[llvm] [GlobalIsel] Combine G_PTR_ADD. (PR #95647)

[Thorsten Schütt via llvm-commits]

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@@ -7437,3 +7437,212 @@ bool CombinerHelper::matchNonNegZext(const MachineOperand &MO,
 
   return false;
 }
+
+bool CombinerHelper::matchPtrAddWithSub(const MachineOperand &MO,
+                                        BuildFnTy &MatchInfo) {
+  GPtrAdd *Inner = cast<GPtrAdd>(MRI.getVRegDef(MO.getReg()));
+  GSub *Sub = cast<GSub>(MRI.getVRegDef(Inner->getOffsetReg()));
+
+  // sub(x, c) -> add(x, -c)
+
+  // one-use check
+  if (!MRI.hasOneNonDBGUse(Sub->getReg(0)))
+    return false;
+
+  // Cannot fail due to pattern.
+  std::optional<APInt> MaybeImm = getIConstantVRegVal(Sub->getRHSReg(), MRI);
+  if (!MaybeImm)
+    return false;
+
+  LLT ConstTy = MRI.getType(Inner->getOffsetReg());
+
+  if (!isConstantLegalOrBeforeLegalizer(ConstTy))
+    return false;
+
+  Register Dst = MO.getReg();
+  LLT DstTy = MRI.getType(Dst);
+
+  MatchInfo = [=](MachineIRBuilder &B) {
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tschuett wrote:

In the match part, I can say a MachineOperand is a constant, e.g., 0. I bet in MIR that I would need to create a G_CONSTANT.

https://github.com/llvm/llvm-project/pull/95647