[PATCH] D26449: DAGCombiner: fix combine of trunc and select

[Asaf Badouh via llvm-commits]

AsafBadouh created this revision.
AsafBadouh added reviewers: zvi, eladcohen, igorb, craig.topper.
AsafBadouh added a subscriber: llvm-commits.
AsafBadouh set the repository for this revision to rL LLVM.

the pattern catch the following:
// trunc (select c, a, b) -> select c, (trunc a), (trunc b)

in case select has more than one use the exist pattern will add extra select.

i16 t1 = select c, a , b
i16 t2 = srl t1, 3
i8 t3 = trunc i16 t1 to i8
-->
i8 a1 = trunc i16 a to i8
i8 b1 = trunc i16 a to i8
i8 t3 = select c, a1 , b1
i16 t1 = select c, a , b
i16 t2 = srl t1, 3


Repository:
  rL LLVM

https://reviews.llvm.org/D26449

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/select.ll


Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7307,8 +7307,8 @@
   // trunc (select c, a, b) -> select c, (trunc a), (trunc b)
   if (N0.getOpcode() == ISD::SELECT) {
     EVT SrcVT = N0.getValueType();
-    if ((!LegalOperations || TLI.isOperationLegal(ISD::SELECT, SrcVT)) &&
-        TLI.isTruncateFree(SrcVT, VT)) {
+    if ((!LegalOperations || TLI.isOperationLegal(ISD::SELECT, SrcVT)) && 
+        N0.hasOneUse() && TLI.isTruncateFree(SrcVT, VT)) {
       SDLoc SL(N0);
       SDValue Cond = N0.getOperand(0);
       SDValue TruncOp0 = DAG.getNode(ISD::TRUNCATE, SL, VT, N0.getOperand(1));
Index: test/CodeGen/X86/select.ll
===================================================================
--- test/CodeGen/X86/select.ll
+++ test/CodeGen/X86/select.ll
@@ -515,6 +515,33 @@
   ret i32 %tmp2
 }
 
+
+define i16 @trunc_select_negative(i8 %a, i16 %b) {
+entry:
+; CHECK-LABEL: trunc_select:
+; CHECK:       ## BB#0:
+; CHECK: cmovew
+; CHECK: cmovew
+; CHECK-NOT: cmovew
+; CHECK: retq
+  %b.tr = trunc i16 %b to i8
+  %and.0 = xor i8 %b.tr, %a
+  %xor.0 = and i8 %and.0, 1
+  %shr.0 = lshr i8 %a, 1
+  %cond = icmp eq i8 %xor.0, 0
+  %shr.1 = lshr i16 %b, 1
+  %0 = xor i16 %shr.1, -432
+  %1 = select i1 %cond, i16 %shr.1, i16 %0
+  %b.tr.1 = trunc i16 %1 to i8
+  %and.1 = xor i8 %b.tr.1, %shr.0
+  %xor.1 = and i8 %and.1, 1
+  %cond.1 = icmp eq i8 %xor.1, 0
+  %shr.2 = lshr i16 %1, 1
+  %2 = xor i16 %shr.2, -231
+  %3 = select i1 %cond.1, i16 %shr.2, i16 %2
+  ret i16 %3
+}
+
 define void @test19() {
 ; This is a massive reduction of an llvm-stress test case that generates
 ; interesting chains feeding setcc and eventually a f32 select operation. This


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