[llvm] [SCCP] Relax two-instruction range checks (PR #158495)

[Yingwei Zheng via llvm-commits]

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@@ -284,6 +286,63 @@ static Value *simplifyInstruction(SCCPSolver &Solver,
     return Sub;
   }
 
+  // Relax range checks.
+  if (auto *ICmp = dyn_cast<ICmpInst>(&Inst)) {
+    Value *X;
+    auto MatchTwoInstructionExactRangeCheck =
+        [&]() -> std::optional<ConstantRange> {
+      const APInt *RHSC;
+      if (!match(ICmp->getOperand(1), m_APInt(RHSC)))
+        return std::nullopt;
+
+      Value *LHS = ICmp->getOperand(0);
+      ICmpInst::Predicate Pred = ICmp->getPredicate();
+      const APInt *Offset;
+      if (match(LHS, m_OneUse(m_AddLike(m_Value(X), m_APInt(Offset)))))
+        return ConstantRange::makeExactICmpRegion(Pred, *RHSC).sub(*Offset);
+      // Match icmp eq/ne X & NegPow2, C
+      if (ICmp->isEquality()) {
+        const APInt *Mask;
+        if (match(LHS, m_OneUse(m_And(m_Value(X), m_NegatedPower2(Mask)))) &&
+            RHSC->countr_zero() >= Mask->countr_zero()) {
+          ConstantRange CR(*RHSC, *RHSC - *Mask);
+          return Pred == ICmpInst::ICMP_EQ ? CR : CR.inverse();
+        }
+      }
+      return std::nullopt;
+    };
+
+    if (auto CR = MatchTwoInstructionExactRangeCheck()) {
+      ConstantRange LRange = GetRange(X);
+      // Early exit if we know nothing about X.
+      if (LRange.isFullSet())
+        return nullptr;
+      // We are allowed to refine the comparison to either true or false for out
+      // of range inputs. Here we refine the comparison to true, i.e. we relax
+      // the range check.
+      auto NewCR = CR->exactUnionWith(LRange.inverse());
+      // TODO: Check if we can narrow the range check to an equality test.
+      // E.g, for X in [0, 4), X - 3 u< 2 -> X == 3
+      if (!NewCR)
+        return nullptr;
+      // Avoid transforming cases which do not relax the range.
+      // Without this we may revert the transform
+      // (X - C) < Pow2 -> (X & -Pow2) == C.
+      if (NewCR.value() == *CR)
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dtcxzyw wrote:

Dropped. It is no longer useful after adding one-use checks.

https://github.com/llvm/llvm-project/pull/158495