vj-codegen 0.2.0

//! SV `string` lowering (LRM 1800-2023 §6.16). A string value is a 64-bit id
//! of an interned byte string (`vj_sim::strings`): stored like a 64-bit
//! 2-state integral (STATE cell, frame slot, argument), compared for
//! (in)equality by id, and operated on through `vj_runtime::strings_rt`.

use inkwell::AddressSpace;
use inkwell::module::Linkage;
use inkwell::types::{BasicMetadataTypeEnum, BasicType, BasicTypeEnum, StructType};
use inkwell::values::{BasicMetadataValueEnum, FunctionValue, IntValue, PointerValue};
use sv_lang::kinds::ExpressionKind;
use sv_lang::{BinaryOp, Expression, KnownSystemName};
use vj_runtime::strings_rt::{
    SV_RT_STR_CSTR_SYMBOL, SV_RT_STR_FROM_REAL_SYMBOL, SV_RT_STR_INTERN_SYMBOL,
    SV_RT_STR_OP_SYMBOL, SV_RT_STR_SFORMATF_SYMBOL, SV_RT_STR_TO_REAL_SYMBOL,
};
use vj_sim::strings::StrOp;

use crate::error::LoweringError;
use crate::expr::{LowerCtx, LoweredValue, lower_expr, lower_expr_h};
use crate::ops::OpBuilder;
use crate::types::TypeWidth;
use crate::value::{PackedValue, RealValue, ValueRepr};

/// A lowered SV string: its `i64` id.
#[derive(Clone, Copy, Debug)]
pub struct StrValue<'ctx> {
    pub llvm: IntValue<'ctx>,
}

fn rt_fn<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    name: &str,
    ret: Option<BasicTypeEnum<'ctx>>,
    params: &[BasicMetadataTypeEnum<'ctx>],
) -> FunctionValue<'ctx> {
    cx.module.get_function(name).unwrap_or_else(|| {
        let ty = match ret {
            Some(r) => r.fn_type(params, false),
            None => cx.ctx.void_type().fn_type(params, false),
        };
        cx.module.add_function(name, ty, Some(Linkage::External))
    })
}

fn call_i64<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    f: FunctionValue<'ctx>,
    args: &[BasicMetadataValueEnum<'ctx>],
) -> IntValue<'ctx> {
    cx.b.build_call(f, args, "str_call")
        .unwrap()
        .try_as_basic_value()
        .basic()
        .expect("returns a value")
        .into_int_value()
}

/// `sv_rt_str_op(op, a, b, c)`.
pub(crate) fn str_op<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    op: StrOp,
    a: IntValue<'ctx>,
    b: IntValue<'ctx>,
    c: IntValue<'ctx>,
) -> IntValue<'ctx> {
    let i64t = cx.ctx.i64_type();
    let f = rt_fn(
        cx,
        SV_RT_STR_OP_SYMBOL,
        Some(i64t.into()),
        &[
            cx.ctx.i32_type().into(),
            i64t.into(),
            i64t.into(),
            i64t.into(),
        ],
    );
    call_i64(
        cx,
        f,
        &[
            cx.ctx.i32_type().const_int(op as u64, false).into(),
            a.into(),
            b.into(),
            c.into(),
        ],
    )
}

fn i64c<'ctx>(cx: &LowerCtx<'ctx, '_, '_>, v: u64) -> IntValue<'ctx> {
    cx.ctx.i64_type().const_int(v, false)
}

/// A string literal's id: interned on first evaluation, then cached in a
/// private global.
pub(crate) fn literal<'ctx>(cx: &LowerCtx<'ctx, '_, '_>, text: &[u8]) -> StrValue<'ctx> {
    if text.iter().all(|b| *b == 0) {
        return StrValue { llvm: i64c(cx, 0) };
    }
    let i64t = cx.ctx.i64_type();
    let bytes = cx.ctx.const_string(text, false);
    let data = cx.module.add_global(bytes.get_type(), None, "strlit_bytes");
    data.set_initializer(&bytes);
    data.set_constant(true);
    data.set_linkage(Linkage::Private);
    let cache = cx.module.add_global(i64t, None, "strlit_id");
    cache.set_initializer(&i64t.const_zero());
    cache.set_linkage(Linkage::Private);
    let cur_bb = cx.b.get_insert_block().expect("positioned");
    let func = cur_bb.get_parent().expect("in a function");
    let miss = cx.ctx.append_basic_block(func, "strlit_intern");
    let done = cx.ctx.append_basic_block(func, "strlit_done");
    let cached =
        cx.b.build_load(i64t, cache.as_pointer_value(), "strlit_cached")
            .unwrap()
            .into_int_value();
    let hit =
        cx.b.build_int_compare(
            inkwell::IntPredicate::NE,
            cached,
            i64t.const_zero(),
            "strlit_hit",
        )
        .unwrap();
    cx.b.build_conditional_branch(hit, done, miss).unwrap();
    cx.b.position_at_end(miss);
    let ptr_ty = cx.ctx.ptr_type(AddressSpace::default());
    let intern = rt_fn(
        cx,
        SV_RT_STR_INTERN_SYMBOL,
        Some(i64t.into()),
        &[ptr_ty.into(), i64t.into()],
    );
    let id = call_i64(
        cx,
        intern,
        &[
            data.as_pointer_value().into(),
            i64c(cx, text.len() as u64).into(),
        ],
    );
    cx.b.build_store(cache.as_pointer_value(), id).unwrap();
    let miss_end = cx.b.get_insert_block().unwrap();
    cx.b.build_unconditional_branch(done).unwrap();
    cx.b.position_at_end(done);
    let phi = cx.b.build_phi(i64t, "strlit").unwrap();
    phi.add_incoming(&[(&cached, cur_bb), (&id, miss_end)]);
    StrValue {
        llvm: phi.as_basic_value().into_int_value(),
    }
}

