[clang] [LLVM] Fix clang to emit llvm-ir for fadd/fsub atomics (PR #162679)
Amina Chabane via cfe-commits
cfe-commits at lists.llvm.org
Thu Oct 9 08:28:45 PDT 2025
https://github.com/Amichaxx created https://github.com/llvm/llvm-project/pull/162679
Currently, Clang emits CAS loops for atomic fp compound assignments, instead of atomicrmw instructions. The code in CGExprScalar.cpp now checks for both integer and floating-point atomic types and emits atomicrmw fadd/fsub instructions in the LLVM IR.
cc: @efriedma-quic ?
>From 9a8a25e0c124c24f551f35c0bd32a07ed9db893f Mon Sep 17 00:00:00 2001
From: Amichaxx <amina.chabane at arm.com>
Date: Mon, 6 Oct 2025 15:42:18 +0000
Subject: [PATCH] [LLVM] Fix clang to emit llvm-ir for fadd/fsub atomics
Currently, Clang emits CAS loops for atoic fp compound assignments, instead of atomicrmw instructions. The code in CGExprScalar.cpp now checks for both integer and floating-point
atomic types and emits atomicrmw fadd/fsub instructions in the LLVM IR.
---
clang/lib/CodeGen/CGExprScalar.cpp | 25 ++++++++--
clang/test/CodeGen/aarch64-lsfe-atomics.c | 61 +++++++++++++++++++++++
2 files changed, 81 insertions(+), 5 deletions(-)
create mode 100644 clang/test/CodeGen/aarch64-lsfe-atomics.c
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index f319b176513f8..e9f5226bbb548 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -3845,9 +3845,20 @@ LValue ScalarExprEmitter::EmitCompoundAssignLValue(
LValue LHSLV = EmitCheckedLValue(E->getLHS(), CodeGenFunction::TCK_Store);
llvm::PHINode *atomicPHI = nullptr;
+
if (const AtomicType *atomicTy = LHSTy->getAs<AtomicType>()) {
QualType type = atomicTy->getValueType();
- if (!type->isBooleanType() && type->isIntegerType() &&
+ const bool isFloat = type->isFloatingType();
+ const bool isInteger = type->isIntegerType();
+
+ bool isPowerOfTwo = false;
+ if (isFloat || isInteger) {
+ llvm::Type *IRTy = CGF.ConvertType(type);
+ uint64_t StoreBits =
+ CGF.CGM.getDataLayout().getTypeStoreSizeInBits(IRTy);
+ isPowerOfTwo = llvm::isPowerOf2_64(StoreBits);
+ }
+ if (!type->isBooleanType() && (isInteger || isFloat) && isPowerOfTwo &&
!(type->isUnsignedIntegerType() &&
CGF.SanOpts.has(SanitizerKind::UnsignedIntegerOverflow)) &&
CGF.getLangOpts().getSignedOverflowBehavior() !=
@@ -3862,12 +3873,16 @@ LValue ScalarExprEmitter::EmitCompoundAssignLValue(
case BO_ShrAssign:
break;
case BO_AddAssign:
- AtomicOp = llvm::AtomicRMWInst::Add;
- Op = llvm::Instruction::Add;
+ AtomicOp = isFloat ? llvm::AtomicRMWInst::FAdd
+ : llvm::AtomicRMWInst::Add;
+ Op = isFloat ? llvm::Instruction::FAdd
+ : llvm::Instruction::Add;
break;
case BO_SubAssign:
- AtomicOp = llvm::AtomicRMWInst::Sub;
- Op = llvm::Instruction::Sub;
+ AtomicOp = isFloat ? llvm::AtomicRMWInst::FSub
+ : llvm::AtomicRMWInst::Sub;
+ Op = isFloat ? llvm::Instruction::FSub
+ : llvm::Instruction::Sub;
break;
case BO_AndAssign:
AtomicOp = llvm::AtomicRMWInst::And;
