Add comments and test for atomic load/store and mem2reg.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137690 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 16 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/PromoteMemoryToRegister.cpp b/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
index e5a00f4e97..db3e942513 100644
--- a/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
+++ b/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -86,11 +86,15 @@ bool llvm::isAllocaPromotable(const AllocaInst *AI) {
        UI != UE; ++UI) {   // Loop over all of the uses of the alloca
     const User *U = *UI;
     if (const LoadInst *LI = dyn_cast<LoadInst>(U)) {
+      // Note that atomic loads can be transformed; atomic semantics do
+      // not have any meaning for a local alloca.
       if (LI->isVolatile())
         return false;
     } else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
       if (SI->getOperand(0) == AI)
         return false;   // Don't allow a store OF the AI, only INTO the AI.
+      // Note that atomic stores can be transformed; atomic semantics do
+      // not have any meaning for a local alloca.
       if (SI->isVolatile())
         return false;
     } else if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
diff --git a/test/Transforms/Mem2Reg/atomic.ll b/test/Transforms/Mem2Reg/atomic.ll
new file mode 100644
index 0000000000..982c41318b
--- /dev/null
+++ b/test/Transforms/Mem2Reg/atomic.ll
@@ -0,0 +1,12 @@
+; RUN: opt -mem2reg < %s -S | FileCheck %s
+
+; mem2reg is allowed with arbitrary atomic operations (although we only support
+; it for atomic load and store at the moment).
+define i32 @test1(i32 %x) {
+; CHECK: @test1
+; CHECK: ret i32 %x
+  %a = alloca i32
+  store atomic i32 %x, i32* %a seq_cst, align 4
+  %r = load atomic i32* %a seq_cst, align 4
+  ret i32 %r
+}