[llvm] [RISCV] Add initial support of memcmp expansion (PR #107548)

[Philip Reames via llvm-commits]

================
@@ -2113,3 +2113,18 @@ bool RISCVTTIImpl::shouldConsiderAddressTypePromotion(
   }
   return Considerable;
 }
+
+RISCVTTIImpl::TTI::MemCmpExpansionOptions
+RISCVTTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
+  TTI::MemCmpExpansionOptions Options;
+  // FIXME: Vector haven't been tested.
+  Options.AllowOverlappingLoads =
+      (ST->enableUnalignedScalarMem() || ST->enableUnalignedVectorMem());
----------------
preames wrote:

Unless I'm missing something, there is no check for sufficient alignment on *any* load generated by ExpandMemCmp, as a result, we can only legally provide a non-byte size if unaligned access is fully supported.

(This is really bug in ExpandMemCmp - it should be asking if the load is properly aligned.)

https://github.com/llvm/llvm-project/pull/107548