[llvm] [GlobalIsel] Combine G_ADD and G_SUB with constants (PR #97771)

[Thorsten Schütt via llvm-commits]

================
@@ -7457,3 +7457,143 @@ void CombinerHelper::applyExpandFPowI(MachineInstr &MI, int64_t Exponent) {
   Builder.buildCopy(Dst, *Res);
   MI.eraseFromParent();
 }
+
+bool CombinerHelper::matchFoldAPlusC1MinusC2(const MachineInstr &MI,
+                                             BuildFnTy &MatchInfo) {
+  // fold (A+C1)-C2 -> A+(C1-C2)
+  const GSub *Sub = cast<GSub>(&MI);
+  GAdd *Add = cast<GAdd>(MRI.getVRegDef(Sub->getLHSReg()));
+
+  if (!MRI.hasOneNonDBGUse(Add->getReg(0)))
+    return false;
+
+  std::optional<APInt> MaybeC2 = getIConstantVRegVal(Sub->getRHSReg(), MRI);
+  if (!MaybeC2)
+    return false;
+
+  std::optional<APInt> MaybeC1 = getIConstantVRegVal(Add->getRHSReg(), MRI);
+  if (!MaybeC1)
+    return false;
+
+  Register Dst = Sub->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    auto Const = B.buildConstant(DstTy, *MaybeC1 - *MaybeC2);
+    B.buildAdd(Dst, Add->getLHSReg(), Const);
+  };
+
+  return true;
+}
+
+bool CombinerHelper::matchFoldC2MinusAPlusC1(const MachineInstr &MI,
+                                             BuildFnTy &MatchInfo) {
+  // fold C2-(A+C1) -> (C2-C1)-A
+  const GSub *Sub = cast<GSub>(&MI);
+  GAdd *Add = cast<GAdd>(MRI.getVRegDef(Sub->getRHSReg()));
+
+  if (!MRI.hasOneNonDBGUse(Add->getReg(0)))
+    return false;
+
+  std::optional<APInt> MaybeC2 = getIConstantVRegVal(Sub->getLHSReg(), MRI);
+  if (!MaybeC2)
+    return false;
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tschuett wrote:

No.  This must be part of the LLVM Programmers guide. Optionals *must* be checked before access. Even if you claim through some side-channel that they cannot fail. They are optionals.

https://github.com/llvm/llvm-project/pull/97771