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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 #87590 (comment).

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
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bors committed Aug 12, 2021
2 parents 4498e30 + 1fb1643 commit 0fa3190
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Showing 9 changed files with 52 additions and 15 deletions.
5 changes: 5 additions & 0 deletions compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1136,6 +1136,11 @@ pub(crate) fn codegen_intrinsic_call<'tcx>(
};
ret.write_cvalue(fx, CValue::by_val(is_eq_value, ret.layout()));
};

black_box, (c a) {
// FIXME implement black_box semantics
ret.write_cvalue(fx, a);
};
}

if let Some((_, dest)) = destination {
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2 changes: 1 addition & 1 deletion compiler/rustc_codegen_llvm/src/asm.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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,
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26 changes: 26 additions & 0 deletions compiler/rustc_codegen_llvm/src/intrinsic.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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};
Expand Down Expand Up @@ -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,
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2 changes: 1 addition & 1 deletion compiler/rustc_mir/src/interpret/intrinsics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -465,7 +465,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
);
self.copy_op(&self.operand_index(&args[0], index)?, dest)?;
}
sym::likely | sym::unlikely => {
sym::likely | sym::unlikely | sym::black_box => {
// These just return their argument
self.copy_op(&args[0], dest)?;
}
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1 change: 1 addition & 0 deletions compiler/rustc_span/src/symbol.rs
Original file line number Diff line number Diff line change
Expand Up @@ -335,6 +335,7 @@ symbols! {
bitreverse,
bitxor,
bitxor_assign,
black_box,
block,
bool,
borrowck_graphviz_format,
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3 changes: 3 additions & 0 deletions compiler/rustc_typeck/src/check/intrinsic.rs
Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,7 @@ pub fn intrinsic_operation_unsafety(intrinsic: Symbol) -> hir::Unsafety {
| sym::maxnumf64
| sym::type_name
| sym::forget
| sym::black_box
| sym::variant_count => hir::Unsafety::Normal,
_ => hir::Unsafety::Unsafe,
}
Expand Down Expand Up @@ -387,6 +388,8 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
(1, vec![param_ty; 2], tcx.types.bool)
}

sym::black_box => (1, vec![param(0)], param(0)),

other => {
tcx.sess.emit_err(UnrecognizedIntrinsicFunction { span: it.span, name: other });
return;
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20 changes: 8 additions & 12 deletions library/core/src/hint.rs
Original file line number Diff line number Diff line change
Expand Up @@ -152,23 +152,19 @@ pub fn spin_loop() {
/// backend used. Programs cannot rely on `black_box` for *correctness* in any way.
///
/// [`std::convert::identity`]: crate::convert::identity
#[cfg_attr(not(miri), inline)]
#[cfg_attr(miri, inline(never))]
#[inline]
#[unstable(feature = "bench_black_box", issue = "64102")]
#[cfg_attr(miri, allow(unused_mut))]
#[cfg_attr(not(bootstrap), allow(unused_mut))]
pub fn black_box<T>(mut dummy: T) -> T {
// 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.

#[cfg(not(miri))] // This is just a hint, so it is fine to skip in Miri.
#[cfg(bootstrap)]
// SAFETY: the inline assembly is a no-op.
unsafe {
// FIXME: Cannot use `asm!` because it doesn't support MIPS and other architectures.
llvm_asm!("" : : "r"(&mut dummy) : "memory" : "volatile");
dummy
}

dummy
#[cfg(not(bootstrap))]
{
crate::intrinsics::black_box(dummy)
}
}
6 changes: 6 additions & 0 deletions library/core/src/intrinsics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1933,6 +1933,12 @@ extern "rust-intrinsic" {
/// which is UB if any of their inputs are `undef`.)
#[rustc_const_unstable(feature = "const_intrinsic_raw_eq", issue = "none")]
pub fn raw_eq<T>(a: &T, b: &T) -> bool;

/// See documentation of [`std::hint::black_box`] for details.
///
/// [`std::hint::black_box`]: crate::hint::black_box
#[cfg(not(bootstrap))]
pub fn black_box<T>(dummy: T) -> T;
}

// Some functions are defined here because they accidentally got made
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2 changes: 1 addition & 1 deletion src/test/ui/sanitize/memory.rs
Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@
// run-fail
// error-pattern: MemorySanitizer: use-of-uninitialized-value
// error-pattern: Uninitialized value was created by an allocation
// error-pattern: in the stack frame of function 'random'
// error-pattern: in the stack frame of function 'main'
//
// This test case intentionally limits the usage of the std,
// since it will be linked with an uninstrumented version of it.
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