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

[DiamondLovesYou]

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.

[jfb]

lgtm, sorry folks for not seeing the breakage earlier.

llvm/trunk/lib/CodeGen/AtomicExpandPass.cpp

bool llvm::expandAtomicRMWToCmpXchg(AtomicRMWInst *AI,
 
   // The split call above "helpfully" added a branch at the end of BB (to the
   // wrong place), but we want a load. It's easiest to just remove
   // the branch entirely.
   std::prev(BB->end())->eraseFromParent();
   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.
   Builder.SetInsertPoint(LoopBB);
   PHINode *Loaded = Builder.CreatePHI(AI->getType(), 2, "loaded");
   Loaded->addIncoming(InitLoaded, BB);
 
   Value *NewVal =

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

+; 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/trunk/test/Transforms/AtomicExpand/X86/lit.local.cfg

+if not 'X86' in config.root.targets:
+  config.unsupported = True