[llvm] [InstCombine] Div ceil optimizations (PR #190175)

[Yingwei Zheng via llvm-commits]

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@@ -1523,6 +1523,68 @@ static Instruction *foldBoxMultiply(BinaryOperator &I) {
   return nullptr;
 }
 
+// Return true if X + (Y-1) is provably non-wrapping in X's type
+static bool checkDivCeilNUW(Value *X, Value *Y, BinaryOperator &I,
+                            AssumptionCache &AC, DominatorTree &DT) {
+  ConstantRange CRX = computeConstantRange(X, /*ForSigned=*/false,
+                                           /*UseInstrInfo=*/true, &AC, &I, &DT);
+  ConstantRange CRY = computeConstantRange(Y, /*ForSigned=*/false,
+                                           /*UseInstrInfo=*/true, &AC, &I, &DT);
+  APInt MaxX = CRX.getUnsignedMax();
+  APInt MaxY = CRY.getUnsignedMax();
+  unsigned BitWidth = MaxX.getBitWidth();
+  // MaxX + (MaxY - 1) <= UINT_MAX  <==>  MaxX <= UINT_MAX - (MaxY - 1)
+  return !MaxX.ugt(APInt::getMaxValue(BitWidth) - (MaxY - 1));
+}
+
+// Fold the same-type form of the div_ceil idiom:
+//   add(udiv(X, Y), zext(icmp ne(urem(X, Y), 0)))
+//     -> udiv(add nuw(X, Y - 1), Y)
+// where udiv and urem are the same type as the add.
+Instruction *InstCombinerImpl::foldSameTypeDivCeil(BinaryOperator &I) {
+  Value *X, *X2, *Y, *Y2;
+  CmpPredicate Pred;
+
+  auto UDivPat = m_OneUse(m_UDiv(m_Value(X), m_Value(Y)));
+  auto URemPat = m_OneUse(m_URem(m_Value(X2), m_Value(Y2)));
+  auto ICmpPat = m_OneUse(m_ICmp(Pred, URemPat, m_Zero()));
+  auto ZExtCmpPat = m_OneUse(m_ZExt(ICmpPat));
+
+  if (!match(&I, m_c_Add(UDivPat, ZExtCmpPat)) || Pred != ICmpInst::ICMP_NE ||
+      X != X2 || Y != Y2 || !checkDivCeilNUW(X, Y, I, AC, DT))
+    return nullptr;
+
+  Value *YMinusOne = Builder.CreateSub(Y, ConstantInt::get(Y->getType(), 1));
+  Value *NUWAdd = Builder.CreateAdd(X, YMinusOne, "", /*HasNUW=*/true);
+  return BinaryOperator::CreateUDiv(NUWAdd, Y);
+}
+
+// Fold the zext form of the div_ceil idiom:
+//   add(zext(udiv(X, Y)), zext(icmp ne(urem(X, Y), 0)))
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dtcxzyw wrote:

I'd expect some general transformations like `binop(zext(X), zext(Y)) -> zext(binop(X, Y))`.


https://github.com/llvm/llvm-project/pull/190175