[MemorySSA] Special case for assume intrinsics.

[gberry]

Summary

Treat assume intrinsics as not reading/writing memory despite
AA->getModRefInfo() saying they do, as is done in BasicAliasAnalysis.

[• dberlin]

So, should we not just move this into getModRefInfo?

(It seems like general applicable alias knowledge that requires no
computation)

[gberry]

I looked at that briefly, and I think that would have additional impact that we may not want. For example, it looked to me like changing getModRefInfo to ignore llvm.assume would cause LICM to hoist/sink assumes which seems at first glance to potentially be bad.

[• dberlin]

It seems like we should just fix that?
I see why it would do that, but it seems it doesn't separate whether it can
sink thinks with whether it should :)

(Note: there should be no other users, AFAIK, since i added this version of
getModRefInfo)

[gberry]

I can give it a shot. The LICM case might be a false alarm, since I believe it will also check isSafeToSpeculativelyExecute before hoisting/sinking, which should return false for llvm.assume.

[gberry]

Abandoning in favor of http://reviews.llvm.org/D19730

lib/Transforms/Utils/MemorySSA.cpp

 /// \brief Helper function to create new memory accesses
 MemoryUseOrDef *MemorySSA::createNewAccess(Instruction *I,
                                            bool IgnoreNonMemory) {
   // Find out what affect this instruction has on memory.
   ModRefInfo ModRef = AA->getModRefInfo(I);
   bool Def = bool(ModRef & MRI_Mod);
   bool Use = bool(ModRef & MRI_Ref);
 
+  // While the assume intrinsic is marked as arbitrarily writing so that
+  // proper control dependencies will be maintained, it never aliases any
+  // particular memory location.
+  if (match(I, PatternMatch::m_Intrinsic<Intrinsic::assume>())) {
+    Def = false;
+    Use = false;
+  }
+
   // It's possible for an instruction to not modify memory at all. During
   // construction, we ignore them.
   if (IgnoreNonMemory && !Def && !Use)
     return nullptr;
 
   assert((Def || Use) &&
          "Trying to create a memory access with a non-memory instruction");
 

test/Transforms/Util/MemorySSA/assume.ll

+; RUN: opt -basicaa -print-memoryssa -verify-memoryssa -analyze < %s 2>&1 | FileCheck %s
+; RUN: opt -aa-pipeline=basic-aa -passes='print<memoryssa>' -verify-memoryssa -disable-output < %s 2>&1 | FileCheck %s
+;
+; Ensures that assumes are treated as not reading or writing memory.
+
+declare void @llvm.assume(i1)
+
+define i32 @foo(i32* %a, i32* %b, i1 %c) {
+; CHECK: 1 = MemoryDef(liveOnEntry)
+; CHECK-NEXT: store i32 4
+  store i32 4, i32* %a, align 4
+; CHECK: call void @llvm.assume
+  call void @llvm.assume(i1 %c)
+; CHECK: MemoryUse(1)
+; CHECK-NEXT: %1 = load i32
+  %1 = load i32, i32* %a, align 4
+  ret i32 %1
+}