StoreSuccess, ConstantInt::get(Type::getInt32Ty(Ctx), 0), "success");
Builder.CreateCondBr(TryAgain, LoopBB, BarrierBB);
- // Finally, make sure later instructions don't get reordered with a fence if
- // necessary.
+ // Make sure later instructions don't get reordered with a fence if necessary.
Builder.SetInsertPoint(BarrierBB);
insertTrailingFence(Builder, SuccessOrder);
Builder.CreateBr(ExitBB);
+ // Finally, we have control-flow based knowledge of whether the cmpxchg
+ // succeeded or not. We expose this to later passes by converting any
+ // subsequent "icmp eq/ne %loaded, %oldval" into a use of an appropriate PHI.
+
+ // Setup the builder so we can create any PHIs we need.
+ Builder.SetInsertPoint(FailureBB, FailureBB->begin());
+ BasicBlock *SuccessBB = FailureOrder == Monotonic ? BarrierBB : TryStoreBB;
+ PHINode *Success = nullptr, *Failure = nullptr;
+
+ // Look for any users of the cmpxchg that are just comparing the loaded value
+ // against the desired one, and replace them with the CFG-derived version.
+ for (auto User : CI->users()) {
+ ICmpInst *ICmp = dyn_cast<ICmpInst>(User);
+ if (!ICmp)
+ continue;
+
+ // Because we know ICmp uses CI, we only need one operand to be the old
+ // value.
+ if (ICmp->getOperand(0) != CI->getCompareOperand() &&
+ ICmp->getOperand(1) != CI->getCompareOperand())
+ continue;
+
+ if (ICmp->getPredicate() == CmpInst::ICMP_EQ) {
+ if (!Success) {
+ Success = Builder.CreatePHI(Type::getInt1Ty(Ctx), 2);
+ Success->addIncoming(ConstantInt::getTrue(Ctx), SuccessBB);
+ Success->addIncoming(ConstantInt::getFalse(Ctx), LoopBB);
+ }
+ ICmp->replaceAllUsesWith(Success);
+ } else if (ICmp->getPredicate() == CmpInst::ICMP_NE) {
+ if (!Failure) {
+ Failure = Builder.CreatePHI(Type::getInt1Ty(Ctx), 2);
+ Failure->addIncoming(ConstantInt::getFalse(Ctx), SuccessBB);
+ Failure->addIncoming(ConstantInt::getTrue(Ctx), LoopBB);
+ }
+ ICmp->replaceAllUsesWith(Failure);
+ }
+ }
+
CI->replaceAllUsesWith(Loaded);
CI->eraseFromParent();