[MemCpyOpt] Handle memcpy marked as memory(none)

Fixes #71183.

Author: nikic

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -1667,6 +1667,11 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
     return true;
   }
 
+  MemoryUseOrDef *MA = MSSA->getMemoryAccess(M);
+  if (!MA)
+    // Degenerate case: memcpy marked as not accessing memory.
+    return false;
+
   // If copying from a constant, try to turn the memcpy into a memset.
   if (auto *GV = dyn_cast<GlobalVariable>(M->getSource()))
     if (GV->isConstant() && GV->hasDefinitiveInitializer())
@@ -1675,8 +1680,7 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
         IRBuilder<> Builder(M);
         Instruction *NewM = Builder.CreateMemSet(
             M->getRawDest(), ByteVal, M->getLength(), M->getDestAlign(), false);
-        auto *LastDef =
-            cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(M));
+        auto *LastDef = cast<MemoryDef>(MA);
         auto *NewAccess =
             MSSAU->createMemoryAccessAfter(NewM, nullptr, LastDef);
         MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);
@@ -1687,7 +1691,6 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
       }
 
   BatchAAResults BAA(*AA);
-  MemoryUseOrDef *MA = MSSA->getMemoryAccess(M);
   // FIXME: Not using getClobberingMemoryAccess() here due to PR54682.
   MemoryAccess *AnyClobber = MA->getDefiningAccess();
   MemoryLocation DestLoc = MemoryLocation::getForDest(M);

llvm/test/Transforms/MemCpyOpt/memcpy.ll

@@ -723,6 +723,15 @@ define void @byval_param_noalias_metadata(ptr align 4 byval(i32) %ptr) {
   ret void
 }
 
+define void @memcpy_memory_none(ptr %p, ptr %p2, i64 %size) {
+; CHECK-LABEL: @memcpy_memory_none(
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr [[P:%.*]], ptr [[P2:%.*]], i64 [[SIZE:%.*]], i1 false) #[[ATTR6:[0-9]+]]
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memcpy.p0.p0.i64(ptr %p, ptr %p2, i64 %size, i1 false) memory(none)
+  ret void
+}
+
 !0 = !{!0}
 !1 = !{!1, !0}
 !2 = !{!1}