[MemCpyOpt] Allow stack move optimization if one address captured (#165527)

Allow the stack move optimization (which merges two allocas) when the
address of only one alloca is captured (and the provenance is not
captured). Both addresses need to be captured to observe that the
allocas were merged.

Fixes #165484.

Author: nikic

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -1534,8 +1534,8 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
   bool SrcNotDom = false;
 
   auto CaptureTrackingWithModRef =
-      [&](Instruction *AI,
-          function_ref<bool(Instruction *)> ModRefCallback) -> bool {
+      [&](Instruction *AI, function_ref<bool(Instruction *)> ModRefCallback,
+          bool &AddressCaptured) -> bool {
     SmallVector<Instruction *, 8> Worklist;
     Worklist.push_back(AI);
     unsigned MaxUsesToExplore = getDefaultMaxUsesToExploreForCaptureTracking();
@@ -1559,8 +1559,9 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
         if (!Visited.insert(&U).second)
           continue;
         UseCaptureInfo CI = DetermineUseCaptureKind(U, AI);
-        if (capturesAnything(CI.UseCC))
+        if (capturesAnyProvenance(CI.UseCC))
           return false;
+        AddressCaptured |= capturesAddress(CI.UseCC);
 
         if (UI->mayReadOrWriteMemory()) {
           if (UI->isLifetimeStartOrEnd()) {
@@ -1627,7 +1628,9 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
     return true;
   };
 
-  if (!CaptureTrackingWithModRef(DestAlloca, DestModRefCallback))
+  bool DestAddressCaptured = false;
+  if (!CaptureTrackingWithModRef(DestAlloca, DestModRefCallback,
+                                 DestAddressCaptured))
     return false;
   // Bailout if Dest may have any ModRef before Store.
   if (!ReachabilityWorklist.empty() &&
@@ -1653,7 +1656,14 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
     return true;
   };
 
-  if (!CaptureTrackingWithModRef(SrcAlloca, SrcModRefCallback))
+  bool SrcAddressCaptured = false;
+  if (!CaptureTrackingWithModRef(SrcAlloca, SrcModRefCallback,
+                                 SrcAddressCaptured))
+    return false;
+
+  // If both the source and destination address are captured, the fact that they
+  // are no longer two separate allocations may be observed.
+  if (DestAddressCaptured && SrcAddressCaptured)
     return false;
 
   // We can do the transformation. First, move the SrcAlloca to the start of the

llvm/test/Transforms/MemCpyOpt/stack-move.ll

@@ -1729,3 +1729,61 @@ define i32 @test_ret_only_capture() {
   %v = load i32, ptr %a
   ret i32 %v
 }
+
+declare ptr @captures_address_only(ptr captures(address))
+
+; Can transform: Only one address captured.
+define void @test_captures_address_captures_none() {
+; CHECK-LABEL: define void @test_captures_address_captures_none() {
+; CHECK-NEXT:    [[SRC:%.*]] = alloca [[STRUCT_FOO:%.*]], align 4
+; CHECK-NEXT:    store [[STRUCT_FOO]] { i32 10, i32 20, i32 30 }, ptr [[SRC]], align 4
+; CHECK-NEXT:    call void @captures_address_only(ptr [[SRC]])
+; CHECK-NEXT:    call void @use_nocapture(ptr [[SRC]])
+; CHECK-NEXT:    ret void
+;
+  %src = alloca %struct.Foo, align 4
+  %dst = alloca %struct.Foo, align 4
+  store %struct.Foo { i32 10, i32 20, i32 30 }, ptr %src
+  call void @captures_address_only(ptr %src)
+  call void @llvm.memcpy.p0.p0.i64(ptr align 4 %dst, ptr align 4 %src, i64 12, i1 false)
+  call void @use_nocapture(ptr %dst)
+  ret void
+}
+
+; Can transform: Only one address captured.
+define void @test_captures_none_and_captures_address() {
+; CHECK-LABEL: define void @test_captures_none_and_captures_address() {
+; CHECK-NEXT:    [[SRC:%.*]] = alloca [[STRUCT_FOO:%.*]], align 4
+; CHECK-NEXT:    store [[STRUCT_FOO]] { i32 10, i32 20, i32 30 }, ptr [[SRC]], align 4
+; CHECK-NEXT:    call void @use_nocapture(ptr [[SRC]])
+; CHECK-NEXT:    call void @captures_address_only(ptr [[SRC]])
+; CHECK-NEXT:    ret void
+;
+  %src = alloca %struct.Foo, align 4
+  %dst = alloca %struct.Foo, align 4
+  store %struct.Foo { i32 10, i32 20, i32 30 }, ptr %src
+  call void @use_nocapture(ptr %src)
+  call void @llvm.memcpy.p0.p0.i64(ptr align 4 %dst, ptr align 4 %src, i64 12, i1 false)
+  call void @captures_address_only(ptr %dst)
+  ret void
+}
+
+; Cannot transform: Both addresses captured.
+define void @test_captures_address_and_captures_address() {
+; CHECK-LABEL: define void @test_captures_address_and_captures_address() {
+; CHECK-NEXT:    [[SRC:%.*]] = alloca [[STRUCT_FOO:%.*]], align 4
+; CHECK-NEXT:    [[DST:%.*]] = alloca [[STRUCT_FOO]], align 4
+; CHECK-NEXT:    store [[STRUCT_FOO]] { i32 10, i32 20, i32 30 }, ptr [[SRC]], align 4
+; CHECK-NEXT:    call void @captures_address_only(ptr [[SRC]])
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 4 [[DST]], ptr align 4 [[SRC]], i64 12, i1 false)
+; CHECK-NEXT:    call void @captures_address_only(ptr [[DST]])
+; CHECK-NEXT:    ret void
+;
+  %src = alloca %struct.Foo, align 4
+  %dst = alloca %struct.Foo, align 4
+  store %struct.Foo { i32 10, i32 20, i32 30 }, ptr %src
+  call void @captures_address_only(ptr %src)
+  call void @llvm.memcpy.p0.p0.i64(ptr align 4 %dst, ptr align 4 %src, i64 12, i1 false)
+  call void @captures_address_only(ptr %dst)
+  ret void
+}