/// The id of the NUL-terminated C string at `ptr` (a DPI `char*`).
pub(crate) fn intern_c<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    ptr: PointerValue<'ctx>,
) -> StrValue<'ctx> {
    let i64t = cx.ctx.i64_type();
    let f = rt_fn(
        cx,
        SV_RT_STR_INTERN_SYMBOL,
        Some(i64t.into()),
        &[cx.ctx.ptr_type(AddressSpace::default()).into(), i64t.into()],
    );
    StrValue {
        llvm: call_i64(cx, f, &[ptr.into(), i64c(cx, u64::MAX).into()]),
    }
}

/// The id of the `len`-byte string at `ptr` (not necessarily NUL-terminated).
pub(crate) fn intern_bytes<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    ptr: PointerValue<'ctx>,
    len: IntValue<'ctx>,
) -> StrValue<'ctx> {
    let i64t = cx.ctx.i64_type();
    let f = rt_fn(
        cx,
        SV_RT_STR_INTERN_SYMBOL,
        Some(i64t.into()),
        &[cx.ctx.ptr_type(AddressSpace::default()).into(), i64t.into()],
    );
    StrValue {
        llvm: call_i64(cx, f, &[ptr.into(), len.into()]),
    }
}

/// String `s` as a C string pointer.
pub(crate) fn c_str<'ctx>(cx: &LowerCtx<'ctx, '_, '_>, s: StrValue<'ctx>) -> PointerValue<'ctx> {
    let ptr_ty = cx.ctx.ptr_type(AddressSpace::default());
    let f = rt_fn(
        cx,
        SV_RT_STR_CSTR_SYMBOL,
        Some(ptr_ty.into()),
        &[cx.ctx.i64_type().into()],
    );
    cx.b.build_call(f, &[s.llvm.into()], "cstr")
        .unwrap()
        .try_as_basic_value()
        .basic()
        .unwrap()
        .into_pointer_value()
}

/// A string as the 64-bit integral its storage holds.
pub(crate) fn as_packed<'ctx>(cx: &LowerCtx<'ctx, '_, '_>, s: StrValue<'ctx>) -> PackedValue<'ctx> {
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    op.pack(s.llvm, op.i_n(64).const_zero(), 64, cx.repr, false)
}

/// `v` as a packed value, a string as its id (strings are interned, so ids
/// compare like their contents -- what `case` matching needs).
pub(crate) fn packed_or_id<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    v: LoweredValue<'ctx>,
    what: &str,
) -> Result<PackedValue<'ctx>, LoweringError> {
    match v {
        LoweredValue::Str(s) => Ok(as_packed(cx, s)),
        other => other.into_packed(what, &cx.fn_sym),
    }
}

/// The string stored as 64-bit integral `pv` (a string variable's value).
pub(crate) fn from_storage<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    pv: PackedValue<'ctx>,
) -> StrValue<'ctx> {
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    let pv = crate::select::fit_width(&op, pv, 64);
    StrValue {
        llvm: op.lane_val(pv),
    }
}

