[PATCH] D31726: AliasAnalysis: Be less conservative about volatile than atomic.

[Daniel Berlin via Phabricator via llvm-commits]

dberlin created this revision.

getModRefInfo is meant to answer the question "what impact does this
instruction have on a given memory location" (not even another
instruction).

Long debate on this on IRC comes to the conclusion the answer should be "nothing special".

That is, a noalias volatile store does not affect a memory location
just by being volatile.  Note: DSE and GVN and memdep currently
believe this, because memdep just goes behind AA's back after it says
"modref" right now.

see line 635 of memdep. Prior to this patch we would get modref there, then check aliasing,
and if it said noalias, we would continue.

getModRefInfo *already* has this same AA check, it just wasn't being used because volatile was
lumped in with ordering.

(I am separately testing whether this code in memdep is now dead except for the invariant load case)


https://reviews.llvm.org/D31726

Files:
  lib/Analysis/AliasAnalysis.cpp


Index: lib/Analysis/AliasAnalysis.cpp
===================================================================
--- lib/Analysis/AliasAnalysis.cpp
+++ lib/Analysis/AliasAnalysis.cpp
@@ -332,23 +332,22 @@
 
 ModRefInfo AAResults::getModRefInfo(const LoadInst *L,
                                     const MemoryLocation &Loc) {
-  // Be conservative in the face of volatile/atomic.
-  if (!L->isUnordered())
+  // Be conservative in the face of atomic.
+  if (isStrongerThan(L->getOrdering(), AtomicOrdering::NotAtomic))
     return MRI_ModRef;
 
   // If the load address doesn't alias the given address, it doesn't read
   // or write the specified memory.
   if (Loc.Ptr && !alias(MemoryLocation::get(L), Loc))
     return MRI_NoModRef;
-
   // Otherwise, a load just reads.
   return MRI_Ref;
 }
 
 ModRefInfo AAResults::getModRefInfo(const StoreInst *S,
                                     const MemoryLocation &Loc) {
-  // Be conservative in the face of volatile/atomic.
-  if (!S->isUnordered())
+    // Be conservative in the face of atomic.
+  if (isStrongerThan(S->getOrdering(), AtomicOrdering::NotAtomic))
     return MRI_ModRef;
 
   if (Loc.Ptr) {


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