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prevent uninitialized access in black_box for zero-sized-types

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This commit is contained in:
Krasimir Georgiev 2022-11-04 16:27:28 +00:00
parent 47c008e440
commit 0e0bcd95cd
2 changed files with 44 additions and 3 deletions
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15
compiler/rustc_codegen_llvm/src/intrinsic.rs
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@ -340,17 +340,26 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
sym::black_box => {
args[0].val.store(self, result);
let result_val_span = [result.llval];
// We need to "use" the argument in some way LLVM can't introspect, and on
// targets that support it we can typically leverage inline assembly to do
// this. LLVM's interpretation of inline assembly is that it's, well, a black
// box. This isn't the greatest implementation since it probably deoptimizes
// more than we want, but it's so far good enough.
//
// For zero-sized types, the location pointed to by the result may be
// uninitialized. Do not "use" the result in this case; instead just clobber
// the memory.
let (constraint, inputs): (&str, &[_]) = if result.layout.is_zst() {
("~{memory}", &[])
} else {
("r,~{memory}", &result_val_span)
};
crate::asm::inline_asm_call(
self,
"",
"r,~{memory}",
&[result.llval],
constraint,
inputs,
self.type_void(),
true,
false,