[llvm] [llvm] Optimize usub.sat fix for #79690 (PR #151044)

[Yingwei Zheng via llvm-commits]

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@@ -1993,6 +1993,63 @@ Value *InstCombinerImpl::foldSelectWithConstOpToBinOp(ICmpInst *Cmp,
   return BinOp;
 }
 
+/// Folds:
+///   %a_sub = call @llvm.usub.sat(x, IntConst1)
+///   %b_sub = call @llvm.usub.sat(y, IntConst2)
+///   %or = or %a_sub, %b_sub
+///   %cmp = icmp eq %or, 0
+///   %sel = select %cmp, 0, MostSignificantBit
+/// into:
+///   %a_sub' = usub.sat(x, IntConst1 - MostSignificantBit)
+///   %b_sub' = usub.sat(y, IntConst2 - MostSignificantBit)
+///   %or = or %a_sub', %b_sub'
+///   %and = and %or, MostSignificantBit
+/// Likewise, for vector arguments as well.
+static Instruction *foldICmpUSubSatWithAndForMostSignificantBitCmp(
+    SelectInst &SI, ICmpInst *ICI, InstCombiner::BuilderTy &Builder) {
+  if (!SI.hasOneUse() || !ICI->hasOneUse())
+    return nullptr;
+  CmpPredicate Pred;
+  Value *A, *B;
+  const APInt *Constant1, *Constant2;
+  if (!match(SI.getCondition(),
+             m_ICmp(Pred,
+                    m_Or(m_Intrinsic<Intrinsic::usub_sat>(m_Value(A),
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dtcxzyw wrote:

Missing one-use check on `or` and `usub_sat`.


https://github.com/llvm/llvm-project/pull/151044