/// Any value as a string: a string as is, an integral value converted by
/// its bytes (LRM 6.16: most significant first, NUL bytes dropped).
pub(crate) fn to_string<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    v: LoweredValue<'ctx>,
) -> Result<StrValue<'ctx>, LoweringError> {
    match v {
        LoweredValue::Str(s) => Ok(s),
        LoweredValue::Packed(pv) => {
            let op = OpBuilder {
                ctx: cx.ctx,
                b: &cx.b,
            };
            let known = op.lane_val(op.to_2state4(pv));
            if pv.width > 128 {
                // 64-bit chunks, most significant first, each its own
                // NUL-dropping string, concatenated.
                let mut acc: Option<IntValue<'ctx>> = None;
                for k in (0..pv.width.div_ceil(64)).rev() {
                    let cw = (pv.width - 64 * k).min(64);
                    let sh =
                        cx.b.build_right_shift(
                            known,
                            op.i_n(pv.width).const_int(u64::from(64 * k), false),
                            false,
                            "str_chunk_sh",
                        )
                        .unwrap();
                    let chunk =
                        cx.b.build_int_truncate(sh, cx.ctx.i64_type(), "str_chunk")
                            .unwrap();
                    let part = str_op(
                        cx,
                        StrOp::FromBits,
                        chunk,
                        i64c(cx, 0),
                        i64c(cx, u64::from(cw)),
                    );
                    acc = Some(match acc {
                        None => part,
                        Some(prev) => str_op(cx, StrOp::Concat, prev, part, i64c(cx, 0)),
                    });
                }
                return Ok(StrValue {
                    llvm: acc.expect("a value has at least one chunk"),
                });
            }
            let w = pv.width.min(128);
            let v128 = crate::select::fit_width(
                &op,
                PackedValue {
                    llvm: known,
                    width: pv.width,
                    repr: ValueRepr::TwoState,
                    signed: false,
                },
                128,
            )
            .llvm;
            let lo =
                cx.b.build_int_truncate(v128, cx.ctx.i64_type(), "str_lo")
                    .unwrap();
            let hi_sh =
                cx.b.build_right_shift(v128, cx.ctx.i128_type().const_int(64, false), false, "hi")
                    .unwrap();
            let hi =
                cx.b.build_int_truncate(hi_sh, cx.ctx.i64_type(), "str_hi")
                    .unwrap();
            Ok(StrValue {
                llvm: str_op(cx, StrOp::FromBits, lo, hi, i64c(cx, u64::from(w))),
            })
        }
        LoweredValue::Huge(h) => Ok(crate::huge::to_string(cx, h)),
        LoweredValue::Real(_) | LoweredValue::Wide(_) => Err(LoweringError::unsupported_at(
            "a real or >128-bit value converted to a string",
            &cx.fn_sym,
        )),
    }
}

/// Widths above this build a string-to-bits value through memory.
const WORD_LOOP_BITS: u32 = 1024;

/// String `s` as a `width`-bit integral value (LRM 6.16).
pub(crate) fn to_bits<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    s: StrValue<'ctx>,
    width: u32,
    signed: bool,
) -> Result<PackedValue<'ctx>, LoweringError> {
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    if width > 128 {
        // Word by word (`StrOp::ToBitsWord`). Above `WORD_LOOP_BITS` the words
        // are stored into a pooled memory buffer and loaded back as one
        // integer: a per-word `zext`/`shl`/`or` chain over an `i57600`-class
        // value costs LLVM minutes in SelectionDAG combine.
        let t = op.i_n(width);
        let nwords = width.div_ceil(64);
        let word = |k: u32| {
            str_op(
                cx,
                StrOp::ToBitsWord,
                s.llvm,
                i64c(cx, u64::from(width)),
                i64c(cx, u64::from(k)),
            )
        };
        let v = if width > WORD_LOOP_BITS {
            let buf = crate::huge::acquire(cx, nwords);
            let i64t = cx.ctx.i64_type();
            for k in 0..nwords {
                // SAFETY: `k < nwords`, the length of the `[nwords x i64]` buffer.
                let slot = unsafe {
                    cx.b.build_in_bounds_gep(
                        i64t.array_type(nwords),
                        buf,
                        &[i64t.const_zero(), i64t.const_int(u64::from(k), false)],
                        "str_word_slot",
                    )
                    .unwrap()
                };
                cx.b.build_store(slot, word(k)).unwrap();
            }
            let v =
                cx.b.build_load(t, buf, "str_bits")
                    .unwrap()
                    .into_int_value();
            crate::huge::release(buf);
            v
        } else {
            let mut v = t.const_zero();
            for k in 0..nwords {
                let w = cx.b.build_int_z_extend(word(k), t, "str_word").unwrap();
                let w =
                    cx.b.build_left_shift(w, t.const_int(u64::from(64 * k), false), "str_word_sh")
                        .unwrap();
                v = cx.b.build_or(v, w, "str_bits").unwrap();
            }
            v
        };
        return Ok(op.pack(v, t.const_zero(), width, cx.repr, signed));
    }
    let lo = str_op(
        cx,
        StrOp::ToBitsLo,
        s.llvm,
        i64c(cx, u64::from(width)),
        i64c(cx, 0),
    );
    let hi = str_op(
        cx,
        StrOp::ToBitsHi,
        s.llvm,
        i64c(cx, u64::from(width)),
        i64c(cx, 0),
    );
    let i128t = cx.ctx.i128_type();
    let lo = cx.b.build_int_z_extend(lo, i128t, "lo").unwrap();
    let hi = cx.b.build_int_z_extend(hi, i128t, "hi").unwrap();
    let hi =
        cx.b.build_left_shift(hi, i128t.const_int(64, false), "hi_sh")
            .unwrap();
    let v = cx.b.build_or(lo, hi, "bits").unwrap();
    let v = crate::select::fit_width(
        &op,
        PackedValue {
            llvm: v,
            width: 128,
            repr: ValueRepr::TwoState,
            signed: false,
        },
        width,
    )
    .llvm;
    Ok(op.pack(v, op.i_n(width).const_zero(), width, cx.repr, signed))
}

