[AliasSetTracker] Do not treat experimental_guard intrinsic as memory…

… writing instruction

The `experimental_guard` intrinsic has memory write semantics to model the thread-exiting
logic, but does not do any actual writes to memory. Currently, `AliasSetTracker` treats it as a
normal memory write. As result, a loop-invariant load cannot be hoisted out of loop because
the guard may possibly alias with it.

This patch makes `AliasSetTracker` so that it doesn't treat guards as memory writes.

Differential Revision: https://reviews.llvm.org/D50497
Reviewed By: reames

llvm-svn: 339753

llvm/lib/Analysis/AliasSetTracker.cpp

@@ -24,6 +24,7 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/Value.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/AtomicOrdering.h"
@@ -169,7 +170,12 @@ void AliasSet::addUnknownInst(Instruction *I, AliasAnalysis &AA) {
     addRef();
   UnknownInsts.emplace_back(I);
 
-  if (!I->mayWriteToMemory()) {
+  // Guards are marked as modifying memory for control flow modelling purposes,
+  // but don't actually modify any specific memory location.
+  using namespace PatternMatch;
+  bool MayWriteMemory = I->mayWriteToMemory() &&
+                        !match(I, m_Intrinsic<Intrinsic::experimental_guard>());
+  if (!MayWriteMemory) {
     Alias = SetMayAlias;
     Access |= RefAccess;
     return;