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Auto merge of #87916 - nbdd0121:black_box, r=nagisa

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Implement `black_box` using intrinsic

Introduce `black_box` intrinsic, as suggested in https://github.com/rust-lang/rust/pull/87590#discussion_r680468700.

This is still codegenned as empty inline assembly for LLVM. For MIR interpretation and cranelift it's treated as identity.

cc `@Amanieu` as this is related to inline assembly
cc `@bjorn3` for rustc_codegen_cranelift changes
cc `@RalfJung` as this affects MIRI

r? `@nagisa` I suppose
This commit is contained in:
bors 2021-08-12 21:04:07 +00:00
commit 0fa3190394
9 changed files with 52 additions and 15 deletions
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2
compiler/rustc_codegen_llvm/src/asm.rs
View file

@ -425,7 +425,7 @@ impl AsmMethods for CodegenCx<'ll, 'tcx> {
}
}
fn inline_asm_call(
pub(crate) fn inline_asm_call(
bx: &mut Builder<'a, 'll, 'tcx>,
asm: &str,
cons: &str,

26
compiler/rustc_codegen_llvm/src/intrinsic.rs
View file

@ -7,6 +7,7 @@ use crate::type_of::LayoutLlvmExt;
use crate::va_arg::emit_va_arg;
use crate::value::Value;
use rustc_ast as ast;
use rustc_codegen_ssa::base::{compare_simd_types, wants_msvc_seh};
use rustc_codegen_ssa::common::span_invalid_monomorphization_error;
use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
@ -327,6 +328,31 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
}
}
sym::black_box => {
args[0].val.store(self, result);
// 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.
crate::asm::inline_asm_call(
self,
"",
"r,~{memory}",
&[result.llval],
self.type_void(),
true,
false,
ast::LlvmAsmDialect::Att,
&[span],
)
.unwrap_or_else(|| bug!("failed to generate inline asm call for `black_box`"));
// We have copied the value to `result` already.
return;
}
_ if name_str.starts_with("simd_") => {
match generic_simd_intrinsic(self, name, callee_ty, args, ret_ty, llret_ty, span) {
Ok(llval) => llval,