[cxx-interop] Fix metadata mismatch regarding fields of structs #81035

susmonteiro · 2025-04-23T17:14:51Z

In #78467 and #78961, we stopped emitting metadata for private C++ fields. However, this created a mismatch between the fields emitted and the number of fields + their offsets in the StructDescriptor.

rdar://147263490

susmonteiro · 2025-04-23T17:15:06Z

@swift-ci please smoke test

susmonteiro · 2025-04-24T11:00:40Z

@swift-ci please smoke test

Xazax-hun

This is great, thanks for tracking this down! I am not super confident about some details, asked some questions inline. Maybe we should get someone more familiar with type metadata reviewed this PR.

Xazax-hun · 2025-04-24T13:41:49Z

lib/IRGen/GenMeta.cpp

@@ -1845,7 +1845,7 @@ namespace {

    void addLayoutInfo() {
      // uint32_t NumFields;
-      B.addInt32(getNumFields(getType()));


I wonder if we need to do something similar for ClassContextDescriptorBuilder too.

I am less sure about emitInitializeFieldOffsetVector and co. Maybe those are good as is but maybe we want to ask someone familiar with type metadata.

I wonder if we need to do something similar for ClassContextDescriptorBuilder too.

Yes I think you're right. I intend to check if we need a similar fix for classes and add those changes in a future PR

Regarding emitInitializeFieldOffsetVector, I'm also not sure. I'll investigate this further

emitInitializeFieldOffsetVector should include all fields. We are using that information to handle values in value witness functions.

@drexin would it be a problem to have a discrepancy between the fields in emitInitializeFieldOffsetVector and the fields in StructContextDescriptorBuilder/FieldTypeMetadataBuilder?

Hm, I am not sure. cc: @mikeash

Reflection relies on the indexes in the field offset vector and the indexes of the descriptor's fields lining up. In theory you could get away with having additional entries at the end of the field offset vector, but not in the middle, and not more field records than field offsets.

Xazax-hun · 2025-04-24T13:45:37Z

lib/IRGen/StructMetadataVisitor.h

@@ -64,7 +65,8 @@ template <class Impl> class StructMetadataVisitor
    // Struct field offsets.
    asImpl().noteStartOfFieldOffsets();
    for (VarDecl *prop : Target->getStoredProperties())
-      asImpl().addFieldOffset(prop);
+      if (!isPrivateField(prop))


I wonder if we can get out of sync with emitInitializeFieldOffsetVector here.

susmonteiro · 2025-04-24T14:49:47Z

@swift-ci please smoke test

susmonteiro · 2025-04-25T11:52:50Z

@swift-ci please smoke test

Xazax-hun · 2025-04-25T13:06:40Z

test/Interop/Cxx/stdlib/print-stdlib-types.swift

+    // CHECK-linux-gnu: 42
+    // CHECK-linux-gnu: shared_ptr<CInt>()
+printStdSharedPtrOfString()
+// CHECK-macosx: basic_string<CChar, std.__1.char_traits<CChar>, std.__1.allocator<CChar>>()


Alternatively, you could add regexes that match on all platforms. I don't have a preference.

Thanks for the suggestion!
Hopefully we will soon fix the conformance of std::string to CustomStringConvertible. Then we can remove all these os options, since the value of the string will be printed instead of the type name.
That said, I don't mind switching to regex

Good point, the string one is temporary (maybe not the shared_ptr one though). Let's leave it as is.

I think using regexes would be a bit more reliable here, since that would ensure the correct behavior on platforms that aren't listed here, e.g. Android or FreeBSD. The regex could be really permissive: something like {{.*}}.basic_string<CChar, {{.*}}> should do the trick.

Sure, I'll make this change.

susmonteiro · 2025-04-25T15:26:20Z

@swift-ci please smoke test

Xazax-hun · 2025-04-25T15:31:35Z

test/Interop/Cxx/stdlib/print-stdlib-types.swift

+    // CHECK-linux-gnu: 42
+    // CHECK-linux-gnu: shared_ptr<CInt>()
+printStdSharedPtrOfString()
+// CHECK-macosx: basic_string<CChar, std.__1.char_traits<CChar>, std.__1.allocator<CChar>>()


Good point, the string one is temporary (maybe not the shared_ptr one though). Let's leave it as is.

