Fix a miscompile in the DAG combiner. Previously, we would incorrectly
try to reduce the width of this load, and would end up transforming:
(truncate (lshr (sextload i48 <ptr> as i64), 32) to i32)
to
(truncate (zextload i32 <ptr+4> as i64) to i32)
We lost the sext attached to the load while building the narrower i32
load, and replaced it with a zext because lshr always zext's the
results. Instead, bail out of this combine when there is a conflict
between a sextload and a zext narrowing. The rest of the DAG combiner
still optimize the code down to the proper single instruction:
movswl 6(...),%eax
Which is exactly what we wanted. Previously we read past the end *and*
missed the sign extension:
movl 6(...), %eax
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169802 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index e774c13..1c28d6d 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5068,11 +5068,15 @@
// At this point, we must have a load or else we can't do the transform.
if (!isa<LoadSDNode>(N0)) return SDValue();
+ // Because a SRL must be assumed to *need* to zero-extend the high bits
+ // (as opposed to anyext the high bits), we can't combine the zextload
+ // lowering of SRL and an sextload.
+ if (cast<LoadSDNode>(N0)->getExtensionType() == ISD::SEXTLOAD)
+ 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 >= cast<LoadSDNode>(N0)->getMemoryVT().getSizeInBits())
return SDValue();
}