Fix a bug in ReduceLoadWidth that wasn't handling extending

loads properly.  We miscompiled the testcase into:

_test:                                  ## @test
	movl	$128, (%rdi)
	movzbl	1(%rdi), %eax
	ret

Now we get a proper:

_test:                                  ## @test
	movl	$128, (%rdi)
	movsbl	(%rdi), %eax
	movzbl	%ah, %eax
	ret

This fixes PR8757.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122392 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2523bb2451..e4f3027053 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -4241,12 +4241,15 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
           return SDValue();
       }
 
+      // At this point, we must have a load or else we can't do the transform.
+      if (!isa<LoadSDNode>(N0)) return SDValue();
+      
       // If the shift amount is larger than the input type then we're not
       // accessing any of the loaded bytes.  If the load was a zextload/extload
       // then the result of the shift+trunc is zero/undef (handled elsewhere).
       // If the load was a sextload then the result is a splat of the sign bit
       // of the extended byte.  This is not worth optimizing for.
-      if (ShAmt >= VT.getSizeInBits())
+      if (ShAmt >= cast<LoadSDNode>(N0)->getMemoryVT().getSizeInBits())
         return SDValue();
     }
   }