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

[Nimit Sachdeva via llvm-commits]

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@@ -1993,6 +1993,134 @@ 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) {
+  auto *CI = dyn_cast<ICmpInst>(SI.getCondition());
+  if (!CI) {
+    return nullptr;
+  }
+
+  Value *CmpLHS = CI->getOperand(0);
+  Value *CmpRHS = CI->getOperand(1);
+  if (!match(CmpRHS, m_Zero())) {
+    return nullptr;
+  }
+  auto Pred = CI->getPredicate();
+  auto *TrueVal = SI.getTrueValue();
+  auto *FalseVal = SI.getFalseValue();
+
+  if (Pred != ICmpInst::ICMP_EQ)
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nimit25 wrote:

I see, makes sense. I can update this check

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