[llvm] [SCCP] Simplify [us]cmp(X, Y) into X - Y (PR #144717)

[Nikita Popov via llvm-commits]

================
@@ -245,11 +247,45 @@ static Value *simplifyInstruction(SCCPSolver &Solver,
   const APInt *RHSC;
   // Remove masking operations.
   if (match(&Inst, m_And(m_Value(X), m_LowBitMask(RHSC)))) {
-    ConstantRange LRange = GetRange(Inst.getOperand(0));
+    ConstantRange LRange = GetRange(X);
     if (LRange.getUnsignedMax().ule(*RHSC))
       return X;
   }
 
+  // Check if we can simplify [us]cmp(X, Y) to X - Y.
+  if (auto *Cmp = dyn_cast<CmpIntrinsic>(&Inst)) {
+    Intrinsic::ID IID = Cmp->getIntrinsicID();
+    Value *LHS = Cmp->getOperand(0);
+    Value *RHS = Cmp->getOperand(1);
+    unsigned BitWidth = LHS->getType()->getScalarSizeInBits();
+    // Bail out on 1-bit comparisons.
+    // if (BitWidth == 1)
+    //   return nullptr;
+    ConstantRange LRange = GetRange(LHS);
+    if (LRange.isSizeLargerThan(3))
+      return nullptr;
+    ConstantRange RRange = GetRange(RHS);
+    if (RRange.isSizeLargerThan(3))
+      return nullptr;
+    ConstantRange RHSLower = RRange.sub(APInt(BitWidth, 1));
+    ConstantRange RHSUpper = RRange.add(APInt(BitWidth, 1));
+    ICmpInst::Predicate Pred =
+        IID == Intrinsic::scmp ? CmpInst::ICMP_SLE : CmpInst::ICMP_ULE;
+    if (!RHSLower.icmp(Pred, LRange) || !LRange.icmp(Pred, RHSUpper))
+      return nullptr;
+
+    IRBuilder<NoFolder> Builder(&Inst);
+    Value *Sub = Builder.CreateSub(LHS, RHS, Inst.getName(), /*HasNUW=*/false,
+                                   /*HasNSW=*/IID == Intrinsic::scmp);
----------------
nikic wrote:

```suggestion
                                   /*HasNSW=*/Cmp->isSigned());
```

https://github.com/llvm/llvm-project/pull/144717