Fix a miscompile introduced in r220178.

The original code had an implicit assumption that if the test for
allocas or globals was reached, the two pointers were not equal. With my
changes to make the pointer analysis more powerful here, I also had to
guard against circumstances where the results weren't useful. That in
turn violated the assumption and gave rise to a circumstance in which we
could have a store with both the queried pointer and stored pointer
rooted at *the same* alloca. Clearly, we cannot ignore such a store.
There are other things we might do in this code to better handle the
case of both pointers ending up at the same alloca or global, but it
seems best to at least make the test explicit in what it intends to
check.

I've added tests for both the alloca and global case here.

llvm-svn: 220190

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index 65a30f2..bb0d60e 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -220,11 +220,12 @@ Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
         return SI->getOperand(0);
       }
 
-      // If Ptr is an alloca and this is a store to a different alloca, ignore
-      // the store.  This is a trivial form of alias analysis that is important
-      // for reg2mem'd code.
+      // If both StrippedPtr and StorePtr reach all the way to an alloca or
+      // global and they are different, ignore the store. This is a trivial form
+      // of alias analysis that is important for reg2mem'd code.
       if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) &&
-          (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)))
+          (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) &&
+          StrippedPtr != StorePtr)
         continue;
 
       // If we have alias analysis and it says the store won't modify the loaded