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Auto merge of #122048 - erikdesjardins:inbounds, r=oli-obk

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Use GEP inbounds for ZST and DST field offsets

ZST field offsets have been non-`inbounds` since I made [this old layout change](https://github.com/rust-lang/rust/pull/73453/files#diff-160634de1c336f2cf325ff95b312777326f1ab29fec9b9b21d5ee9aae215ecf5). Before that, they would have been `inbounds` due to using `struct_gep`. Using `inbounds` for ZSTs likely doesn't matter for performance, but I'd like to remove the special case.

DST field offsets have been non-`inbounds` since the alignment-aware DST field offset computation was first [implemented](a2557d472e (diff-04fd352da30ca186fe0bb71cc81a503d1eb8a02ca17a3769e1b95981cd20964aR1188)) in 1.6 (back then `GEPi()` would be used for `inbounds`), but I don't think there was any reason for it.

Split out from #121577 / #121665.

r? `@oli-obk`

cc `@RalfJung` -- is there some weird situation where field offsets can't be `inbounds`?

Note that it's fine for `inbounds` offsets to be one-past-the-end, so it's okay even if there's a ZST as 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. [(link)](https://llvm.org/docs/LangRef.html#getelementptr-instruction)

For https://github.com/rust-lang/unsafe-code-guidelines/issues/93, zero-offset GEP is (now) always `inbounds`:

> Note that getelementptr with all-zero indices is always considered to be inbounds, even if the base pointer does not point to an allocated object. [(link)](https://llvm.org/docs/LangRef.html#getelementptr-instruction)
This commit is contained in:
bors 2024-03-08 02:01:51 +00:00
commit 79d246112d
3 changed files with 88 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 }
}