[llvm] [LowerMemIntrinsics] Avoid udiv/urem when type size is a power of 2 (PR #81238)

[Pierre van Houtryve via llvm-commits]

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@@ -155,6 +156,26 @@ void llvm::createMemCpyLoopKnownSize(
          "Bytes copied should match size in the call!");
 }
 
+// \returns \p Len udiv \p OpSize, checking for optimization opportunities.
+static Value *getRuntimeLoopCount(const DataLayout &DL, IRBuilderBase &B,
+                                  Value *Len, Value *OpSize,
+                                  unsigned OpSizeVal) {
+  // For powers of 2, we can lshr by log2 instead of using udiv.
+  if (isPowerOf2_32(OpSizeVal))
+    return B.CreateLShr(Len, Log2_32(OpSizeVal));
+  return B.CreateUDiv(Len, OpSize);
+}
+
+// \returns \p Len urem \p OpSize, checking for optimization opportunities.
+static Value *getRuntimeLoopRemainder(const DataLayout &DL, IRBuilderBase &B,
+                                      Value *Len, Value *OpSize,
+                                      unsigned OpSizeVal) {
+  // For powers of 2, we can and by (OpSizeVal - 1) instead of using urem.
+  if (isPowerOf2_32(OpSizeVal))
+    return B.CreateAnd(Len, OpSizeVal - 1);
+  return B.CreateURem(Len, OpSize);
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Pierre-vh wrote:

No we don't, I just noticed that the type size is always a power of 2 because it's controlled by TTI
I can add a CL switch to no do the optimization + add another set of CHECK lines if you want.

https://github.com/llvm/llvm-project/pull/81238