The memcpy optimizer was happily doing call slot forwarding when the new memory

was less aligned than the old.  In the testcase this results in an overaligned
memset: the memset alignment was correct for the original memory but is too much
for the new memory.  Fix this by either increasing the alignment of the new
memory or bailing out if that isn't possible.  Should fix the gcc-4.7 self-host
buildbot failure.

llvm-svn: 165220

1 files changed, 30 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index 35acc792387..68be3b7c77f 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -332,7 +332,7 @@ namespace {
     bool processMemCpy(MemCpyInst *M);
     bool processMemMove(MemMoveInst *M);
     bool performCallSlotOptzn(Instruction *cpy, Value *cpyDst, Value *cpySrc,
-                              uint64_t cpyLen, CallInst *C);
+                              uint64_t cpyLen, unsigned cpyAlign, CallInst *C);
     bool processMemCpyMemCpyDependence(MemCpyInst *M, MemCpyInst *MDep,
                                        uint64_t MSize);
     bool processByValArgument(CallSite CS, unsigned ArgNo);
@@ -509,10 +509,18 @@ bool MemCpyOpt::processStore(StoreInst *SI, BasicBlock::iterator &BBI) {
       }
 
       if (C) {
+        unsigned storeAlign = SI->getAlignment();
+        if (!storeAlign)
+          storeAlign = TD->getABITypeAlignment(SI->getOperand(0)->getType());
+        unsigned loadAlign = LI->getAlignment();
+        if (!loadAlign)
+          loadAlign = TD->getABITypeAlignment(LI->getType());
+
         bool changed = performCallSlotOptzn(LI,
                         SI->getPointerOperand()->stripPointerCasts(),
                         LI->getPointerOperand()->stripPointerCasts(),
-                        TD->getTypeStoreSize(SI->getOperand(0)->getType()), C);
+                        TD->getTypeStoreSize(SI->getOperand(0)->getType()),
+                        std::min(storeAlign, loadAlign), C);
         if (changed) {
           MD->removeInstruction(SI);
           SI->eraseFromParent();
@@ -559,7 +567,8 @@ bool MemCpyOpt::processMemSet(MemSetInst *MSI, BasicBlock::iterator &BBI) {
 /// the call write its result directly into the destination of the memcpy.
 bool MemCpyOpt::performCallSlotOptzn(Instruction *cpy,
                                      Value *cpyDest, Value *cpySrc,
-                                     uint64_t cpyLen, CallInst *C) {
+                                     uint64_t cpyLen, unsigned cpyAlign,
+                                     CallInst *C) {
   // The general transformation to keep in mind is
   //
   //   call @func(..., src, ...)
@@ -596,6 +605,16 @@ bool MemCpyOpt::performCallSlotOptzn(Instruction *cpy,
   if (cpyLen < srcSize)
     return false;
 
+  // Check that dest points to memory that is at least as aligned as src.
+  unsigned srcAlign = srcAlloca->getAlignment();
+  if (!srcAlign)
+    srcAlign = TD->getABITypeAlignment(srcAlloca->getAllocatedType());
+  bool isDestSufficientlyAligned = srcAlign <= cpyAlign;
+  // If dest is not aligned enough and we can't increase its alignment then
+  // bail out.
+  if (!isDestSufficientlyAligned && !isa<AllocaInst>(cpyDest))
+    return false;
+
   // Check that accessing the first srcSize bytes of dest will not cause a
   // trap.  Otherwise the transform is invalid since it might cause a trap
   // to occur earlier than it otherwise would.
@@ -687,6 +706,12 @@ bool MemCpyOpt::performCallSlotOptzn(Instruction *cpy,
   if (!changedArgument)
     return false;
 
+  // If the destination wasn't sufficiently aligned then increase its alignment.
+  if (!isDestSufficientlyAligned) {
+    assert(isa<AllocaInst>(cpyDest) && "Can only increase alloca alignment!");
+    cast<AllocaInst>(cpyDest)->setAlignment(srcAlign);
+  }
+
   // Drop any cached information about the call, because we may have changed
   // its dependence information by changing its parameter.
   MD->removeInstruction(C);
@@ -813,7 +838,8 @@ bool MemCpyOpt::processMemCpy(MemCpyInst *M) {
   if (DepInfo.isClobber()) {
     if (CallInst *C = dyn_cast<CallInst>(DepInfo.getInst())) {
       if (performCallSlotOptzn(M, M->getDest(), M->getSource(),
-                               CopySize->getZExtValue(), C)) {
+                               CopySize->getZExtValue(), M->getAlignment(),
+                               C)) {
         MD->removeInstruction(M);
         M->eraseFromParent();
         return true;