[DAGCombiner] Fix misuse of getZeroExtendInReg in SimplifySelectCC.

[topperc]

If VT has less bits than SCC, using a ZeroExtendInReg isn't going to fix it. That's an AND instruction. We need to truncate the value instead.

This should be ok because we already checked that the boolean contents is ZeroOrOne so the setcc can only produce 0 or 1.

No test because I found this while trying to make i32 legal for RISC-V 64 which I'm not ready to upload yet. You can see in the coverage report that this line isn't tested today.

https://lab.llvm.org/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp.html#L27270

[llvmbot]

@llvm/pr-subscribers-llvm-selectiondag

Author: Craig Topper (topperc)

Changes

If VT has less bits than SCC, using a ZeroExtendInReg isn't going to fix it. That's an AND instruction. We need to truncate the value instead.

This should be ok because we already checked that the boolean contents is ZeroOrOne so the setcc can only produce 0 or 1.

No test because I found this while trying to make i32 legal for RISC-V 64 which I'm not ready to upload yet. You can see in the coverage report that this line isn't tested today.

https://lab.llvm.org/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp.html#L27270

---

Full diff: https://github.com/llvm/llvm-project/pull/70066.diff

1 Files Affected:

• (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+1-4)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index c0a737b70c460e7..ca5bd4952866886 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -27274,10 +27274,7 @@ SDValue DAGCombiner::SimplifySelectCC(const SDLoc &DL, SDValue N0, SDValue N1,
     // zext (setcc n0, n1)
     if (LegalTypes) {
       SCC = DAG.getSetCC(DL, CmpResVT, N0, N1, CC);
-      if (VT.bitsLT(SCC.getValueType()))
-        Temp = DAG.getZeroExtendInReg(SCC, SDLoc(N2), VT);
-      else
-        Temp = DAG.getNode(ISD::ZERO_EXTEND, SDLoc(N2), VT, SCC);
+      Temp = DAG.getZExtOrTrunc(SCC, SDLoc(N2), VT);
     } else {
       SCC = DAG.getSetCC(SDLoc(N0), MVT::i1, N0, N1, CC);
       Temp = DAG.getNode(ISD::ZERO_EXTEND, SDLoc(N2), VT, SCC);

[efriedma-quic]

LGTM