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Rollup merge of #122619 - erikdesjardins:cast, r=compiler-errors

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Fix some unsoundness with PassMode::Cast ABI

Fixes #122617

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Matthias Krüger 2024-04-03 22:11:00 +02:00 • committed by GitHub
commit bc8415b9e6
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8 changed files with 476 additions and 89 deletions
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32
compiler/rustc_codegen_ssa/src/mir/block.rs
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@ -1505,9 +1505,35 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
if by_ref && !arg.is_indirect() {
// Have to load the argument, maybe while casting it.
if let PassMode::Cast { cast: ty, .. } = &arg.mode {
let llty = bx.cast_backend_type(ty);
llval = bx.load(llty, llval, align.min(arg.layout.align.abi));
if let PassMode::Cast { cast, pad_i32: _ } = &arg.mode {
// The ABI mandates that the value is passed as a different struct representation.
// Spill and reload it from the stack to convert from the Rust representation to
// the ABI representation.
let scratch_size = cast.size(bx);
let scratch_align = cast.align(bx);
// Note that the ABI type may be either larger or smaller than the Rust type,
// due to the presence or absence of trailing padding. For example:
// - On some ABIs, the Rust layout { f64, f32, <f32 padding> } may omit padding
// when passed by value, making it smaller.
// - On some ABIs, the Rust layout { u16, u16, u16 } may be padded up to 8 bytes
// when passed by value, making it larger.
let copy_bytes = cmp::min(scratch_size.bytes(), arg.layout.size.bytes());
// Allocate some scratch space...
let llscratch = bx.alloca(bx.cast_backend_type(cast), scratch_align);
bx.lifetime_start(llscratch, scratch_size);
// ...memcpy the value...
bx.memcpy(
llscratch,
scratch_align,
llval,
align,
bx.const_usize(copy_bytes),
MemFlags::empty(),
);
// ...and then load it with the ABI type.
let cast_ty = bx.cast_backend_type(cast);
llval = bx.load(cast_ty, llscratch, scratch_align);
bx.lifetime_end(llscratch, scratch_size);
} else {
// We can't use `PlaceRef::load` here because the argument
// may have a type we don't treat as immediate, but the ABI