/// A string-typed expression kind the generic lowering cannot handle:
/// conversions to `string`, concatenation, replication.
pub(crate) fn lower_string_expr<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    e: Expression<'d>,
) -> Result<Option<StrValue<'ctx>>, LoweringError> {
    let kind = e.kind();
    // A stream into a string (`s = {>>{bytes}}`): its bytes.
    let stream = if kind == ExpressionKind::Streaming {
        Some(e)
    } else if kind == ExpressionKind::Conversion {
        // `string'(queue_or_dyn_array)`: the elements' bytes, like a stream.
        e.conversion_operand().filter(|i| {
            i.kind() == ExpressionKind::Streaming || crate::stream::is_collection_operand(*i)
        })
    } else {
        None
    };
    if let Some(inner) = stream {
        return Ok(Some(crate::stream::stream_to_string(cx, inner)?));
    }
    Ok(Some(if kind == ExpressionKind::StringLiteral {
        literal(cx, e.string_literal_value().unwrap_or_default().as_bytes())
    } else if kind == ExpressionKind::Conversion {
        let inner = e.conversion_operand().ok_or_else(|| {
            LoweringError::unsupported_at("conversion missing operand", &cx.fn_sym)
        })?;
        if inner.kind() == ExpressionKind::StringLiteral {
            literal(
                cx,
                inner.string_literal_value().unwrap_or_default().as_bytes(),
            )
        } else {
            to_string(cx, lower_expr_h(cx, inner)?)?
        }
    } else if let Some(text) = (kind == ExpressionKind::Call
        && e.is_system_call() == Some(true)
        // slang folds `$sformatf` from the arguments its format consumes and
        // never looks at the rest, so `$sformatf("a %d", 1, x)` folds to
        // "a 1" with `x` unread, and a constant extra drops from the text. An
        // argument no specifier consumes prints in the default radix like
        // `$display`'s (LRM 21.2.1, 21.3.3): left to the run-time formatter.
        && !e.system_subroutine().is_some_and(|s| {
            matches!(
                s.known_name_id(),
                KnownSystemName::SFormatF | KnownSystemName::PSPrintF
            )
        }))
    .then(|| e.constant_value())
    .flatten()
    .and_then(|cv| cv.as_str().map(str::to_owned))
    {
        // A system call that folded to a string at elaboration (`$typename`,
        // LRM 20.6.1). Never a user function: slang folds a side-effect-free
        // call such as `h()`, skipping its `$display`s and static state
        // (LRM 13.4.1).
        literal(cx, text.as_bytes())
    } else if kind == ExpressionKind::Concatenation {
        let mut acc: Option<IntValue<'ctx>> = None;
        for part in e.operands() {
            let s = to_string(cx, lower_expr_h(cx, part)?)?;
            acc = Some(match acc {
                None => s.llvm,
                Some(a) => str_op(cx, StrOp::Concat, a, s.llvm, i64c(cx, 0)),
            });
        }
        StrValue {
            llvm: acc.unwrap_or_else(|| i64c(cx, 0)),
        }
    } else if kind == ExpressionKind::Replication {
        let ops = e.operands();
        let (count, inner) = match ops.as_slice() {
            [count, inner] => (*count, *inner),
            _ => {
                return Err(LoweringError::unsupported_at(
                    "string replication needs a count and an operand",
                    &cx.fn_sym,
                ));
            }
        };
        let n = lower_expr(cx, count)?.into_packed("replication count", &cx.fn_sym)?;
        let op = OpBuilder {
            ctx: cx.ctx,
            b: &cx.b,
        };
        let n = crate::select::fit_width(&op, op.to_2state4(n), 64);
        let s = to_string(cx, lower_expr_h(cx, inner)?)?;
        StrValue {
            llvm: str_op(cx, StrOp::Replicate, s.llvm, op.lane_val(n), i64c(cx, 0)),
        }
    } else {
        // Every other string-typed kind (a variable, a call, ...) lowers
        // generically to the string's stored id.
        return Ok(None);
    }))
}

/// A comparison with string operands: `==`/`!=` (and `===`/`!==`) by id,
/// the relational operators by `compare`'s sign. `None` for any other
/// operator.
pub(crate) fn lower_string_binary<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    e: Expression<'d>,
    bop: BinaryOp,
) -> Result<Option<LoweredValue<'ctx>>, LoweringError> {
    let (Some(l), Some(r)) = (e.left(), e.right()) else {
        return Ok(None);
    };
    let (a, b) = (
        to_string(cx, lower_expr_h(cx, l)?)?,
        to_string(cx, lower_expr_h(cx, r)?)?,
    );
    use inkwell::IntPredicate as P;
    let (pred, by_compare) = match bop {
        BinaryOp::Equality | BinaryOp::CaseEquality | BinaryOp::WildcardEquality => (P::EQ, false),
        BinaryOp::Inequality | BinaryOp::CaseInequality | BinaryOp::WildcardInequality => {
            (P::NE, false)
        }
        BinaryOp::LessThan => (P::SLT, true),
        BinaryOp::LessThanEqual => (P::SLE, true),
        BinaryOp::GreaterThan => (P::SGT, true),
        BinaryOp::GreaterThanEqual => (P::SGE, true),
        _ => return Ok(None),
    };
    let bit = if by_compare {
        let c = str_op(cx, StrOp::Compare, a.llvm, b.llvm, i64c(cx, 0));
        cx.b.build_int_compare(pred, c, i64c(cx, 0), "str_rel")
            .unwrap()
    } else {
        cx.b.build_int_compare(pred, a.llvm, b.llvm, "str_eq")
            .unwrap()
    };
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    Ok(Some(LoweredValue::Packed(op.pack(
        bit,
        op.i_n(1).const_zero(),
        1,
        cx.repr,
        false,
    ))))
}

