[llvm] [LegalizeTypes] Expand 128-bit UDIV/UREM by constant via Chunk Addition (PR #146238)

[Matt Arsenault via llvm-commits]

================
@@ -8030,6 +8028,80 @@ bool TargetLowering::expandDIVREMByConstant(SDNode *N,
                               DAG.getConstant(0, dl, HiLoVT));
       Sum = DAG.getNode(ISD::ADD, dl, HiLoVT, Sum, Carry);
     }
+
+  } else {
+    // If we cannot split in two halves. Let's look for a smaller chunk
+    // width where (1 << ChunkWidth) mod Divisor == 1.
+    // This ensures that the sum of all such chunks modulo Divisor
+    // is equivalent to the original value modulo Divisor.
+    const APInt &Divisor = CN->getAPIntValue();
+    unsigned BitWidth = VT.getScalarSizeInBits();
+    unsigned BestChunkWidth = 0;
+
+    // We restrict to small chunk sizes (e.g., ≤ 32 bits) to ensure that all
+    // operations remain legal on most targets.
+    unsigned MaxChunk = 32;
+    for (int i = MaxChunk; i >= 1; --i) {
+      APInt ChunkMaxPlus1 = APInt::getOneBitSet(BitWidth, i);
+      if (ChunkMaxPlus1.urem(Divisor).isOne()) {
+        BestChunkWidth = i;
+        break;
+      }
+    }
+
+    // If we found a good chunk width, slice the number and sum the pieces.
+    if (BestChunkWidth > 0) {
+      EVT ChunkVT = EVT::getIntegerVT(*DAG.getContext(), BestChunkWidth);
+
+      if (!LL)
----------------
arsenm wrote:

Braces, but I'm not really sure why this is conditional in the first place 

https://github.com/llvm/llvm-project/pull/146238