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

[Yingwei Zheng via llvm-commits]

================
@@ -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.
----------------
dtcxzyw wrote:

Use `///` for header comments.

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