Merging r199351:

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r199351 | aschwaighofer | 2014-01-15 23:53:18 -0500 (Wed, 15 Jan 2014) | 5 lines

BasicAA: We need to check both access sizes when comparing a gep and an
underlying object of unknown size.

Fixes PR18460.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_34@206053 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 9 insertions, 1 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index f1a9dd991c..d0e186c5f2 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1009,7 +1009,15 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
         return NoAlias;
       }
     } else {
-      if (V1Size != UnknownSize) {
+      // We have the situation where:
+      // +                +
+      // | BaseOffset     |
+      // ---------------->|
+      // |-->V1Size       |-------> V2Size
+      // GEP1             V2
+      // We need to know that V2Size is not unknown, otherwise we might have
+      // stripped a gep with negative index ('gep <ptr>, -1, ...).
+      if (V1Size != UnknownSize && V2Size != UnknownSize) {
         if (-(uint64_t)GEP1BaseOffset < V1Size)
           return PartialAlias;
         return NoAlias;