/// `s inside {c0, [lo:hi], ...}` with a string left operand (LRM 11.4.13):
/// a plain candidate matches by string equality, a `[lo:hi]` range by the
/// string relational operators (6.16, lexicographic `compare`). A string is
/// never X, so the result is a known 0/1.
pub(crate) fn lower_string_inside<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    left: Expression<'d>,
    candidates: Vec<Expression<'d>>,
) -> Result<LoweredValue<'ctx>, LoweringError> {
    use inkwell::IntPredicate as P;
    if candidates.is_empty() {
        return Err(LoweringError::unsupported_at(
            "inside with an empty candidate list",
            &cx.fn_sym,
        ));
    }
    let a = to_string(cx, lower_expr_h(cx, left)?)?;
    let missing = |which: &str| {
        LoweringError::unsupported_at(&format!("inside range missing {which} bound"), &cx.fn_sym)
    };
    let mut acc: Option<IntValue<'ctx>> = None;
    for cand in candidates {
        let hit = if cand.kind() == ExpressionKind::ValueRange {
            crate::expr::refuse_tolerance_range(cand, &cx.fn_sym)?;
            let lo = cand.left().ok_or_else(|| missing("low"))?;
            let hi = cand.right().ok_or_else(|| missing("high"))?;
            let lo = to_string(cx, lower_expr_h(cx, lo)?)?;
            let hi = to_string(cx, lower_expr_h(cx, hi)?)?;
            let zero = i64c(cx, 0);
            let c_lo = str_op(cx, StrOp::Compare, a.llvm, lo.llvm, zero);
            let c_hi = str_op(cx, StrOp::Compare, a.llvm, hi.llvm, zero);
            let ge = cx.b.build_int_compare(P::SGE, c_lo, zero, "in_lo").unwrap();
            let le = cx.b.build_int_compare(P::SLE, c_hi, zero, "in_hi").unwrap();
            cx.b.build_and(ge, le, "in_rng").unwrap()
        } else {
            let c = to_string(cx, lower_expr_h(cx, cand)?)?;
            cx.b.build_int_compare(P::EQ, a.llvm, c.llvm, "in_eq")
                .unwrap()
        };
        acc = Some(match acc {
            None => hit,
            Some(prev) => cx.b.build_or(prev, hit, "in_or").unwrap(),
        });
    }
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    Ok(LoweredValue::Packed(op.pack(
        acc.expect("candidates non-empty"),
        op.i_n(1).const_zero(),
        1,
        cx.repr,
        false,
    )))
}

/// The length of the string-typed expression `e`, as an `i64`.
pub(crate) fn len_of<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    e: Expression<'d>,
) -> Result<IntValue<'ctx>, LoweringError> {
    let s = to_string(cx, lower_expr_h(cx, e)?)?;
    Ok(str_op(cx, StrOp::Len, s.llvm, i64c(cx, 0), i64c(cx, 0)))
}

/// `s[i]`: the byte at `i` (0 out of range), for a string-typed base.
pub(crate) fn lower_getc<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    base: Expression<'d>,
    index: Expression<'d>,
) -> Result<LoweredValue<'ctx>, LoweringError> {
    let s = to_string(cx, lower_expr_h(cx, base)?)?;
    let i = index_i64(cx, index)?;
    let c = str_op(cx, StrOp::Getc, s.llvm, i, i64c(cx, 0));
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    let byte = cx.b.build_int_truncate(c, op.i_n(8), "getc").unwrap();
    Ok(LoweredValue::Packed(op.pack(
        byte,
        op.i_n(8).const_zero(),
        8,
        cx.repr,
        false,
    )))
}

pub(crate) fn index_i64<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    e: Expression<'d>,
) -> Result<IntValue<'ctx>, LoweringError> {
    let pv = lower_expr(cx, e)?.into_packed("a string index", &cx.fn_sym)?;
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    let known = op.to_2state4(pv);
    let v = op.lane_val(known);
    Ok(if pv.width < 64 {
        if pv.signed {
            cx.b.build_int_s_extend(v, cx.ctx.i64_type(), "idx")
                .unwrap()
        } else {
            cx.b.build_int_z_extend(v, cx.ctx.i64_type(), "idx")
                .unwrap()
        }
    } else {
        crate::select::fit_width(&op, known, 64).llvm
    })
}

