Frame::of(&func, &allocation, &Layout::new(&SYSV, REGS));

        // Thirty eight values on the stack is three hundred and four bytes, and the red zone is a
        // hundred and twenty eight.
        assert_eq!(frame.size(), 304);
        assert_eq!(frame.slot(0), Some(0));
        assert_eq!(frame.slot(37), Some(296));
    }

    #[test]
    fn a_function_that_calls_something_leaves_the_stack_pointer_where_a_call_wants_it() {
        let (func, allocation) = pressure(&SYSV, 4, 2);
        let base = Layout::new(&SYSV, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { leaf: false, ..base });

        // Sixteen bytes of spills, and the call that reached this function left the stack pointer
        // eight bytes off, so the frame is eight bytes wider than the spills need and every call
        // this function makes is correctly aligned.
        assert_eq!(frame.size(), 24);
        assert_eq!((frame.slot(0), frame.slot(1)), (Some(0), Some(8)));
        assert_eq!(frame.incoming(), Incoming::from_stack(32));
    }

    #[test]
    fn a_push_is_counted_in_the_alignment_the_frame_has_to_produce() {
        let (func, allocation) = pressure(&SYSV, 12, 12);
        let base = Layout::new(&SYSV, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { leaf: false, ..base });

        // Twelve values reach into the preserved end of the allocation order, so three registers
        // are pushed, and three pushes plus the return address is a multiple of sixteen already.
        // The frame is empty and stays empty rather than being padded for the sake of it.
        assert_eq!(named(frame.saved_int()), ["rbx", "r12", "r13"]);
        assert_eq!(frame.size(), 0);
        assert_eq!(frame.incoming(), Incoming::from_stack(32));
    }

    #[test]
    fn the_registers_a_call_leaves_alone_are_saved_in_the_order_the_convention_lists_them() {
        let (func, allocation) = pressure(&SYSV, 13, 13);
        let frame = Frame::of(&func, &allocation, &Layout::new(&SYSV, REGS));

        // Four of them now, in the convention's order rather than the order the allocator handed
        // them out in, so that two functions saving the same registers get the same prologue.
        assert_eq!(named(frame.saved_int()), ["rbx", "r12", "r13", "r14"]);
    }

    #[test]
    fn a_function_that_keeps_a_frame_pointer_does_not_save_it_twice() {
        let mut names = Interner::new();
        let mut func = Func::new(names.intern("f"));
        let opcode = Opcode::new(names.intern("x64.nop"));
        let block = func.create_block();
        // An instruction that names the frame pointer register outright, which is what a lowering
        // rule for something that has to use it produces.
        func.build(block, opcode).operand(Operand::write(Reg::physical(RBP), GPR)).finish();
        let allocation = rucc_regalloc::run(&mut func, &env(&SYSV, 4), "test", true);
        let base = Layout::new(&SYSV, REGS);

        let kept = Frame::of(&func, &allocation, &Layout { frame_pointer: true, ..base });
        let dropped = Frame::of(&func, &allocation, &base);

        // `rbp` is a register SysV preserves, so a function that leaves it alone saves it in the
        // ordinary way, and a function that keeps a frame pointer in it saves it as part of
        // setting the frame pointer up instead.
        assert_eq!(named(dropped.saved_int()), ["rbp"]);
        assert_eq!(named(kept.saved_int()), Vec::<&str>::new());
        assert!(kept.frame_pointer());
    }

    #[test]
    fn locals_are_placed_widest_alignment_first_and_reported_in_the_order_they_arrived() {
        let (func, allocation) = pressure(&SYSV, 2, 4);
        let locals = [
            Local { size: 1, align: 1 },
            Local { size: 16, align: 16 },
            Local { size: 8, align: 8 },
        ];
        let base = Layout::new(&SYSV, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { locals: &locals, ..base });

        // The sixteen byte one is placed first, so nothing is padded to reach it, and the one
        // byte one goes last where the padding after it costs nothing.
        assert_eq!((frame.local(1), frame.local(2), frame.local(0)), (Some(0), Some(16), Some(24)));
        assert_eq!(frame.local(3), None);
        // A local wanting sixteen byte alignment is more than the stack pointer has for free, so
        // the frame is taken rather than the red zone used, and it is padded to keep the local
        // where it was put.
        assert_eq!(frame.size(), 40);
        assert_eq!(frame.realign(), None);
    }

    #[test]
    fn a_local_is_counted_from_the_call_frame_address_wherever_the_frame_was_put() {
        let (func, allocation) = pressure(&SYSV, 2, 4);
        let base = Layout::new(&SYSV, REGS);

        let locals = [
            Local { size: 1, align: 1 },
            Local { size: 16, align: 16 },
            Local { size: 8, align: 8 },
        ];
        let taken = Frame::of(&func, &allocation, &Layout { locals: &locals, ..base });

