[llvm] [InstCombine] Decompose an icmp into multiple ranges (PR #69855)

[via llvm-commits]

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@@ -1334,6 +1334,101 @@ Value *InstCombinerImpl::foldAndOrOfICmpsUsingRanges(ICmpInst *ICmp1,
   return Builder.CreateICmp(NewPred, NewV, ConstantInt::get(Ty, NewC));
 }
 
+/// Decompose icmp into intersection or union of ranges.
+static bool decomposeICmpIntoRangeSet(SmallVectorImpl<ConstantRange> &Set,
+                                      ICmpInst *ICmp, Value *X, bool IsAnd) {
+  // icmp eq/ne (X & mask), 0
+  ICmpInst::Predicate Pred;
+  const APInt *Mask;
+  if (match(ICmp,
+            m_ICmp(Pred, m_And(m_Specific(X), m_APInt(Mask)), m_Zero())) &&
+      ICmp->isEquality()) {
+    if (Mask->popcount() == Mask->getBitWidth() - 1) {
+      auto Zero = APInt::getZero(Mask->getBitWidth());
+      auto Val = ~*Mask;
+      if (IsAnd) {
+        if (Pred == ICmpInst::ICMP_EQ) {
+          Set.push_back(ConstantRange(Zero, Val + 1));
+          Set.push_back(ConstantRange(Val, APInt(Mask->getBitWidth(), 1)));
+        } else {
+          Set.push_back(ConstantRange(Zero).inverse());
+          Set.push_back(ConstantRange(Val).inverse());
+        }
+      } else {
+        if (Pred == ICmpInst::ICMP_EQ) {
+          Set.push_back(Val);
+          Set.push_back(Zero);
+        } else {
+          Set.push_back(ConstantRange(APInt(Mask->getBitWidth(), 1), Val));
+          Set.push_back(ConstantRange(Val + 1, Zero));
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goldsteinn wrote:

What these ranges repesent need some comments.

https://github.com/llvm/llvm-project/pull/69855