[MemorySSA] Update MSSA for non-conventional AA.

Summary:
Regularly when moving an instruction that may not read or write memory,
the instruction is not modelled in MSSA, so not action is necessary.
For a non-conventional AA pipeline, MSSA needs to explicitly check when
creating accesses, so as to not model instructions that may not read and
write memory.

Reviewers: george.burgess.iv

Subscribers: Prazek, sanjoy.google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67562

llvm-svn: 372137

Author: alinas

llvm/lib/Analysis/MemorySSA.cpp

@@ -1736,9 +1736,15 @@ MemoryUseOrDef *MemorySSA::createNewAccess(Instruction *I,
   // 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 || isa<DbgInfoIntrinsic>(II))
+    if (II->getIntrinsicID() == Intrinsic::assume)
       return nullptr;
 
+  // Using a nonstandard AA pipelines might leave us with unexpected modref
+  // results for I, so add a check to not model instructions that may not read
+  // from or write to memory. This is necessary for correctness.
+  if (!I->mayReadFromMemory() && !I->mayWriteToMemory())
+    return nullptr;
+
   bool Def, Use;
   if (Template) {
     Def = dyn_cast_or_null<MemoryDef>(Template) != nullptr;

llvm/test/Analysis/MemorySSA/loop-rotate-disablebasicaa.ll

@@ -0,0 +1,26 @@
+; RUN: opt -disable-basicaa -print-memoryssa -disable-output %s 2>&1 | FileCheck %s
+
+; Note: if @foo is modelled as a MemoryDef, this test will assert with -loop-rotate, due to MemorySSA not
+; being preserved when moving instructions that may not read from or write to memory.
+
+; CHECK-LABEL: @main
+; CHECK-NOT: MemoryDef
+define void @main() {
+entry:
+  br label %for.cond120
+
+for.cond120:                                      ; preds = %for.body127, %entry
+  call void @foo()
+  br i1 undef, label %for.body127, label %for.cond.cleanup126
+
+for.cond.cleanup126:                              ; preds = %for.cond120
+  unreachable
+
+for.body127:                                      ; preds = %for.cond120
+  %0 = load i16**, i16*** undef, align 1
+  br label %for.cond120
+}
+
+declare void @foo() readnone
+
+