[PATCH] D97401: [basicaa] Recurse through a single phi input

[Philip Reames via Phabricator via llvm-commits]

reames updated this revision to Diff 326555.
reames added a comment.

Update to most restrictive form.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D97401/new/

https://reviews.llvm.org/D97401

Files:
  llvm/lib/Analysis/BasicAliasAnalysis.cpp
  llvm/test/Analysis/BasicAA/recphi.ll


Index: llvm/test/Analysis/BasicAA/recphi.ll
===================================================================
--- llvm/test/Analysis/BasicAA/recphi.ll
+++ llvm/test/Analysis/BasicAA/recphi.ll
@@ -296,9 +296,9 @@
 ; CHECK: NoAlias:	i8* %a, i8* %p.base
 ; CHECK: NoAlias:	i8* %a, i8* %p.outer
 ; CHECK: NoAlias:	i8* %a, i8* %p.outer.next
-; CHECK: MayAlias:	i8* %a, i8* %p.inner
+; NO-PHI-VALUES: NoAlias:	i8* %a, i8* %p.inner
+; PHI-VALUES: MayAlias:	i8* %a, i8* %p.inner
 ; CHECK: NoAlias:	i8* %a, i8* %p.inner.next
-; TODO: %p.inner does not alias %a
 define void @nested_loop3(i1 %c, i1 %c2, i8* noalias %p.base) {
 entry:
   %a = alloca i8
@@ -351,9 +351,9 @@
 ; CHECK: NoAlias:	i8* %a, i8* %p.base
 ; CHECK: NoAlias:	i8* %a, i8* %p1
 ; CHECK: NoAlias:	i8* %a, i8* %p1.next
-; CHECK: MayAlias:	i8* %a, i8* %p2
+; NO-PHI-VALUES: NoAlias:	i8* %a, i8* %p2
+; PHI-VALUES: MayAlias:	i8* %a, i8* %p2
 ; CHECK: NoAlias:	i8* %a, i8* %p2.next
-; TODO: %p2 does not alias %a
 define void @sibling_loop2(i1 %c, i1 %c2, i8* noalias %p.base) {
 entry:
   %a = alloca i8
Index: llvm/lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1385,13 +1385,19 @@
     // If we don't have PhiInfo then just look at the operands of the phi itself
     // FIXME: Remove this once we can guarantee that we have PhiInfo always
     SmallPtrSet<Value *, 4> UniqueSrc;
+    Value *OnePhi = nullptr;
     for (Value *PV1 : PN->incoming_values()) {
-      if (isa<PHINode>(PV1))
-        // If any of the source itself is a PHI, return MayAlias conservatively
-        // to avoid compile time explosion. The worst possible case is if both
-        // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
-        // and 'n' are the number of PHI sources.
-        return MayAlias;
+      if (isa<PHINode>(PV1)) {
+        if (OnePhi && OnePhi != PV1) {
+          // To control potential compile time explosion, we choose to be
+          // conserviate when we have more than one Phi input.  It is important
+          // that we handle the single phi case as that lets us handle LCSSA
+          // phi nodes and (combined with the recursive phi handling) simple
+          // pointer induction variable patterns.
+          return MayAlias;
+        }
+        OnePhi = PV1;
+      }
 
       if (CheckForRecPhi(PV1))
         continue;
@@ -1399,6 +1405,11 @@
       if (UniqueSrc.insert(PV1).second)
         V1Srcs.push_back(PV1);
     }
+
+    if (OnePhi && UniqueSrc.size() > 1)
+      // Out of an abundance of caution, allow only the trivial lcssa and
+      // recursive phi cases.
+      return MayAlias;
   }
 
   // If V1Srcs is empty then that means that the phi has no underlying non-phi


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