bjoernager/rust
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MaybeUninit::assume_init_read should have noundef load metadata

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I was looking into `array::IntoIter` optimization, and noticed that it wasn't annotating the loads with `noundef` for simple things like `array::IntoIter<i32, N>`.

Turned out to be a more general problem as `MaybeUninit::assume_init_read` isn't marking the load as initialized (<https://rust.godbolt.org/z/Mxd8TPTnv>), which is unfortunate since that's basically its reason to exist.

This PR lowers `ptr::read(p)` to `copy *p` in MIR, which fortuitiously also improves the IR we give to LLVM for things like `mem::replace`.
This commit is contained in:
Scott McMurray 2023-03-11 15:32:54 -08:00
parent 19c53768af
commit b2c717fa33
14 changed files with 214 additions and 39 deletions
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24
compiler/rustc_mir_transform/src/lower_intrinsics.rs
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@ -149,6 +149,30 @@ impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
terminator.kind = TerminatorKind::Goto { target };
}
}
sym::read_via_copy => {
let Ok([arg]) = <[_; 1]>::try_from(std::mem::take(args)) else {
span_bug!(terminator.source_info.span, "Wrong number of arguments");
};
let derefed_place =
if let Some(place) = arg.place() && let Some(local) = place.as_local() {
tcx.mk_place_deref(local.into())
} else {
span_bug!(terminator.source_info.span, "Only passing a local is supported");
};
block.statements.push(Statement {
source_info: terminator.source_info,
kind: StatementKind::Assign(Box::new((
*destination,
Rvalue::Use(Operand::Copy(derefed_place)),
))),
});
if let Some(target) = *target {
terminator.kind = TerminatorKind::Goto { target };
} else {
// Reading something uninhabited means this is unreachable.
terminator.kind = TerminatorKind::Unreachable;
}
}
sym::discriminant_value => {
if let (Some(target), Some(arg)) = (*target, args[0].place()) {
let arg = tcx.mk_place_deref(arg);