//! Stack slots.
//!
//! The `StackSlotData` struct keeps track of a single stack slot in a function.
//!
use entity::{Iter, IterMut, Keys, PrimaryMap};
use ir::{StackSlot, Type};
use packed_option::PackedOption;
use std::cmp;
use std::fmt;
use std::ops::{Index, IndexMut};
use std::slice;
use std::str::FromStr;
use std::vec::Vec;
/// The size of an object on the stack, or the size of a stack frame.
///
/// We don't use `usize` to represent object sizes on the target platform because Cranelift supports
/// cross-compilation, and `usize` is a type that depends on the host platform, not the target
/// platform.
pub type StackSize = u32;
/// A stack offset.
///
/// The location of a stack offset relative to a stack pointer or frame pointer.
pub type StackOffset = i32;
/// The minimum size of a spill slot in bytes.
///
/// ISA implementations are allowed to assume that small types like `b1` and `i8` get a full 4-byte
/// spill slot.
const MIN_SPILL_SLOT_SIZE: StackSize = 4;
/// Get the spill slot size to use for `ty`.
fn spill_size(ty: Type) -> StackSize {
cmp::max(MIN_SPILL_SLOT_SIZE, ty.bytes())
}
/// The kind of a stack slot.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum StackSlotKind {
/// A spill slot. This is a stack slot created by the register allocator.
SpillSlot,
/// An explicit stack slot. This is a chunk of stack memory for use by the `stack_load`
/// and `stack_store` instructions.
ExplicitSlot,
/// An incoming function argument.
///
/// If the current function has more arguments than fits in registers, the remaining arguments
/// are passed on the stack by the caller. These incoming arguments are represented as SSA
/// values assigned to incoming stack slots.
IncomingArg,
/// An outgoing function argument.
///
/// When preparing to call a function whose arguments don't fit in registers, outgoing argument
/// stack slots are used to represent individual arguments in the outgoing call frame. These
/// stack slots are only valid while setting up a call.
OutgoingArg,
/// An emergency spill slot.
///
/// Emergency slots are allocated late when the register's constraint solver needs extra space
/// to shuffle registers around. The are only used briefly, and can be reused.
EmergencySlot,
}
impl FromStr for StackSlotKind {
type Err = ();
fn from_str(s: &str) -> Result<Self, ()> {
use self::StackSlotKind::*;
match s {
"explicit_slot" => Ok(ExplicitSlot),
"spill_slot" => Ok(SpillSlot),
"incoming_arg" => Ok(IncomingArg),
"outgoing_arg" => Ok(OutgoingArg),
"emergency_slot" => Ok(EmergencySlot),
_ => Err(()),
}
}
}
impl fmt::Display for StackSlotKind {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
use self::StackSlotKind::*;
f.write_str(match *self {
ExplicitSlot => "explicit_slot",
SpillSlot => "spill_slot",
IncomingArg => "incoming_arg",
OutgoingArg => "outgoing_arg",
EmergencySlot => "emergency_slot",
})
}
}
/// Contents of a stack slot.
#[derive(Clone, Debug)]
pub struct StackSlotData {
/// The kind of stack slot.
pub kind: StackSlotKind,
/// Size of stack slot in bytes.
pub size: StackSize,
/// Offset of stack slot relative to the stack pointer in the caller.
///
/// On x86, the base address is the stack pointer *before* the return address was pushed. On
/// RISC ISAs, the base address is the value of the stack pointer on entry to the function.
///
/// For `OutgoingArg` stack slots, the offset is relative to the current function's stack
/// pointer immediately before the call.
pub offset: Option<StackOffset>,
}
impl StackSlotData {
/// Create a stack slot with the specified byte size.
pub fn new(kind: StackSlotKind, size: StackSize) -> Self {
Self {
kind,
size,
offset: None,
}
}
/// Get the alignment in bytes of this stack slot given the stack pointer alignment.
pub fn alignment(&self, max_align: StackSize) -> StackSize {
debug_assert!(max_align.is_power_of_two());
// We want to find the largest power of two that divides both `self.size` and `max_align`.
// That is the same as isolating the rightmost bit in `x`.
let x = self.size | max_align;
// C.f. Hacker's delight.
x & x.wrapping_neg()
}
}
impl fmt::Display for StackSlotData {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "{} {}", self.kind, self.size)?;
if let Some(offset) = self.offset {
write!(f, ", offset {}", offset)?;
}
Ok(())
}
}
/// Stack frame manager.
///
/// Keep track of all the stack slots used by a function.
