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

[Matt Arsenault via llvm-commits]

Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>,
Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/97771 at github.com>


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@@ -7490,3 +7490,153 @@ bool CombinerHelper::matchNonNegZext(const MachineOperand &MO,
 
   return false;
 }
+
+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;
+
+  // Cannot fail due to pattern.
+  std::optional<APInt> MaybeC2 = getIConstantVRegVal(Sub->getRHSReg(), MRI);
+  if (!MaybeC2)
+    return false;
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arsenm wrote:

You are still checking it despite the comment that it cannot fail 

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