[llvm] [ExpandMemCmp][RISCV] Expand memcmp/bcmp for aligned pointers on strict-align targets (PR #209738)

[Pengcheng Wang via llvm-commits]

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@@ -3630,23 +3630,30 @@ bool RISCVTTIImpl::isProfitableToSinkOperands(
 RISCVTTIImpl::TTI::MemCmpExpansionOptions
 RISCVTTIImpl::enableMemCmpExpansion(bool OptSize, bool IsZeroCmp) const {
   TTI::MemCmpExpansionOptions Options;
-  // TODO: Enable expansion when unaligned access is not supported after we fix
-  // issues in ExpandMemcmp.
-  if (!ST->enableUnalignedScalarMem())
-    return Options;
 
   if (!ST->hasStdExtZbb() && !ST->hasStdExtZbkb() && !IsZeroCmp)
     return Options;
 
-  Options.AllowOverlappingLoads = true;
+  // Even if the target does not support unaligned scalar memory access,
+  // expansion is still possible when both pointers are statically known to be
+  // sufficiently aligned. ExpandMemCmp queries the target for each load size
+  // and keeps only the ones the target can actually access at the known
+  // per-call-site alignment, falling back to the libcall when none fits.
+  // Overlapping loads and merged tail expansions produce accesses that need
+  // not be naturally aligned, so they are only offered when unaligned scalar
+  // access is supported.
+  bool UnalignedScalar = ST->enableUnalignedScalarMem();
+  Options.AllowOverlappingLoads = UnalignedScalar;
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wangpc-pp wrote:

Yeah I agree. I did a quick fix and it is more complicated than I thought. I can fix it in a follow-up.

https://github.com/llvm/llvm-project/pull/209738