bjoernager/rust
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Force LLVM to use CMOV for binary search

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Since https://reviews.llvm.org/D118118, LLVM will no longer turn CMOVs
into branches if it comes from a `select` marked with an `unpredictable`
metadata attribute.

This PR introduces `core::intrinsics::select_unpredictable` which emits
such a `select` and uses it in the implementation of `binary_search_by`.
This commit is contained in:
Amanieu d'Antras 2024-07-26 19:36:21 +01:00
parent 355efacf0d
commit 4f78f9fbb0
7 changed files with 107 additions and 1 deletions
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31
compiler/rustc_codegen_llvm/src/intrinsic.rs
View file

@ -11,7 +11,7 @@ use rustc_codegen_ssa::base::{compare_simd_types, wants_msvc_seh, wants_wasm_eh}
use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
use rustc_codegen_ssa::errors::{ExpectedPointerMutability, InvalidMonomorphization};
use rustc_codegen_ssa::mir::operand::{OperandRef, OperandValue};
use rustc_codegen_ssa::mir::place::PlaceRef;
use rustc_codegen_ssa::mir::place::{PlaceRef, PlaceValue};
use rustc_codegen_ssa::traits::*;
use rustc_hir as hir;
use rustc_middle::mir::BinOp;
@ -203,6 +203,35 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
}
sym::unlikely => self
.call_intrinsic("llvm.expect.i1", &[args[0].immediate(), self.const_bool(false)]),
sym::select_unpredictable => {
let cond = args[0].immediate();
assert_eq!(args[1].layout, args[2].layout);
let select = |bx: &mut Self, true_val, false_val| {
let result = bx.select(cond, true_val, false_val);
bx.set_unpredictable(&result);
result
};
match (args[1].val, args[2].val) {
(OperandValue::Ref(true_val), OperandValue::Ref(false_val)) => {
assert!(true_val.llextra.is_none());
assert!(false_val.llextra.is_none());
assert_eq!(true_val.align, false_val.align);
let ptr = select(self, true_val.llval, false_val.llval);
let selected =
OperandValue::Ref(PlaceValue::new_sized(ptr, true_val.align));
selected.store(self, result);
return Ok(());
}
(OperandValue::Immediate(_), OperandValue::Immediate(_))
| (OperandValue::Pair(_, _), OperandValue::Pair(_, _)) => {
let true_val = args[1].immediate_or_packed_pair(self);
let false_val = args[2].immediate_or_packed_pair(self);
select(self, true_val, false_val)
}
(OperandValue::ZeroSized, OperandValue::ZeroSized) => return Ok(()),
_ => span_bug!(span, "Incompatible OperandValue for select_unpredictable"),
}
}
sym::catch_unwind => {
catch_unwind_intrinsic(
self,