[llvm-commits] [PATCH] Preserve load's alignment after IC xform

[Cameron McInally]

Hey guys,

I found an Instruction Combining transform that does not preserve the
alignment of the load being xformed...

Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp (revision 154024)
+++ lib/Transforms/InstCombine/InstructionCombining.cpp (working copy)
@@ -1401,7 +1401,9 @@
       Value *GEP = Builder->CreateInBoundsGEP(L->getPointerOperand(),
Indices);
       // Returning the load directly will cause the main loop to insert it
in
       // the wrong spot, so use ReplaceInstUsesWith().
-      return ReplaceInstUsesWith(EV, Builder->CreateLoad(GEP));
+      LoadInst *NewLoad = Builder->CreateLoad(GEP);
+      NewLoad->setAlignment(L->getAlignment());
+      return ReplaceInstUsesWith(EV, NewLoad);
     }
   // We could simplify extracts from other values. Note that nested
extracts may
   // already be simplified implicitly by the above: extract (extract
(insert) )


I stumbled upon this in LLVM 2.9, but the issue still exists in trunk. It's
possible that there's a better solution to the problem in trunk, but I have
not spent time investigating.

Tx,
Cameron
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