[X86] Prefer lock or over mfence (llvm#106555)

vchuravy · gbaraldi · commit 1dcaa8d726df · 2025-08-04T18:33:37.000-03:00
Originally discussed in https://reviews.llvm.org/D129947 LLVM currently emits `mfence` for `__atomic_thread_fence(seq_cst)`. On modern CPUs lock or is more efficient and provides the same sequential consistency. GCC 11 made this switch as well (see https://gcc.gnu.org/pipermail/gcc-cvs/2020-July/314418.html) and https://reviews.llvm.org/D61863 and https://reviews.llvm.org/D58632 moved into this direction as well, but didn't touch fence seq_cst. This switches to `lock or` on all x64 systems, and leaves `__builtin_ia32_mfence` for folks who want this precise instruction. (cherry picked from commit b334321)
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -31790,7 +31790,7 @@ X86TargetLowering::lowerIdempotentRMWIntoFencedLoad(AtomicRMWInst *AI) const {
   // especially clever.
 
   // Use `fence seq_cst` over `llvm.x64.sse2.mfence` here to get the correct
-  // lowering for SSID == SyncScope::SingleThread and !hasMFence
+  // lowering for SSID == SyncScope::SingleThread and avoidMFence || !hasMFence
   Builder.CreateFence(AtomicOrdering::SequentiallyConsistent, SSID);
 
   // Finally we can emit the atomic load.
@@ -31879,7 +31879,7 @@ static SDValue LowerATOMIC_FENCE(SDValue Op, const X86Subtarget &Subtarget,
   // cross-thread fence.
   if (FenceOrdering == AtomicOrdering::SequentiallyConsistent &&
       FenceSSID == SyncScope::System) {
-    if (Subtarget.hasMFence())
+    if (!Subtarget.avoidMFence() && Subtarget.hasMFence())
       return DAG.getNode(X86ISD::MFENCE, dl, MVT::Other, Op.getOperand(0));
 
     SDValue Chain = Op.getOperand(0);
diff --git a/llvm/lib/Target/X86/X86Subtarget.h b/llvm/lib/Target/X86/X86Subtarget.h
@@ -280,6 +280,9 @@ class X86Subtarget final : public X86GenSubtargetInfo {
   /// supports it.
   bool hasMFence() const { return hasSSE2() || is64Bit(); }
 
+  /// Avoid use of `mfence` for`fence seq_cst`, and instead use `lock or`.
+  bool avoidMFence() const { return is64Bit(); }
+
   const Triple &getTargetTriple() const { return TargetTriple; }
 
   bool isTargetDarwin() const { return TargetTriple.isOSDarwin(); }
diff --git a/llvm/test/CodeGen/X86/atomic-idempotent.ll b/llvm/test/CodeGen/X86/atomic-idempotent.ll
@@ -14,7 +14,7 @@
 define i8 @add8(ptr %p) #0 {
 ; X64-LABEL: add8:
 ; X64:       # %bb.0:
-; X64-NEXT:    mfence
+; X64-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    movzbl (%rdi), %eax
 ; X64-NEXT:    retq
 ;
@@ -47,7 +47,7 @@ define i8 @add8(ptr %p) #0 {
 define i16 @or16(ptr %p) #0 {
 ; X64-LABEL: or16:
 ; X64:       # %bb.0:
-; X64-NEXT:    mfence
+; X64-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    movzwl (%rdi), %eax
 ; X64-NEXT:    retq
 ;
@@ -80,7 +80,7 @@ define i16 @or16(ptr %p) #0 {
 define i32 @xor32(ptr %p) #0 {
 ; X64-LABEL: xor32:
 ; X64:       # %bb.0:
-; X64-NEXT:    mfence
+; X64-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    movl (%rdi), %eax
 ; X64-NEXT:    retq
 ;
@@ -113,7 +113,7 @@ define i32 @xor32(ptr %p) #0 {
 define i64 @sub64(ptr %p) #0 {
 ; X64-LABEL: sub64:
 ; X64:       # %bb.0:
-; X64-NEXT:    mfence
+; X64-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    movq (%rdi), %rax
 ; X64-NEXT:    retq
 ;
@@ -265,7 +265,7 @@ define i128 @or128(ptr %p) #0 {
 define i32 @and32 (ptr %p) #0 {
 ; X64-LABEL: and32:
 ; X64:       # %bb.0:
-; X64-NEXT:    mfence
+; X64-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    movl (%rdi), %eax
 ; X64-NEXT:    retq
 ;
diff --git a/llvm/test/CodeGen/X86/atomic-unordered.ll b/llvm/test/CodeGen/X86/atomic-unordered.ll
@@ -2096,7 +2096,7 @@ define i64 @nofold_fence(ptr %p) {
 ; CHECK-LABEL: nofold_fence:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movq (%rdi), %rax
-; CHECK-NEXT:    mfence
+; CHECK-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    addq $15, %rax
 ; CHECK-NEXT:    retq
   %v = load atomic i64, ptr %p unordered, align 8
@@ -2170,7 +2170,7 @@ define i64 @fold_constant_fence(i64 %arg) {
 ; CHECK-LABEL: fold_constant_fence:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movq Constant(%rip), %rax
-; CHECK-NEXT:    mfence
+; CHECK-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    addq %rdi, %rax
 ; CHECK-NEXT:    retq
   %v = load atomic i64, ptr @Constant unordered, align 8
@@ -2197,7 +2197,7 @@ define i64 @fold_invariant_fence(ptr dereferenceable(8) %p, i64 %arg) {
 ; CHECK-LABEL: fold_invariant_fence:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movq (%rdi), %rax
-; CHECK-NEXT:    mfence
+; CHECK-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    addq %rsi, %rax
 ; CHECK-NEXT:    retq
   %v = load atomic i64, ptr %p unordered, align 8, !invariant.load !{}
@@ -2321,7 +2321,7 @@ define i1 @fold_cmp_over_fence(ptr %p, i32 %v1) {
 ; CHECK-O0-LABEL: fold_cmp_over_fence:
 ; CHECK-O0:       # %bb.0:
 ; CHECK-O0-NEXT:    movl (%rdi), %eax
-; CHECK-O0-NEXT:    mfence
+; CHECK-O0-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-O0-NEXT:    cmpl %eax, %esi
 ; CHECK-O0-NEXT:    jne .LBB116_2
 ; CHECK-O0-NEXT:  # %bb.1: # %taken
@@ -2335,7 +2335,7 @@ define i1 @fold_cmp_over_fence(ptr %p, i32 %v1) {
 ; CHECK-O3-LABEL: fold_cmp_over_fence:
 ; CHECK-O3:       # %bb.0:
 ; CHECK-O3-NEXT:    movl (%rdi), %eax
-; CHECK-O3-NEXT:    mfence
+; CHECK-O3-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-O3-NEXT:    cmpl %eax, %esi
 ; CHECK-O3-NEXT:    jne .LBB116_2
 ; CHECK-O3-NEXT:  # %bb.1: # %taken
diff --git a/llvm/test/CodeGen/X86/implicit-null-check.ll b/llvm/test/CodeGen/X86/implicit-null-check.ll
@@ -1,3 +1,4 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
 ; RUN: llc -verify-machineinstrs -O3 -mtriple=x86_64-apple-macosx -enable-implicit-null-checks < %s | FileCheck %s
 
