[llvm] [InstCombine] Handle ceil division idiom (PR #100977)

[via llvm-commits]

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@@ -1250,6 +1250,75 @@ static Instruction *foldToUnsignedSaturatedAdd(BinaryOperator &I) {
   return nullptr;
 }
 
+static Value *foldCeilIdioms(BinaryOperator &I, InstCombinerImpl &IC) {
+  assert(I.getOpcode() == Instruction::Add && "Expecting add instruction.");
+  Value *A, *B;
+  ICmpInst::Predicate Pred;
+  auto &ICB = IC.Builder;
+
+  // Fold the log2 ceil idiom:
+  // zext (ctpop(A) >u/!= 1) + (ctlz (A, true) ^ (BW - 1))
+  //      -> BW - ctlz (A - 1, false)
+  const APInt *XorC;
+  if (match(&I,
+            m_c_Add(
+                m_ZExt(m_ICmp(Pred, m_Intrinsic<Intrinsic::ctpop>(m_Value(A)),
+                              m_One())),
+                m_OneUse(m_ZExtOrSelf(m_OneUse(m_Xor(
+                    m_OneUse(m_TruncOrSelf(m_OneUse(
+                        m_Intrinsic<Intrinsic::ctlz>(m_Deferred(A), m_One())))),
+                    m_APInt(XorC))))))) &&
+      (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_NE) &&
+      *XorC == A->getType()->getScalarSizeInBits() - 1) {
+    Value *Sub = ICB.CreateAdd(A, Constant::getAllOnesValue(A->getType()));
+    Value *Ctlz = ICB.CreateIntrinsic(Intrinsic::ctlz, {A->getType()},
+                                      {Sub, ICB.getFalse()});
+    Value *Ret = ICB.CreateSub(
+        ConstantInt::get(A->getType(), A->getType()->getScalarSizeInBits()),
+        Ctlz, "", /*HasNUW*/ true, /*HasNSW*/ true);
+    return ICB.CreateZExtOrTrunc(Ret, I.getType());
+  }
+
+  // Fold the ceil division idiom:
+  // add (udiv (sub A, Bias), B), Bias
+  //      -> udiv (add A, B - 1), B)
+  // with Bias = A != 0; A + B not to overflow
+  auto MatchDivision = [&IC](Instruction *Div, Value *&DivOp0, Value *&DivOp1) {
+    if (match(Div, m_UDiv(m_Value(DivOp0), m_Value(DivOp1))))
+      return true;
+
+    Value *N;
+    const APInt *C;
+    if (match(Div, m_LShr(m_Value(DivOp0), m_Value(N))) &&
+        match(N,
+              m_OneUse(m_Sub(m_APInt(C), m_Intrinsic<Intrinsic::ctlz>(
+                                             m_Specific(DivOp1), m_Zero())))) &&
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goldsteinn wrote:

Think `DivOp1` can be undefined here.

https://github.com/llvm/llvm-project/pull/100977