[llvm] [InstCombine] Fold icmp with clamp into unsigned bound check (PR #161303)

[Yingwei Zheng via llvm-commits]

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@@ -5780,6 +5780,47 @@ Instruction *InstCombinerImpl::foldICmpWithMinMax(Instruction &I,
   return nullptr;
 }
 
+// Transform patterns like:
+//   icmp eq/ne X, min(max(X, Lo), Hi)
+// Into:
+//   (X - Lo) u< (Hi - Lo + 1)
+Instruction *InstCombinerImpl::foldICmpWithClamp(ICmpInst &I, Value *X,
+                                                 MinMaxIntrinsic *Min) {
+  if (!I.isEquality() || !Min->hasOneUse())
+    return nullptr;
+
+  const APInt *Lo = nullptr, *Hi = nullptr;
+  if (Min->isSigned()) {
+    if (!match(Min->getLHS(), m_OneUse(m_SMax(m_Specific(X), m_APInt(Lo)))) ||
+        !match(Min->getRHS(), m_APInt(Hi)) || !Lo->slt(*Hi))
+      return nullptr;
+  } else {
+    if (!match(Min->getLHS(), m_OneUse(m_UMax(m_Specific(X), m_APInt(Lo)))) ||
+        !match(Min->getRHS(), m_APInt(Hi)) || !Lo->ult(*Hi))
+      return nullptr;
+  }
+
+  // If Hi is the maximum value, the min operation becomes redundant and
+  // will be removed by other optimizations.
+  if ((Min->isSigned() && (Lo->isMinSignedValue() || Hi->isMaxSignedValue())) ||
+      (!Min->isSigned() && (Lo->isMinValue() || Hi->isMaxValue())))
+    return nullptr;
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dtcxzyw wrote:

Does it affect the correctness or cause regressions? IIRC these checks are unnecessary.

https://github.com/llvm/llvm-project/pull/161303