/// `s[i] = c` for a string-typed `s`: `s` becomes `s` with `putc(i, c)`.
/// Returns `Ok(false)` when `lhs` is not such a target.
pub(crate) fn assign_char<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    lhs: Expression<'d>,
    value: PackedValue<'ctx>,
) -> Result<bool, LoweringError> {
    if lhs.kind() != ExpressionKind::ElementSelect {
        return Ok(false);
    }
    let (Some(base), Some(index)) = (lhs.select_value(), lhs.selector()) else {
        return Ok(false);
    };
    if !base.expr_type().is_some_and(|t| t.is_string()) {
        return Ok(false);
    }
    let s = to_string(cx, lower_expr_h(cx, base)?)?;
    let i = index_i64(cx, index)?;
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    let c = crate::select::fit_width(&op, op.to_2state4(value), 64);
    let new = str_op(cx, StrOp::Putc, s.llvm, i, op.lane_val(c));
    crate::select::lower_assign_to(cx, base, as_packed(cx, StrValue { llvm: new }))?;
    Ok(true)
}

/// A string method or string-producing system function call (`s.len()`,
/// `s.toupper()`, `$sformatf(...)`, ...), `None` for any other system call.
/// The void methods (`putc`, `itoa`, ...) store into the object and yield a
/// placeholder value.
pub(crate) fn lower_string_sysfunc<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    name: KnownSystemName,
    e: Expression<'d>,
) -> Option<Result<LoweredValue<'ctx>, LoweringError>> {
    use KnownSystemName as K;
    let is_method = matches!(
        name,
        K::Len
            | K::Getc
            | K::Putc
            | K::ToUpper
            | K::ToLower
            | K::Compare
            | K::ICompare
            | K::Substr
            | K::AToI
            | K::AToHex
            | K::AToOct
            | K::AToBin
            | K::AToReal
            | K::IToA
            | K::HexToA
            | K::OctToA
            | K::BinToA
            | K::RealToA
            | K::Left
            | K::Right
            | K::Low
            | K::High
            | K::Size
    );
    let args = e.operands();
    if is_method
        && !args
            .first()
            .and_then(|a| a.expr_type())
            .is_some_and(|t| t.is_string())
    {
        return None;
    }
    if !is_method
        && !matches!(
            name,
            K::SFormatF
                | K::PSPrintF
                | K::SFormat
                | K::SWrite
                | K::SWriteB
                | K::SWriteO
                | K::SWriteH
        )
    {
        return None;
    }
    Some(lower_string_call(cx, name, e))
}

