Fix a long-standing miscompile in the load analysis that was uncovered

by my refactoring of this code.

The method isSafeToLoadUnconditionally assumes that the load will
proceed with the preferred type alignment. Given that, it has to ensure
that the alloca or global is at least that aligned. It has always done
this historically when a datalayout is present, but has never checked it
when the datalayout is absent. When I refactored the code in r220156,
I exposed this path when datalayout was present and that turned the
latent bug into a patent bug.

This fixes the issue by just removing the special case which allows
folding things without datalayout. This isn't worth the complexity of
trying to tease apart when it is or isn't safe without actually knowing
the preferred alignment.

llvm-svn: 220161

1 files changed, 0 insertions, 11 deletions

diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index f5eb7055726..4838d856ae3 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -73,11 +73,6 @@ bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
   Type *BaseType = nullptr;
   unsigned BaseAlign = 0;
   if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
-    // Loading directly from an alloca is trivially safe. We can't even look
-    // through pointer casts here though, as that might change the size loaded.
-    if (AI == V)
-      return true;
-
     // An alloca is safe to load from as load as it is suitably aligned.
     BaseType = AI->getAllocatedType();
     BaseAlign = AI->getAlignment();
@@ -86,12 +81,6 @@ bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
     // overridden. Their size may change or they may be weak and require a test
     // to determine if they were in fact provided.
     if (!GV->mayBeOverridden()) {
-      // Loading directly from the non-overridden global is trivially safe. We
-      // can't even look through pointer casts here though, as that might change
-      // the size loaded.
-      if (GV == V)
-        return true;
-
       BaseType = GV->getType()->getElementType();
       BaseAlign = GV->getAlignment();
     }