[llvm] [DAGCombiner] Add basic support for `trunc nsw/nuw` (PR #113808)

[Yingwei Zheng via llvm-commits]

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@@ -13807,23 +13809,27 @@ SDValue DAGCombiner::visitSIGN_EXTEND(SDNode *N) {
     unsigned OpBits   = Op.getScalarValueSizeInBits();
     unsigned MidBits  = N0.getScalarValueSizeInBits();
     unsigned DestBits = VT.getScalarSizeInBits();
-    unsigned NumSignBits = DAG.ComputeNumSignBits(Op);
 
-    if (OpBits == DestBits) {
-      // Op is i32, Mid is i8, and Dest is i32.  If Op has more than 24 sign
-      // bits, it is already ready.
-      if (NumSignBits > DestBits-MidBits)
+    if (N0->getFlags().hasNoSignedWrap() ||
+        DAG.ComputeNumSignBits(Op) > OpBits - MidBits) {
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dtcxzyw wrote:

Unlike in InstCombine, we don't infer poison generating flags in SDAG. Without recursive analysis we may miss some optimization opportunities.


https://github.com/llvm/llvm-project/pull/113808