        // The frame is forty bytes and the return address is eight more, so the call frame address
        // is forty eight above the stack pointer and every local is that much less than wherever
        // the layout put it. The one byte one is nearest, at the top of the frame.
        assert_eq!(taken.incoming(), Incoming::from_stack(48));
        assert_eq!(taken.from_frame_base(1), Some(-48));
        assert_eq!(taken.from_frame_base(2), Some(-32));
        assert_eq!(taken.from_frame_base(0), Some(-24));
        assert_eq!(taken.from_frame_base(3), None);

        // And a leaf small enough to live in the red zone takes no frame at all, so its stack
        // pointer is still one return address below the call frame address and its local is below
        // that. The same subtraction answers both, which is the point of doing it this way.
        let one = [Local { size: 8, align: 8 }];
        let free = Frame::of(&func, &allocation, &Layout { locals: &one, ..base });

        assert_eq!(free.size(), 0);
        assert_eq!(free.incoming(), Incoming::from_stack(8));
        assert_eq!(free.from_frame_base(0), Some(-16));
    }

    #[test]
    fn a_realigned_frame_is_no_constant_distance_from_the_call_frame_address() {
        let (func, allocation) = pressure(&SYSV, 2, 4);
        let locals = [Local { size: 64, align: 32 }];
        let base = Layout::new(&SYSV, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { locals: &locals, ..base });

        // The prologue rounds the stack pointer down to a multiple of thirty two, which throws
        // away however far the caller left it from there, so how far the local is below the call
        // frame address is a different number every time the function is called.
        assert_eq!(frame.realign(), Some(32));
        assert_eq!(frame.local(0), Some(0));
        assert_eq!(frame.from_frame_base(0), None);
    }

    #[test]
    fn a_local_wanting_more_alignment_than_a_call_gives_makes_the_prologue_force_it() {
        let (func, allocation) = pressure(&SYSV, 2, 4);
        let locals = [Local { size: 64, align: 32 }];
        let base = Layout::new(&SYSV, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { locals: &locals, ..base });

        assert_eq!(frame.realign(), Some(32));
        assert_eq!(frame.local(0), Some(0));
        assert_eq!(frame.size(), 64);
        // Forcing the alignment throws away how far the caller's stack pointer was from where the
        // prologue wanted it, so a frame pointer is needed and the caller's stack is reached
        // through it instead: one word for the saved frame pointer and one for the return address.
        assert!(frame.frame_pointer());
        assert_eq!(frame.incoming(), Incoming::from_frame(16));
    }

    #[test]
    fn the_canary_is_above_every_byte_a_local_or_a_spill_reaches() {
        let (func, allocation) = pressure(&SYSV, 4, 2);
        let locals = [Local { size: 16, align: 16 }, Local { size: 8, align: 8 }];
        let base = Layout::new(&SYSV, REGS);
        let there = Layout { leaf: false, locals: &locals, protect: true, ..base };
        let frame = Frame::of(&func, &allocation, &there);

        // Two spill slots at the bottom, then the two locals, then the canary above all four. That
        // order is the whole mechanism: a write that runs off the end of either local passes the
        // canary before it reaches the saved registers and the return address.
        let canary = frame.canary().expect("a protected frame has a slot");
        for below in [frame.slot(0), frame.slot(1), frame.local(0), frame.local(1)] {
            assert!(below.expect("a slot that was asked for") < canary);
        }
        assert_eq!(canary, 40);
        // Forty eight bytes of areas, and then the eight that put the stack pointer back where a
        // call wants it, because the arm the check fails on makes one.
        assert_eq!(frame.size(), 56);
        assert_eq!((frame.size() + SYSV.return_address) % SYSV.stack_align, 0);
    }

    #[test]
    fn a_frame_with_no_protector_has_no_slot_for_a_canary() {
        let (func, allocation) = pressure(&SYSV, 2, 4);
        let frame = Frame::of(&func, &allocation, &Layout::new(&SYSV, REGS));

        assert_eq!(frame.canary(), None);
    }

    #[test]
    fn a_call_reads_its_stack_arguments_from_the_bottom_of_the_frame() {
        let (func, allocation) = pressure(&SYSV, 4, 2);
        let base = Layout::new(&SYSV, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { leaf: false, outgoing: 24, ..base });

        // The outgoing area is at the stack pointer, because that is where the callee will look
        // for it, and the spills sit above it.
        assert_eq!(frame.outgoing(), 24);
        assert_eq!((frame.slot(0), frame.slot(1)), (Some(24), Some(32)));
        assert_eq!(frame.size(), 40);
    }