fn lower_string_call<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    name: KnownSystemName,
    e: Expression<'d>,
) -> Result<LoweredValue<'ctx>, LoweringError> {
    use KnownSystemName as K;
    let args = e.operands();
    let arg = |i: usize| {
        args.get(i).copied().ok_or_else(|| {
            LoweringError::unsupported_at("string method missing an argument", &cx.fn_sym)
        })
    };
    // An output argument (`$sformat`'s destination) arrives as
    // `dest = <result>`: its target is the assignment's left side.
    let target = |i: usize| -> Result<Expression<'d>, LoweringError> {
        let a = arg(i)?;
        Ok(if a.kind() == ExpressionKind::Assignment {
            a.left().unwrap_or(a)
        } else {
            a
        })
    };
    let op = OpBuilder {
        ctx: cx.ctx,
        b: &cx.b,
    };
    let int = |v: IntValue<'ctx>, width: u32, signed: bool| {
        let t =
            cx.b.build_int_truncate_or_bit_cast(v, op.i_n(width), "str_int")
                .unwrap();
        LoweredValue::Packed(op.pack(t, op.i_n(width).const_zero(), width, cx.repr, signed))
    };
    let this =
        || -> Result<StrValue<'ctx>, LoweringError> { to_string(cx, lower_expr_h(cx, arg(0)?)?) };
    let zero = i64c(cx, 0);
    let placeholder = || {
        LoweredValue::Packed(op.pack(
            op.i_n(1).const_zero(),
            op.i_n(1).const_zero(),
            1,
            cx.repr,
            false,
        ))
    };
    // `this = new` for the void methods.
    let store_this = |new: IntValue<'ctx>| -> Result<LoweredValue<'ctx>, LoweringError> {
        crate::select::lower_assign_to(cx, target(0)?, as_packed(cx, StrValue { llvm: new }))?;
        Ok(placeholder())
    };
    Ok(match name {
        K::Len | K::Size => int(str_op(cx, StrOp::Len, this()?.llvm, zero, zero), 32, true),
        // Array-query functions on a string (LRM 20.7): it is indexed
        // `0 .. len-1`, so `$left`/`$low` are 0 and `$right`/`$high` are
        // `len - 1` (-1 for the empty string), both run-time values.
        K::Left | K::Low => int(zero, 32, true),
        K::Right | K::High => {
            let len = str_op(cx, StrOp::Len, this()?.llvm, zero, zero);
            let last = cx.b.build_int_sub(len, i64c(cx, 1), "str_high").unwrap();
            int(last, 32, true)
        }
        K::Getc => return lower_getc(cx, arg(0)?, arg(1)?),
        K::ToUpper => LoweredValue::Str(StrValue {
            llvm: str_op(cx, StrOp::ToUpper, this()?.llvm, zero, zero),
        }),
        K::ToLower => LoweredValue::Str(StrValue {
            llvm: str_op(cx, StrOp::ToLower, this()?.llvm, zero, zero),
        }),
        K::Compare | K::ICompare => {
            let other = to_string(cx, lower_expr_h(cx, arg(1)?)?)?;
            let which = if name == K::Compare {
                StrOp::Compare
            } else {
                StrOp::ICompare
            };
            int(str_op(cx, which, this()?.llvm, other.llvm, zero), 32, true)
        }
        K::Substr => {
            let s = this()?;
            let (i, j) = (index_i64(cx, arg(1)?)?, index_i64(cx, arg(2)?)?);
            LoweredValue::Str(StrValue {
                llvm: str_op(cx, StrOp::Substr, s.llvm, i, j),
            })
        }
        K::AToI | K::AToHex | K::AToOct | K::AToBin => {
            let which = match name {
                K::AToI => StrOp::AToI,
                K::AToHex => StrOp::AToHex,
                K::AToOct => StrOp::AToOct,
                _ => StrOp::AToBin,
            };
            int(str_op(cx, which, this()?.llvm, zero, zero), 32, true)
        }
        K::AToReal => {
            let f = rt_fn(
                cx,
                SV_RT_STR_TO_REAL_SYMBOL,
                Some(cx.ctx.f64_type().into()),
                &[cx.ctx.i64_type().into()],
            );
            let r =
                cx.b.build_call(f, &[this()?.llvm.into()], "atoreal")
                    .unwrap()
                    .try_as_basic_value()
                    .basic()
                    .unwrap()
                    .into_float_value();
            LoweredValue::Real(RealValue { llvm: r })
        }
        K::Putc => {
            let s = this()?;
            let i = index_i64(cx, arg(1)?)?;
            let c = lower_expr(cx, arg(2)?)?.into_packed("putc character", &cx.fn_sym)?;
            let c = crate::select::fit_width(&op, op.to_2state4(c), 64);
            store_this(str_op(cx, StrOp::Putc, s.llvm, i, op.lane_val(c)))?
        }
        K::IToA | K::HexToA | K::OctToA | K::BinToA => {
            let v = lower_expr(cx, arg(1)?)?.into_packed("itoa value", &cx.fn_sym)?;
            let width = v.width.min(64);
            let bits = crate::select::fit_width(&op, op.to_2state4(v), 64);
            let which = match name {
                K::IToA => StrOp::IToA,
                K::HexToA => StrOp::HexToA,
                K::OctToA => StrOp::OctToA,
                _ => StrOp::BinToA,
            };
            let flags = u64::from(width) | if v.signed { 0x100 } else { 0 };
            store_this(str_op(cx, which, op.lane_val(bits), zero, i64c(cx, flags)))?
        }
        K::RealToA => {
            let r = lower_expr(cx, arg(1)?)?.into_real("realtoa value", &cx.fn_sym)?;
            let f = rt_fn(
                cx,
                SV_RT_STR_FROM_REAL_SYMBOL,
                Some(cx.ctx.i64_type().into()),
                &[cx.ctx.f64_type().into()],
            );
            let id = call_i64(cx, f, &[crate::real::as_f64(cx, r.llvm).into()]);
            store_this(id)?
        }
        K::SFormatF | K::PSPrintF => LoweredValue::Str(sformat(cx, &args, false, e)?),
        K::SFormat => {
            // `$sformat(dest, fmt, args...)`.
            let s = sformat(cx, &args[1..], false, e)?;
            crate::select::lower_assign_to(cx, target(0)?, dest_value(cx, target(0)?, s)?)?;
            placeholder()
        }
        K::SWrite | K::SWriteB | K::SWriteO | K::SWriteH => {
            // `$swrite(dest, args...)`: `$write`'s argument list, the
            // default radix from the task's name.
            let s = sformat(cx, &args[1..], true, e)?;
            crate::select::lower_assign_to(cx, target(0)?, dest_value(cx, target(0)?, s)?)?;
            placeholder()
        }
        _ => unreachable!("filtered by lower_string_sysfunc"),
    })
}

/// The value `$sformat`/`$swrite` store into `dest`: the string itself, or
/// its bytes for an integral destination.
fn dest_value<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    dest: Expression<'d>,
    s: StrValue<'ctx>,
) -> Result<PackedValue<'ctx>, LoweringError> {
    let ty = dest
        .expr_type()
        .ok_or_else(|| LoweringError::unsupported_at("$sformat target has no type", &cx.fn_sym))?;
    if ty.is_string() {
        Ok(as_packed(cx, s))
    } else {
        to_bits(cx, s, ty.value_width(), ty.is_signed())
    }
}

