[llvm] [InstCombine] Try the flipped strictness of predicate in `foldICmpShlConstant` (PR #92773)

[Yingwei Zheng via llvm-commits]

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@@ -2414,14 +2414,37 @@ Instruction *InstCombinerImpl::foldICmpShlConstant(ICmpInst &Cmp,
   // free on the target. It has the additional benefit of comparing to a
   // smaller constant that may be more target-friendly.
   unsigned Amt = ShiftAmt->getLimitedValue(TypeBits - 1);
-  if (Shl->hasOneUse() && Amt != 0 && C.countr_zero() >= Amt &&
-      DL.isLegalInteger(TypeBits - Amt)) {
-    Type *TruncTy = IntegerType::get(Cmp.getContext(), TypeBits - Amt);
-    if (auto *ShVTy = dyn_cast<VectorType>(ShType))
-      TruncTy = VectorType::get(TruncTy, ShVTy->getElementCount());
-    Constant *NewC =
-        ConstantInt::get(TruncTy, C.ashr(*ShiftAmt).trunc(TypeBits - Amt));
-    return new ICmpInst(Pred, Builder.CreateTrunc(X, TruncTy), NewC);
+  if (Shl->hasOneUse() && Amt != 0 && DL.isLegalInteger(TypeBits - Amt)) {
+    auto FoldICmpShlToICmpTrunc = [&](ICmpInst::Predicate Pred,
+                                      const APInt &C) -> Instruction * {
+      if (C.countr_zero() < Amt)
+        return nullptr;
+      Type *TruncTy = ShType->getWithNewBitWidth(TypeBits - Amt);
+      Constant *NewC =
+          ConstantInt::get(TruncTy, C.ashr(*ShiftAmt).trunc(TypeBits - Amt));
+      return new ICmpInst(
+          Pred, Builder.CreateTrunc(X, TruncTy, "", Shl->hasNoSignedWrap()),
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dtcxzyw wrote:

> If you don't have `nsw` and have `nuw`, you can just use `lshr` and propagate `nuw`? If you have both you have to drop either `nuw` or `nsw`. https://alive2.llvm.org/ce/z/9tjTDb

Yeah, I don't know which one is better when we have both flags :(
BTW, as the motivation cases (see https://github.com/dtcxzyw/llvm-opt-benchmark/pull/616) have no flags on shl, I think it is ok to just keep it simple.


https://github.com/llvm/llvm-project/pull/92773