[llvm] [DAGCombiner] Fold subtraction if above a constant threshold to `umin` (PR #135194)

[Björn Pettersson via llvm-commits]

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@@ -12074,6 +12078,17 @@ SDValue DAGCombiner::visitSELECT(SDNode *N) {
 
     if (SDValue NewSel = SimplifySelect(DL, N0, N1, N2))
       return NewSel;
+
+    // (select (ugt x, C), (add x, ~C), x) -> (umin (add x, ~C), x)
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bjope wrote:

I think this shows the problem: https://alive2.llvm.org/ce/z/fmKmiZ

One either need to freeze the UMIN operands, or make sure that N1 and N2 is guaranteed not to be poison when doing this transform. Maybe it is enough to freeze the ADD operand (as the comparison would be poison if x is poison)?

https://github.com/llvm/llvm-project/pull/135194