/// Formats `args` (`[fmt, values...]`, or with `arg_list` a `$write`-style
/// argument list) into a new string.
fn sformat<'ctx, 'd>(
    cx: &LowerCtx<'ctx, 'd, '_>,
    args: &[Expression<'d>],
    arg_list: bool,
    call: Expression<'d>,
) -> Result<StrValue<'ctx>, LoweringError> {
    let literal_format = args.first().is_some_and(|f| {
        let mut f = *f;
        while f.kind() == ExpressionKind::Conversion {
            match f.conversion_operand() {
                Some(inner) => f = inner,
                None => break,
            }
        }
        f.kind() == ExpressionKind::StringLiteral
    });
    let (fmt_ptr, vals) = if arg_list || literal_format {
        // `$swrite`: arguments like `$write`'s (LRM 1800-2023 §21.3.3); a
        // literal format expands like `$display`'s (`%p` of an aggregate).
        crate::systask::display_format_and_values(cx, args, call)?
    } else {
        let (fmt, rest) = args.split_first().ok_or_else(|| {
            LoweringError::unsupported_at("$sformatf needs a format string", &cx.fn_sym)
        })?;
        let fmt_ptr = crate::systask::lower_display_format(cx, *fmt, call)?;
        let mut vals = Vec::with_capacity(rest.len());
        for v in rest {
            vals.push(if v.kind() == ExpressionKind::StringLiteral {
                LoweredValue::Str(literal(
                    cx,
                    v.string_literal_value().unwrap_or_default().as_bytes(),
                ))
            } else {
                lower_expr(cx, *v)?
            });
        }
        (fmt_ptr, vals)
    };
    sformat_with(cx, fmt_ptr, vals)
}

/// The string the already-assembled format `fmt_ptr` and its argument values
/// `vals` print as (the shared tail of `$sformatf`/`$swrite`; `%p` of a class
/// handle builds its text with it too, `crate::classes::format_object`).
pub(crate) fn sformat_with<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    fmt_ptr: PointerValue<'ctx>,
    vals: Vec<LoweredValue<'ctx>>,
) -> Result<StrValue<'ctx>, LoweringError> {
    let rtarg_ty = crate::systask::rtarg_llvm_type(cx.ctx);
    let (args_ptr, n) = crate::systask::build_rtarg_array(cx, rtarg_ty, vals)?;
    let ptr_ty = cx.ctx.ptr_type(AddressSpace::default());
    let f = rt_fn(
        cx,
        SV_RT_STR_SFORMATF_SYMBOL,
        Some(cx.ctx.i64_type().into()),
        &[ptr_ty.into(), ptr_ty.into(), cx.ctx.i32_type().into()],
    );
    Ok(StrValue {
        llvm: call_i64(cx, f, &[fmt_ptr.into(), args_ptr.into(), n.into()]),
    })
}

/// A `%s` display argument: `RtArgKind::Str` (2) with the string's bytes.
pub(crate) fn store_str_rtarg<'ctx>(
    cx: &LowerCtx<'ctx, '_, '_>,
    rtarg_ty: StructType<'ctx>,
    elem_ptr: PointerValue<'ctx>,
    s: StrValue<'ctx>,
) {
    let ctx = cx.ctx;
    let b = &cx.b;
    let i32_ty = ctx.i32_type();
    let ptr_ty = ctx.ptr_type(AddressSpace::default());
    let field = |idx: u32, name: &str| b.build_struct_gep(rtarg_ty, elem_ptr, idx, name).unwrap();
    let len = str_op(cx, StrOp::Len, s.llvm, i64c(cx, 0), i64c(cx, 0));
    let len32 = b.build_int_truncate(len, i32_ty, "str_len").unwrap();
    let data = c_str(cx, s);
    b.build_store(field(0, "rtarg_kind"), i32_ty.const_int(2, false))
        .unwrap();
    b.build_store(
        field(1, "rtarg_width"),
        b.build_int_mul(len32, i32_ty.const_int(8, false), "w")
            .unwrap(),
    )
    .unwrap();
    b.build_store(field(2, "rtarg_signed"), ctx.i8_type().const_zero())
        .unwrap();
    b.build_store(field(3, "rtarg_narrow"), ctx.i128_type().const_zero())
        .unwrap();
    b.build_store(field(4, "rtarg_real"), ctx.f64_type().const_zero())
        .unwrap();
    b.build_store(field(5, "rtarg_wide_aval"), ptr_ty.const_null())
        .unwrap();
    b.build_store(field(6, "rtarg_wide_bval"), ptr_ty.const_null())
        .unwrap();
    b.build_store(field(7, "rtarg_wide_words"), i32_ty.const_zero())
        .unwrap();
    b.build_store(field(8, "rtarg_str_ptr"), data).unwrap();
    b.build_store(field(9, "rtarg_str_len"), len32).unwrap();
}