diff --git a/clang/test/CodeGen/aarch64-lsfe-atomics.c b/clang/test/CodeGen/aarch64-lsfe-atomics.c
new file mode 100644
index 0000000000000..957e960b3fe3a
--- /dev/null
+++ b/clang/test/CodeGen/aarch64-lsfe-atomics.c
@@ -0,0 +1,61 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 6
+// RUN: %clang_cc1 %s -emit-llvm -o - -triple=aarch64-linux-gnu | FileCheck %s --check-prefix=CHECK-LLVM
+
+_Atomic(float) f;
+_Atomic(double) d;
+
+// CHECK-LLVM-LABEL: define dso_local void @test_float_add(
+// CHECK-LLVM-SAME: float noundef [[VAL:%.*]]) #[[ATTR0:[0-9]+]] {
+// CHECK-LLVM-NEXT: [[ENTRY:.*:]]
+// CHECK-LLVM-NEXT: [[VAL_ADDR:%.*]] = alloca float, align 4
+// CHECK-LLVM-NEXT: store float [[VAL]], ptr [[VAL_ADDR]], align 4
+// CHECK-LLVM-NEXT: [[TMP0:%.*]] = load float, ptr [[VAL_ADDR]], align 4
+// CHECK-LLVM-NEXT: [[TMP1:%.*]] = atomicrmw fadd ptr @f, float [[TMP0]] seq_cst, align 4
+// CHECK-LLVM-NEXT: [[TMP2:%.*]] = fadd float [[TMP1]], [[TMP0]]
+// CHECK-LLVM-NEXT: ret void
+//
+void test_float_add(float val) {
+ f += val;
+}
+
+// CHECK-LLVM-LABEL: define dso_local void @test_double_add(
+// CHECK-LLVM-SAME: double noundef [[VAL:%.*]]) #[[ATTR0]] {
+// CHECK-LLVM-NEXT: [[ENTRY:.*:]]
+// CHECK-LLVM-NEXT: [[VAL_ADDR:%.*]] = alloca double, align 8
+// CHECK-LLVM-NEXT: store double [[VAL]], ptr [[VAL_ADDR]], align 8
+// CHECK-LLVM-NEXT: [[TMP0:%.*]] = load double, ptr [[VAL_ADDR]], align 8
+// CHECK-LLVM-NEXT: [[TMP1:%.*]] = atomicrmw fadd ptr @d, double [[TMP0]] seq_cst, align 8
+// CHECK-LLVM-NEXT: [[TMP2:%.*]] = fadd double [[TMP1]], [[TMP0]]
+// CHECK-LLVM-NEXT: ret void
+//
+void test_double_add(double val) {
+ d += val;
+}
+
+// CHECK-LLVM-LABEL: define dso_local void @test_float_sub(
+// CHECK-LLVM-SAME: float noundef [[VAL:%.*]]) #[[ATTR0]] {
+// CHECK-LLVM-NEXT: [[ENTRY:.*:]]
+// CHECK-LLVM-NEXT: [[VAL_ADDR:%.*]] = alloca float, align 4
+// CHECK-LLVM-NEXT: store float [[VAL]], ptr [[VAL_ADDR]], align 4
+// CHECK-LLVM-NEXT: [[TMP0:%.*]] = load float, ptr [[VAL_ADDR]], align 4
+// CHECK-LLVM-NEXT: [[TMP1:%.*]] = atomicrmw fsub ptr @f, float [[TMP0]] seq_cst, align 4
+// CHECK-LLVM-NEXT: [[TMP2:%.*]] = fsub float [[TMP1]], [[TMP0]]
+// CHECK-LLVM-NEXT: ret void
+//
+void test_float_sub(float val) {
+ f -= val;
+}
+
+// CHECK-LLVM-LABEL: define dso_local void @test_double_sub(
+// CHECK-LLVM-SAME: double noundef [[VAL:%.*]]) #[[ATTR0]] {
+// CHECK-LLVM-NEXT: [[ENTRY:.*:]]
+// CHECK-LLVM-NEXT: [[VAL_ADDR:%.*]] = alloca double, align 8
+// CHECK-LLVM-NEXT: store double [[VAL]], ptr [[VAL_ADDR]], align 8
+// CHECK-LLVM-NEXT: [[TMP0:%.*]] = load double, ptr [[VAL_ADDR]], align 8
+// CHECK-LLVM-NEXT: [[TMP1:%.*]] = atomicrmw fsub ptr @d, double [[TMP0]] seq_cst, align 8
+// CHECK-LLVM-NEXT: [[TMP2:%.*]] = fsub double [[TMP1]], [[TMP0]]
+// CHECK-LLVM-NEXT: ret void
+//
+void test_double_sub(double val){
+ d -= val;
+}
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