MemorySSA: Revert r269678 and r268068; replace with special casing in MemorySSA.

It turns out that too many passes are relying on alias analysis results
for control dependencies. Until we fix that by introducing a more accurate
modelling of control dependencies, special case assume in MemorySSA instead.

Also introduce tests to ensure we don't regress the FunctionAttrs or LICM
passes.

Differential Revision: http://reviews.llvm.org/D20658

llvm-svn: 270823

2 files changed, 8 insertions, 9 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 6d166da75b9..f1b60c6432b 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -580,20 +580,11 @@ FunctionModRefBehavior BasicAAResult::getModRefBehavior(ImmutableCallSite CS) {
 
 /// Returns the behavior when calling the given function. For use when the call
 /// site is not known.
-/// NOTE: Because of the special case handling of llvm.assume below, the result
-/// of this function may not match similar results derived from function
-/// attributes (e.g. "readnone").
 FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
   // If the function declares it doesn't access memory, we can't do better.
   if (F->doesNotAccessMemory())
     return FMRB_DoesNotAccessMemory;
 
-  // While the assume intrinsic is marked as arbitrarily writing so that
-  // proper control dependencies will be maintained, it never aliases any
-  // actual memory locations.
-  if (F->getIntrinsicID() == Intrinsic::assume)
-    return FMRB_DoesNotAccessMemory;
-
   FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
 
   // If the function declares it only reads memory, go with that.
diff --git a/llvm/lib/Transforms/Utils/MemorySSA.cpp b/llvm/lib/Transforms/Utils/MemorySSA.cpp
index b87a6d611b1..9d8c0884fdb 100644
--- a/llvm/lib/Transforms/Utils/MemorySSA.cpp
+++ b/llvm/lib/Transforms/Utils/MemorySSA.cpp
@@ -354,6 +354,14 @@ MemorySSAWalker *MemorySSA::buildMemorySSA(AliasAnalysis *AA,
 
 /// \brief Helper function to create new memory accesses
 MemoryUseOrDef *MemorySSA::createNewAccess(Instruction *I) {
+  // The assume intrinsic has a control dependency which we model by claiming
+  // that it writes arbitrarily. Ignore that fake memory dependency here.
+  // FIXME: Replace this special casing with a more accurate modelling of
+  // assume's control dependency.
+  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
+    if (II->getIntrinsicID() == Intrinsic::assume)
+      return nullptr;
+
   // Find out what affect this instruction has on memory.
   ModRefInfo ModRef = AA->getModRefInfo(I);
   bool Def = bool(ModRef & MRI_Mod);