bjoernager/rust
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use GEP inbounds for ZST and DST field offsets

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For the former, it's fine for `inbounds` offsets to be one-past-the-end,
so it's okay even if the ZST is the last field in the layout:

> The base pointer has an in bounds address of an allocated object,
> which means that it points into an allocated object, or to its end.

https://llvm.org/docs/LangRef.html#getelementptr-instruction

For the latter, even DST fields must always be inside the layout
(or to its end for ZSTs), so using inbounds is also fine there.
This commit is contained in:
Erik Desjardins 2024-02-28 00:09:24 -05:00
parent 7606c13961
commit 8ebd307d2a
3 changed files with 73 additions and 9 deletions
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7
compiler/rustc_codegen_ssa/src/mir/place.rs
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@ -104,10 +104,6 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
let mut simple = || {
let llval = if offset.bytes() == 0 {
self.llval
} else if field.is_zst() {
// FIXME(erikdesjardins): it should be fine to use inbounds for ZSTs too;
// keeping this logic for now to preserve previous behavior.
bx.ptradd(self.llval, bx.const_usize(offset.bytes()))
} else {
bx.inbounds_ptradd(self.llval, bx.const_usize(offset.bytes()))
};
@ -168,8 +164,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
debug!("struct_field_ptr: DST field offset: {:?}", offset);
// Adjust pointer.
// FIXME(erikdesjardins): should be able to use inbounds here too.
let ptr = bx.ptradd(self.llval, offset);
let ptr = bx.inbounds_ptradd(self.llval, offset);
PlaceRef { llval: ptr, llextra: self.llextra, layout: field, align: effective_field_align }
}