1 files changed, 56 insertions, 0 deletions
diff --git a/llvm/test/CodeGen/X86/atomic_idempotent.ll b/llvm/test/CodeGen/X86/atomic_idempotent.ll
new file mode 100644
index 00000000000..1afc535133d
--- /dev/null
+++ b/llvm/test/CodeGen/X86/atomic_idempotent.ll
@@ -0,0 +1,56 @@
+; RUN: llc < %s -march=x86-64 -verify-machineinstrs | FileCheck %s --check-prefix=CHECK --check-prefix=X64
+; RUN: llc < %s -march=x86 -mattr=+sse2 -verify-machineinstrs | FileCheck %s --check-prefix=CHECK --check-prefix=X32
+
+; On x86, an atomic rmw operation that does not modify the value in memory
+; (such as atomic add 0) can be replaced by an mfence followed by a mov.
+; This is explained (with the motivation for such an optimization) in
+; http://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf
+
+define i8 @add8(i8* %p) {
+; CHECK-LABEL: add8
+; CHECK: mfence
+; CHECK: movb
+  %1 = atomicrmw add i8* %p, i8 0 monotonic
+  ret i8 %1
+}
+
+define i16 @or16(i16* %p) {
+; CHECK-LABEL: or16
+; CHECK: mfence
+; CHECK: movw
+  %1 = atomicrmw or i16* %p, i16 0 acquire
+  ret i16 %1
+}
+
+define i32 @xor32(i32* %p) {
+; CHECK-LABEL: xor32
+; CHECK: mfence
+; CHECK: movl
+  %1 = atomicrmw xor i32* %p, i32 0 release
+  ret i32 %1
+}
+
+define i64 @sub64(i64* %p) {
+; CHECK-LABEL: sub64
+; X64: mfence
+; X64: movq
+; X32-NOT: mfence
+  %1 = atomicrmw sub i64* %p, i64 0 seq_cst
+  ret i64 %1
+}
+
+define i128 @or128(i128* %p) {
+; CHECK-LABEL: or128
+; CHECK-NOT: mfence
+  %1 = atomicrmw or i128* %p, i128 0 monotonic
+  ret i128 %1
+}
+
+; For 'and', the idempotent value is (-1)
+define i32 @and32 (i32* %p) {
+; CHECK-LABEL: and32
+; CHECK: mfence
+; CHECK: movl
+  %1 = atomicrmw and i32* %p, i32 -1 acq_rel
+  ret i32 %1
+}