Misclicked.

susmonteiro · 2025-04-25T15:43:48Z

@swift-ci please smoke test

susmonteiro · 2025-04-28T09:12:25Z

@swift-ci Please test macOS platform

egorzhdan

This looks great, thanks!
I only have a couple of comments on testing.

egorzhdan · 2025-05-06T12:19:50Z

test/Interop/Cxx/stdlib/Inputs/stdlib-types.h

@@ -0,0 +1,24 @@
+#ifndef TEST_INTEROP_CXX_STDLIB_INPUTS_STD_TYPES_H


Can we reuse an existing test header in this directory, e.g. std-string-and-vector.h or std-vector.h and std-unique-ptr.h, instead of adding a new one?

egorzhdan · 2025-05-06T12:21:31Z

test/Interop/Cxx/stdlib/print-stdlib-types.swift

+    // CHECK-linux-gnu: 42
+    // CHECK-linux-gnu: shared_ptr<CInt>()
+printStdSharedPtrOfString()
+// CHECK-macosx: basic_string<CChar, std.__1.char_traits<CChar>, std.__1.allocator<CChar>>()


I think using regexes would be a bit more reliable here, since that would ensure the correct behavior on platforms that aren't listed here, e.g. Android or FreeBSD. The regex could be really permissive: something like {{.*}}.basic_string<CChar, {{.*}}> should do the trick.

susmonteiro · 2025-05-07T15:19:16Z

@swift-ci please smoke test

susmonteiro added the c++ interop Feature: Interoperability with C++ label Apr 23, 2025

susmonteiro force-pushed the susmonteiro/metadata-private-fields branch from f36ff56 to a39010b Compare April 24, 2025 10:59

susmonteiro marked this pull request as ready for review April 24, 2025 11:03

susmonteiro requested review from zoecarver, egorzhdan, Xazax-hun, j-hui, fahadnayyar, hnrklssn and rjmccall as code owners April 24, 2025 11:03

Xazax-hun reviewed Apr 24, 2025

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susmonteiro force-pushed the susmonteiro/metadata-private-fields branch from a39010b to a49738e Compare April 24, 2025 14:49

susmonteiro force-pushed the susmonteiro/metadata-private-fields branch from a49738e to abd2549 Compare April 25, 2025 11:52

Xazax-hun reviewed Apr 25, 2025

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susmonteiro force-pushed the susmonteiro/metadata-private-fields branch from abd2549 to 2a99a7d Compare April 25, 2025 15:25

Xazax-hun previously approved these changes Apr 25, 2025

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susmonteiro force-pushed the susmonteiro/metadata-private-fields branch from 2a99a7d to bdacdd0 Compare April 25, 2025 15:42

egorzhdan requested a review from tbkka May 1, 2025 14:37

egorzhdan reviewed May 6, 2025

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[cxx-interop] Fix metadata mismatch regarding fields of structs

3efa921

susmonteiro force-pushed the susmonteiro/metadata-private-fields branch from bdacdd0 to 3efa921 Compare May 7, 2025 15:18

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[cxx-interop] Fix metadata mismatch regarding fields of structs #81035

[cxx-interop] Fix metadata mismatch regarding fields of structs #81035

susmonteiro commented Apr 23, 2025 •

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susmonteiro commented May 7, 2025

		@@ -0,0 +1,24 @@
		#ifndef TEST_INTEROP_CXX_STDLIB_INPUTS_STD_TYPES_H

[cxx-interop] Fix metadata mismatch regarding fields of structs #81035

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[cxx-interop] Fix metadata mismatch regarding fields of structs #81035

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susmonteiro commented May 7, 2025

susmonteiro commented Apr 23, 2025 •

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