#[derive(Clone, Debug)]
pub struct StackSlots {
/// All allocated stack slots.
slots: PrimaryMap<StackSlot, StackSlotData>,
/// All the outgoing stack slots, ordered by offset.
outgoing: Vec<StackSlot>,
/// All the emergency slots.
emergency: Vec<StackSlot>,
/// The total size of the stack frame.
///
/// This is the distance from the stack pointer in the current function to the stack pointer in
/// the calling function, so it includes a pushed return address as well as space for outgoing
/// call arguments.
///
/// This is computed by the `layout()` method.
pub frame_size: Option<StackSize>,
}
/// Stack slot manager functions that behave mostly like an entity map.
impl StackSlots {
/// Create an empty stack slot manager.
pub fn new() -> Self {
Self {
slots: PrimaryMap::new(),
outgoing: Vec::new(),
emergency: Vec::new(),
frame_size: None,
}
}
/// Clear out everything.
pub fn clear(&mut self) {
self.slots.clear();
self.outgoing.clear();
self.emergency.clear();
self.frame_size = None;
}
/// Allocate a new stack slot.
///
/// This function should be primarily used by the text format parser. There are more convenient
/// functions for creating specific kinds of stack slots below.
pub fn push(&mut self, data: StackSlotData) -> StackSlot {
self.slots.push(data)
}
/// Check if `ss` is a valid stack slot reference.
pub fn is_valid(&self, ss: StackSlot) -> bool {
self.slots.is_valid(ss)
}
/// Set the offset of a stack slot.
pub fn set_offset(&mut self, ss: StackSlot, offset: StackOffset) {
self.slots[ss].offset = Some(offset);
}
/// Get an iterator over all the stack slot keys.
pub fn iter(&self) -> Iter<StackSlot, StackSlotData> {
self.slots.iter()
}
/// Get an iterator over all the stack slot keys, mutable edition.
pub fn iter_mut(&mut self) -> IterMut<StackSlot, StackSlotData> {
self.slots.iter_mut()
}
/// Get an iterator over all the stack slot records.
pub fn values(&self) -> slice::Iter<StackSlotData> {
self.slots.values()
}
/// Get an iterator over all the stack slot records, mutable edition.
pub fn values_mut(&mut self) -> slice::IterMut<StackSlotData> {
self.slots.values_mut()
}
/// Get an iterator over all the stack slot keys.
pub fn keys(&self) -> Keys<StackSlot> {
self.slots.keys()
}
/// Get a reference to the next stack slot that would be created by `push()`.
///
/// This should just be used by the parser.
pub fn next_key(&self) -> StackSlot {
self.slots.next_key()
}
}
impl Index<StackSlot> for StackSlots {
type Output = StackSlotData;
fn index(&self, ss: StackSlot) -> &StackSlotData {
&self.slots[ss]
}
}
impl IndexMut<StackSlot> for StackSlots {
fn index_mut(&mut self, ss: StackSlot) -> &mut StackSlotData {
&mut self.slots[ss]
}
}
/// Higher-level stack frame manipulation functions.
impl StackSlots {
/// Create a new spill slot for spilling values of type `ty`.
pub fn make_spill_slot(&mut self, ty: Type) -> StackSlot {
self.push(StackSlotData::new(StackSlotKind::SpillSlot, spill_size(ty)))
}
/// Create a stack slot representing an incoming function argument.
pub fn make_incoming_arg(&mut self, ty: Type, offset: StackOffset) -> StackSlot {
let mut data = StackSlotData::new(StackSlotKind::IncomingArg, ty.bytes());
debug_assert!(offset <= StackOffset::max_value() - data.size as StackOffset);
data.offset = Some(offset);
self.push(data)
}
/// Get a stack slot representing an outgoing argument.
///
/// This may create a new stack slot, or reuse an existing outgoing stack slot with the
/// requested offset and size.
///
/// The requested offset is relative to this function's stack pointer immediately before making
/// the call.
pub fn get_outgoing_arg(&mut self, ty: Type, offset: StackOffset) -> StackSlot {
let size = ty.bytes();
// Look for an existing outgoing stack slot with the same offset and size.
let inspos = match self.outgoing.binary_search_by_key(&(offset, size), |&ss| {
(self[ss].offset.unwrap(), self[ss].size)
}) {
Ok(idx) => return self.outgoing[idx],
Err(idx) => idx,
};
// No existing slot found. Make one and insert it into `outgoing`.
let mut data = StackSlotData::new(StackSlotKind::OutgoingArg, size);
debug_assert!(offset <= StackOffset::max_value() - size as StackOffset);
data.offset = Some(offset);
let ss = self.slots.push(data);
self.outgoing.insert(inspos, ss);
ss
}
/// Get an emergency spill slot that can be used to store a `ty` value.
