[PATCH] D88778: [MemCpyOpt] Fix MemorySSA preservation

[Nikita Popov via Phabricator via llvm-commits]

nikic updated this revision to Diff 297330.
nikic added a comment.

Next try...


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D88778/new/

https://reviews.llvm.org/D88778

Files:
  llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
  llvm/test/Transforms/MemCpyOpt/preserve-memssa.ll


Index: llvm/test/Transforms/MemCpyOpt/preserve-memssa.ll
===================================================================
--- llvm/test/Transforms/MemCpyOpt/preserve-memssa.ll
+++ llvm/test/Transforms/MemCpyOpt/preserve-memssa.ll
@@ -148,6 +148,21 @@
   ret void
 }
 
+define void @test8(%t* noalias %src, %t* %dst) {
+; CHECK-LABEL: @test8(
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast %t* [[SRC:%.*]] to i8*
+; CHECK-NEXT:    [[TMP2:%.*]] = bitcast %t* [[DST:%.*]] to i8*
+; CHECK-NEXT:    [[TMP3:%.*]] = bitcast %t* [[SRC]] to i8*
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 1 [[TMP2]], i8* align 1 [[TMP3]], i64 8224, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* align 1 [[TMP1]], i8 0, i64 8224, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %1 = load %t, %t* %src
+  store %t zeroinitializer, %t* %src
+  store %t %1, %t* %dst
+  ret void
+}
+
 declare void @clobber()
 
 ; Function Attrs: argmemonly nounwind willreturn
Index: llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -478,7 +478,7 @@
       Args.insert(Ptr);
 
   // Instruction to lift before P.
-  SmallVector<Instruction*, 8> ToLift;
+  SmallVector<Instruction *, 8> ToLift{SI};
 
   // Memory locations of lifted instructions.
   SmallVector<MemoryLocation, 8> MemLocs{StoreLoc};
@@ -544,10 +544,40 @@
       }
   }
 
-  // We made it, we need to lift
+  // Find MSSA insertion point. Normally P will always have a corresponding
+  // memory access before which we can insert. However, with non-standard AA
+  // pipelines, there may be a mismatch between AA and MSSA, in which case we
+  // will scan for a memory access before P. In either case, we know for sure
+  // that at least the load will have a memory access.
+  // TODO: Simplify this once P will be determined by MSSA, in which case the
+  // discrepancy can no longer occur.
+  MemoryUseOrDef *MemInsertPoint = nullptr;
+  if (MSSAU) {
+    if (MemoryUseOrDef *MA = MSSAU->getMemorySSA()->getMemoryAccess(P)) {
+      MemInsertPoint = cast<MemoryUseOrDef>(--MA->getIterator());
+    } else {
+      const Instruction *ConstP = P;
+      for (const Instruction &I : make_range(++ConstP->getReverseIterator(),
+                                             ++LI->getReverseIterator())) {
+        if (MemoryUseOrDef *MA = MSSAU->getMemorySSA()->getMemoryAccess(&I)) {
+          MemInsertPoint = MA;
+          break;
+        }
+      }
+    }
+  }
+
+  // We made it, we need to lift.
   for (auto *I : llvm::reverse(ToLift)) {
     LLVM_DEBUG(dbgs() << "Lifting " << *I << " before " << *P << "\n");
     I->moveBefore(P);
+    if (MSSAU) {
+      assert(MemInsertPoint && "Must have found insert point");
+      if (MemoryUseOrDef *MA = MSSAU->getMemorySSA()->getMemoryAccess(I)) {
+        MSSAU->moveAfter(MA, MemInsertPoint);
+        MemInsertPoint = MA;
+      }
+    }
   }
 
   return true;
@@ -631,9 +661,8 @@
                             << *M << "\n");
 
           if (MSSAU) {
-            assert(isa<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(P)));
             auto *LastDef =
-                cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(P));
+                cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(SI));
             auto *NewAccess =
                 MSSAU->createMemoryAccessAfter(M, LastDef, LastDef);
             MSSAU->insertDef(cast<MemoryDef>(NewAccess), /*RenameUses=*/true);


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