IR: add "cmpxchg weak" variant to support permitted failure.
This commit adds a weak variant of the cmpxchg operation, as described
in C++11. A cmpxchg instruction with this modifier is permitted to
fail to store, even if the comparison indicated it should.
As a result, cmpxchg instructions must return a flag indicating
success in addition to their original iN value loaded. Thus, for
uniformity *all* cmpxchg instructions now return "{ iN, i1 }". The
second flag is 1 when the store succeeded.
At the DAG level, a new ATOMIC_CMP_SWAP_WITH_SUCCESS node has been
added as the natural representation for the new cmpxchg instructions.
It is a strong cmpxchg.
By default this gets Expanded to the existing ATOMIC_CMP_SWAP during
Legalization, so existing backends should see no change in behaviour.
If they wish to deal with the enhanced node instead, they can call
setOperationAction on it. Beware: as a node with 2 results, it cannot
be selected from TableGen.
Currently, no use is made of the extra information provided in this
patch. Test updates are almost entirely adapting the input IR to the
new scheme.
Summary for out of tree users:
------------------------------
+ Legacy Bitcode files are upgraded during read.
+ Legacy assembly IR files will be invalid.
+ Front-ends must adapt to different type for "cmpxchg".
+ Backends should be unaffected by default.
llvm-svn: 210903
diff --git a/llvm/lib/CodeGen/AtomicExpandLoadLinkedPass.cpp b/llvm/lib/CodeGen/AtomicExpandLoadLinkedPass.cpp
index fbb6e96..6f93ced 100644
--- a/llvm/lib/CodeGen/AtomicExpandLoadLinkedPass.cpp
+++ b/llvm/lib/CodeGen/AtomicExpandLoadLinkedPass.cpp
@@ -299,41 +299,44 @@
// 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;
+ PHINode *Success = Builder.CreatePHI(Type::getInt1Ty(Ctx), 2);
+ Success->addIncoming(ConstantInt::getTrue(Ctx), SuccessBB);
+ Success->addIncoming(ConstantInt::getFalse(Ctx), LoopBB);
// 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.
+ SmallVector<ExtractValueInst *, 2> PrunedInsts;
for (auto User : CI->users()) {
- ICmpInst *ICmp = dyn_cast<ICmpInst>(User);
- if (!ICmp)
+ ExtractValueInst *EV = dyn_cast<ExtractValueInst>(User);
+ if (!EV)
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;
+ assert(EV->getNumIndices() == 1 && EV->getIndices()[0] <= 1 &&
+ "weird extraction from { iN, i1 }");
- 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);
- }
+ if (EV->getIndices()[0] == 0)
+ EV->replaceAllUsesWith(Loaded);
+ else
+ EV->replaceAllUsesWith(Success);
+
+ PrunedInsts.push_back(EV);
}
- CI->replaceAllUsesWith(Loaded);
- CI->eraseFromParent();
+ // We can remove the instructions now we're no longer iterating through them.
+ for (auto EV : PrunedInsts)
+ EV->eraseFromParent();
+ if (!CI->use_empty()) {
+ // Some use of the full struct return that we don't understand has happened,
+ // so we've got to reconstruct it properly.
+ Value *Res;
+ Res = Builder.CreateInsertValue(UndefValue::get(CI->getType()), Loaded, 0);
+ Res = Builder.CreateInsertValue(Res, Success, 1);
+
+ CI->replaceAllUsesWith(Res);
+ }
+
+ CI->eraseFromParent();
return true;
}