[llvm] [InstCombine] Prefer to keep power-of-2 constants when combining ashr exact and slt/ult of a constant (PR #86111)

[Alex Bradbury via llvm-commits]

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@@ -2482,10 +2482,21 @@ Instruction *InstCombinerImpl::foldICmpShrConstant(ICmpInst &Cmp,
   // those conditions rather than checking them. This is difficult because of
   // undef/poison (PR34838).
   if (IsAShr && Shr->hasOneUse()) {
+    if (IsExact && (Pred == CmpInst::ICMP_SLT || Pred == CmpInst::ICMP_ULT) &&
+        !C.isMinSignedValue() && (C - 1).isPowerOf2()) {
+      // When C - 1 is a power of two and the transform can be legally
+      // performed, prefer this form so the produced constant is close to a
+      // power of two.
+      // icmp slt/ult (ashr exact X, ShAmtC), C
+      // --> icmp slt/ult (C - 1) << ShAmtC) -1
+      APInt ShiftedC = (C - 1).shl(ShAmtVal) + 1;
+      return new ICmpInst(Pred, X, ConstantInt::get(ShrTy, ShiftedC));
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asb wrote:

Thank you, let me take a closer look.

https://github.com/llvm/llvm-project/pull/86111