    /// Moving everything up by the size of the outgoing area is what would break its alignment,
    /// so the area is padded to the widest alignment anything above it wanted. The area itself
    /// still starts at the stack pointer, because that is the one thing about it that is not this
    /// frame's to choose.
    #[test]
    fn what_is_above_the_outgoing_area_keeps_the_alignment_it_asked_for() {
        let (func, allocation) = pressure(&SYSV, 2, 4);
        let locals = [Local { size: 16, align: 16 }];
        let base = Layout::new(&SYSV, REGS);
        let there = Layout { leaf: false, outgoing: 8, locals: &locals, ..base };
        let frame = Frame::of(&func, &allocation, &there);

        assert_eq!(frame.outgoing(), 8);
        assert_eq!(frame.local(0), Some(16));
        assert_eq!(frame.size(), 40);
        // A call leaves the stack pointer one return address short of aligned and nothing was
        // pushed on top of that, so the frame is what puts it back and the local lands aligned.
        assert_eq!((frame.size() + SYSV.return_address) % SYSV.stack_align, 0);
    }

    #[test]
    fn a_windows_call_gets_the_thirty_two_bytes_below_it_even_when_it_passes_nothing() {
        let (func, allocation) = pressure(&WIN64, 2, 4);
        let base = Layout::new(&WIN64, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { leaf: false, ..base });

        // Windows has no red zone and every caller reserves thirty two bytes below the call for
        // the callee to spill its register arguments into.
        assert_eq!(frame.outgoing(), 32);
        assert_eq!(frame.size(), 40);
        assert_eq!(frame.incoming(), Incoming::from_stack(48));
    }

    #[test]
    fn a_windows_frame_pointer_is_established_after_the_frame_rather_than_before_it() {
        let (func, allocation) = pressure(&WIN64, 4, 2);
        let base = Layout::new(&WIN64, REGS);
        let kept = Frame::of(&func, &allocation, &Layout { frame_pointer: true, ..base });
        let dropped = Frame::of(&func, &allocation, &base);

        // The unwind record that platform reads cannot describe the other order, so the prologue
        // pushes, takes the frame and only then points the pointer at it. What that buys is that
        // the pointer holds what the stack pointer holds, so a frame with one and a frame without
        // one are the same frame with the same numbers in it.
        assert!(kept.frame_pointer());
        assert!(kept.late());
        assert!(!dropped.frame_pointer());
        assert_eq!(kept.size(), dropped.size());
        assert_eq!((kept.slot(0), kept.slot(1)), (dropped.slot(0), dropped.slot(1)));
        assert_eq!(kept.incoming(), Incoming::from_stack(dropped.incoming().at + 8));
    }

    #[test]
    fn a_windows_frame_that_grows_keeps_the_numbers_it_had_and_changes_the_register() {
        let (func, allocation) = pressure(&WIN64, 4, 2);
        let base = Layout::new(&WIN64, REGS);
        let there = Layout { leaf: false, frame_pointer: true, ..base };
        let still = Frame::of(&func, &allocation, &there);
        let grown = Frame::of(&func, &allocation, &Layout { grows: true, ..there });

        // A frame that grows keeps a pointer whatever the flags asked for, and on this platform
        // that pointer is established late, which means it is a copy of the stack pointer as the
        // body finds it. So every distance the frame had already worked out from the stack pointer
        // is the same distance from the pointer, and growing changes which register the offsets are
        // counted from and nothing else. That is the whole of why this frame needs no adjustment.
        assert!(grown.grows());
        assert!(grown.late());
        assert_eq!(grown.size(), still.size());
        assert_eq!(grown.outgoing(), still.outgoing());
        assert_eq!((grown.slot(0), grown.slot(1)), (still.slot(0), still.slot(1)));
        assert_eq!(grown.incoming(), Incoming::from_frame(still.incoming().at));
    }

    #[test]
    fn a_realigned_frame_on_windows_keeps_the_early_order_it_has_no_choice_about() {
        let (func, allocation) = pressure(&WIN64, 2, 4);
        let locals = [Local { size: 64, align: 32 }];
        let base = Layout::new(&WIN64, REGS);
        let frame = Frame::of(&func, &allocation, &Layout { locals: &locals, ..base });

        // Forcing the alignment leaves the pushes at no constant distance from anything, so the
        // pointer has to go up before the mask and the platform's answer does not apply. Such a
        // frame is described by nothing and the assembler refuses it by name, which is
        // `tamnd/rucc#1422`.
        assert_eq!(frame.realign(), Some(32));
        assert!(frame.frame_pointer());
        assert!(!frame.late());
        assert_eq!(frame.incoming(), Incoming::from_frame(16));
    }

    #[test]
    fn a_slot_is_as_wide_as_the_widest_thing_of_its_class() {
        let base = Layout::new(&SYSV, REGS);

        assert_eq!(width(&base, GPR), 8);
        assert_eq!(width(&base, XMM), 16);
        // A long double is eighty bits and takes sixteen bytes, because an address has to be a
        // multiple of the size of what is at it.
        assert_eq!(width(&base, REGS.class_named("x87").expect("a class")), 16);
    }
}