BasicAA: Only query twice if the result of the more general query was MayAlias

This is a compile time optimization. Before the patch we would do two traversals
on each call to aliasGEP - one with a set size parameter one with UnknownSize.
We can do better by first checking the result of the alias query with
UnknownSize.
Only if this one returns MayAlias do we query a second time using size and type.

This recovers an about 7% compile time regression on spec/ammp.

radar://12349960

llvm-svn: 178045

1 files changed, 5 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 4139336f268..ae6da1af0c4 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -851,9 +851,13 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
   // pointers, figure out if the indexes to the GEP tell us anything about the
   // derived pointer.
   if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
+    // Do the base pointers alias?
+    AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
+                                       UnderlyingV2, UnknownSize, 0);
+
     // Check for geps of non-aliasing underlying pointers where the offsets are
     // identical.
-    if (V1Size == V2Size) {
+    if ((BaseAlias == MayAlias) && V1Size == V2Size) {
       // Do the base pointers alias assuming type and size.
       AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size,
                                                 V1TBAAInfo, UnderlyingV2,
@@ -881,10 +885,6 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
         GEP1VariableIndices.clear();
       }
     }
-
-    // Do the base pointers alias?
-    AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
-                                       UnderlyingV2, UnknownSize, 0);
     
     // If we get a No or May, then return it immediately, no amount of analysis
     // will improve this situation.