MemCpyOpt: replace an AA query with MSSA query (NFC) (#108535)

Fix a long-standing TODO.

Author: artagnon

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -638,6 +638,7 @@ bool MemCpyOptPass::processStoreOfLoad(StoreInst *SI, LoadInst *LI,
   if (!LI->isSimple() || !LI->hasOneUse() || LI->getParent() != SI->getParent())
     return false;
 
+  BatchAAResults BAA(*AA);
   auto *T = LI->getType();
   // Don't introduce calls to memcpy/memmove intrinsics out of thin air if
   // the corresponding libcalls are not available.
@@ -647,19 +648,17 @@ bool MemCpyOptPass::processStoreOfLoad(StoreInst *SI, LoadInst *LI,
       (EnableMemCpyOptWithoutLibcalls ||
        (TLI->has(LibFunc_memcpy) && TLI->has(LibFunc_memmove)))) {
     MemoryLocation LoadLoc = MemoryLocation::get(LI);
-
-    // We use alias analysis to check if an instruction may store to
-    // the memory we load from in between the load and the store. If
-    // such an instruction is found, we try to promote there instead
-    // of at the store position.
-    // TODO: Can use MSSA for this.
-    Instruction *P = SI;
-    for (auto &I : make_range(++LI->getIterator(), SI->getIterator())) {
-      if (isModSet(AA->getModRefInfo(&I, LoadLoc))) {
-        P = &I;
-        break;
-      }
-    }
+    MemoryUseOrDef *LoadAccess = MSSA->getMemoryAccess(LI),
+                   *StoreAccess = MSSA->getMemoryAccess(SI);
+
+    // We use MSSA to check if an instruction may store to the memory we load
+    // from in between the load and the store. If such an instruction is found,
+    // we try to promote there instead of at the store position.
+    auto *Clobber = MSSA->getWalker()->getClobberingMemoryAccess(
+        StoreAccess->getDefiningAccess(), LoadLoc, BAA);
+    Instruction *P = MSSA->dominates(LoadAccess, Clobber)
+                         ? cast<MemoryUseOrDef>(Clobber)->getMemoryInst()
+                         : SI;
 
     // If we found an instruction that may write to the loaded memory,
     // we can try to promote at this position instead of the store
@@ -707,7 +706,6 @@ bool MemCpyOptPass::processStoreOfLoad(StoreInst *SI, LoadInst *LI,
   // Detect cases where we're performing call slot forwarding, but
   // happen to be using a load-store pair to implement it, rather than
   // a memcpy.
-  BatchAAResults BAA(*AA);
   auto GetCall = [&]() -> CallInst * {
     // We defer this expensive clobber walk until the cheap checks
     // have been done on the source inside performCallSlotOptzn.

llvm/test/Transforms/MemCpyOpt/fca2memcpy.ll

@@ -141,4 +141,19 @@ define void @throwing_call(ptr noalias %src, ptr %dst) {
   ret void
 }
 
+define void @loop_memoryphi(ptr %a, ptr %b) {
+; CHECK-LABEL: @loop_memoryphi(
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    call void @llvm.memmove.p0.p0.i64(ptr align 8 [[B:%.*]], ptr align 8 [[A:%.*]], i64 16, i1 false)
+; CHECK-NEXT:    br label [[LOOP]]
+;
+  br label %loop
+
+loop:
+  %v = load { i64, i64 }, ptr %a
+  store { i64, i64 } %v, ptr %b
+  br label %loop
+}
+
 declare void @call()