 define i32 @imp_null_check_load(ptr %x) {
@@ -465,7 +466,7 @@ define i32 @imp_null_check_load_fence2(ptr %x) {
 ; CHECK-NEXT:    testq %rdi, %rdi
 ; CHECK-NEXT:    je LBB17_1
 ; CHECK-NEXT:  ## %bb.2: ## %not_null
-; CHECK-NEXT:    mfence
+; CHECK-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movl (%rdi), %eax
 ; CHECK-NEXT:    retq
 ; CHECK-NEXT:  LBB17_1: ## %is_null
diff --git a/llvm/test/CodeGen/X86/membarrier.ll b/llvm/test/CodeGen/X86/membarrier.ll
@@ -6,9 +6,9 @@ define i32 @t() {
 ; CHECK-LABEL: t:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movl $1, -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    mfence
+; CHECK-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    lock decl -{{[0-9]+}}(%rsp)
-; CHECK-NEXT:    mfence
+; CHECK-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    xorl %eax, %eax
 ; CHECK-NEXT:    retq
   %i = alloca i32, align 4
diff --git a/llvm/test/CodeGen/X86/mfence.ll b/llvm/test/CodeGen/X86/mfence.ll
@@ -5,10 +5,15 @@
 ; It doesn't matter if an x86-64 target has specified "no-sse2"; we still can use mfence.
 
 define void @test() {
-; CHECK-LABEL: test:
-; CHECK:       # %bb.0:
-; CHECK-NEXT:    mfence
-; CHECK-NEXT:    ret{{[l|q]}}
+; X86-LABEL: test:
+; X86:       # %bb.0:
+; X86-NEXT:    mfence
+; X86-NEXT:    retl
+;
+; X64-LABEL: test:
+; X64:       # %bb.0:
+; X64-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
+; X64-NEXT:    retq
   fence seq_cst
   ret void
 }
@@ -23,10 +28,25 @@ define i32 @fence(ptr %ptr) {
 ;
 ; X64-LABEL: fence:
 ; X64:       # %bb.0:
-; X64-NEXT:    mfence
+; X64-NEXT:    lock orl $0, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    movl (%rdi), %eax
 ; X64-NEXT:    retq
   %atomic = atomicrmw add ptr %ptr, i32 0 seq_cst
   ret i32 %atomic
 }
 
+define void @mfence() nounwind {
+; X32-LABEL: mfence:
+; X32:       # %bb.0:
+; X32-NEXT:    mfence
+; X32-NEXT:    retl
+;
+; CHECK-LABEL: mfence:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    mfence
+; CHECK-NEXT:    ret{{[l|q]}}
+  call void @llvm.x86.sse2.mfence()
+  ret void
+}
+declare void @llvm.x86.sse2.mfence() nounwind readnone
+
