Fix an alignment error in `llvm::expandAtomicRMWToCmpXchg` without breaking the build where X86 isn't enabled.

Summary: Divide the primitive size in bits by eight so the initial load's alignment is in bytes as expected. Tested with the included unit test.

Reviewers: rengolin, jfb

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11804

llvm-svn: 244229

llvm/lib/CodeGen/AtomicExpandPass.cpp

@@ -550,7 +550,7 @@ bool llvm::expandAtomicRMWToCmpXchg(AtomicRMWInst *AI,
   Builder.SetInsertPoint(BB);
   LoadInst *InitLoaded = Builder.CreateLoad(Addr);
   // Atomics require at least natural alignment.
-  InitLoaded->setAlignment(AI->getType()->getPrimitiveSizeInBits());
+  InitLoaded->setAlignment(AI->getType()->getPrimitiveSizeInBits() / 8);
   Builder.CreateBr(LoopBB);
 
   // Start the main loop block now that we've taken care of the preliminaries.

llvm/test/Transforms/AtomicExpand/X86/expand-atomic-rmw-initial-load.ll

@@ -0,0 +1,11 @@
+; RUN: opt -S %s -atomic-expand -mtriple=i686-linux-gnu | FileCheck %s
+
+; This file tests the function `llvm::expandAtomicRMWToCmpXchg`.
+; It isn't technically target specific, but is exposed through a pass that is.
+
+define i8 @test_initial_load(i8* %ptr, i8 %value) {
+  %res = atomicrmw nand i8* %ptr, i8 %value seq_cst
+  ret i8 %res
+}
+; CHECK-LABEL: @test_initial_load
+; CHECK-NEXT:    %1 = load i8, i8* %ptr, align 1

llvm/test/Transforms/AtomicExpand/X86/lit.local.cfg

@@ -0,0 +1,2 @@
+if not 'X86' in config.root.targets:
+  config.unsupported = True