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

[Yingwei Zheng via llvm-commits]

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@@ -1993,6 +1991,137 @@ 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 && Pred != llvm::ICmpInst::ICMP_NE)
+    return nullptr;
+
+  // Match: icmp eq (or (usub.sat A, IntConst1), (usub.sat B, IntConst2)), 0
+  Value *A, *B;
+  ConstantInt *IntConst1, *IntConst2, *PossibleMSBInt;
+
+  if (match(CmpLHS, m_Or(m_Intrinsic<Intrinsic::usub_sat>(
+                             m_Value(A), m_ConstantInt(IntConst1)),
----------------
dtcxzyw wrote:

Use `m_APInt`, then you can avoid repeating code to handle vector splats.
If you want to handle non-splat vectors as well, use constant folding helpers. But it is not enforced: https://llvm.org/docs/InstCombineContributorGuide.html#guidelines-for-reviewers.
IIRC the original motivating case only has constant splats.


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