[PATCH] D16598: DAGCombiner: don't do (zext (trunc x)) -> (and x, 65535) if !hasOneUse(trunc x) and zext is free

escha via llvm-commits llvm-commits at lists.llvm.org
Tue Jan 26 12:54:33 PST 2016

escha created this revision.
escha added a reviewer: resistor.
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This came up in our out of tree target and results in a lot of cases where 'x' ends up used by both (trunc x) and (and x, 65535), causing x to no longer have one use when it really should (and ending up materializing wasted instructions later).




Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6111,8 +6111,10 @@
   // fold (zext (truncate x)) -> (and x, mask)
+  // Only do this if zext isn't free or (truncate x) has one use.
   if (N0.getOpcode() == ISD::TRUNCATE &&
-      (!LegalOperations || TLI.isOperationLegal(ISD::AND, VT))) {
+      (!LegalOperations || TLI.isOperationLegal(ISD::AND, VT)) &&
+      (N0.hasOneUse() || !TLI.isZExtFree(N0.getValueType(), VT))) {
     // fold (zext (truncate (load x))) -> (zext (smaller load x))
     // fold (zext (truncate (srl (load x), c))) -> (zext (smaller load (x+c/n)))

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