///
/// This may allocate a new slot, or it may reuse an existing emergency spill slot, excluding
/// any slots in the `in_use` list.
pub fn get_emergency_slot(
&mut self,
ty: Type,
in_use: &[PackedOption<StackSlot>],
) -> StackSlot {
let size = spill_size(ty);
// Find the smallest existing slot that can fit the type.
if let Some(&ss) = self.emergency
.iter()
.filter(|&&ss| self[ss].size >= size && !in_use.contains(&ss.into()))
.min_by_key(|&&ss| self[ss].size)
{
return ss;
}
// Alternatively, use the largest available slot and make it larger.
if let Some(&ss) = self.emergency
.iter()
.filter(|&&ss| !in_use.contains(&ss.into()))
.max_by_key(|&&ss| self[ss].size)
{
self.slots[ss].size = size;
return ss;
}
// No existing slot found. Make one and insert it into `emergency`.
let data = StackSlotData::new(StackSlotKind::EmergencySlot, size);
let ss = self.slots.push(data);
self.emergency.push(ss);
ss
}
}
#[cfg(test)]
mod tests {
use super::*;
use ir::types;
use ir::Function;
use std::string::ToString;
#[test]
fn stack_slot() {
let mut func = Function::new();
let ss0 = func.create_stack_slot(StackSlotData::new(StackSlotKind::IncomingArg, 4));
let ss1 = func.create_stack_slot(StackSlotData::new(StackSlotKind::SpillSlot, 8));
assert_eq!(ss0.to_string(), "ss0");
assert_eq!(ss1.to_string(), "ss1");
assert_eq!(func.stack_slots[ss0].size, 4);
assert_eq!(func.stack_slots[ss1].size, 8);
assert_eq!(func.stack_slots[ss0].to_string(), "incoming_arg 4");
assert_eq!(func.stack_slots[ss1].to_string(), "spill_slot 8");
}
#[test]
fn outgoing() {
let mut sss = StackSlots::new();
let ss0 = sss.get_outgoing_arg(types::I32, 8);
let ss1 = sss.get_outgoing_arg(types::I32, 4);
let ss2 = sss.get_outgoing_arg(types::I64, 8);
assert_eq!(sss[ss0].offset, Some(8));
assert_eq!(sss[ss0].size, 4);
assert_eq!(sss[ss1].offset, Some(4));
assert_eq!(sss[ss1].size, 4);
assert_eq!(sss[ss2].offset, Some(8));
assert_eq!(sss[ss2].size, 8);
assert_eq!(sss.get_outgoing_arg(types::I32, 8), ss0);
assert_eq!(sss.get_outgoing_arg(types::I32, 4), ss1);
assert_eq!(sss.get_outgoing_arg(types::I64, 8), ss2);
}
#[test]
fn alignment() {
let slot = StackSlotData::new(StackSlotKind::SpillSlot, 8);
assert_eq!(slot.alignment(4), 4);
assert_eq!(slot.alignment(8), 8);
assert_eq!(slot.alignment(16), 8);
let slot2 = StackSlotData::new(StackSlotKind::ExplicitSlot, 24);
assert_eq!(slot2.alignment(4), 4);
assert_eq!(slot2.alignment(8), 8);
assert_eq!(slot2.alignment(16), 8);
assert_eq!(slot2.alignment(32), 8);
}
#[test]
fn emergency() {
let mut sss = StackSlots::new();
let ss0 = sss.get_emergency_slot(types::I32, &[]);
assert_eq!(sss[ss0].size, 4);
// When a smaller size is requested, we should simply get the same slot back.
assert_eq!(sss.get_emergency_slot(types::I8, &[]), ss0);
assert_eq!(sss[ss0].size, 4);
assert_eq!(sss.get_emergency_slot(types::F32, &[]), ss0);
assert_eq!(sss[ss0].size, 4);
// Ask for a larger size and the slot should grow.
assert_eq!(sss.get_emergency_slot(types::F64, &[]), ss0);
assert_eq!(sss[ss0].size, 8);
// When one slot is in use, we should get a new one.
let ss1 = sss.get_emergency_slot(types::I32, &[None.into(), ss0.into()]);
assert_eq!(sss[ss0].size, 8);
assert_eq!(sss[ss1].size, 4);
// Now we should get the smallest fit of the two available slots.
assert_eq!(sss.get_emergency_slot(types::F32, &[]), ss1);
assert_eq!(sss.get_emergency_slot(types::F64, &[]), ss0);
}
}