Phi speculation improvement for BasicAA

This is a partial solution to PR14351. It removes some of the special
significance of the first incoming phi value in the phi aliasing checking logic
in BasicAA. In the context of a loop, the old logic assumes that the first
incoming value is the interesting one (meaning that it is the one that comes
from outside the loop), but this is often not the case.  With this change, we
now test first the incoming value that comes from a block other than the parent
of the phi being tested.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168245 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 12 insertions, 3 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 4bb93ee88a..e8b3063588 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1065,9 +1065,15 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
       if (PN > V2)
         std::swap(Locs.first, Locs.second);
 
+      // Find the first incoming phi value not from its parent.
+      unsigned f = 0;
+      while (PN->getIncomingBlock(f) == PN->getParent() &&
+             f < PN->getNumIncomingValues()-1)
+        ++f;
+
       AliasResult Alias =
-        aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
-                   PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
+        aliasCheck(PN->getIncomingValue(f), PNSize, PNTBAAInfo,
+                   PN2->getIncomingValueForBlock(PN->getIncomingBlock(f)),
                    V2Size, V2TBAAInfo);
       if (Alias == MayAlias)
         return MayAlias;
@@ -1096,7 +1102,10 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
         ArePhisAssumedNoAlias = true;
       }
 
-      for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
+      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
+        if (i == f)
+          continue;
+
         AliasResult ThisAlias =
           aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
                      PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),