From 6b633c4b14850df376d5cec571699018772f358e Mon Sep 17 00:00:00 2001 From: Tomas Popela Date: Tue, 17 Sep 2019 19:48:48 +0000 Subject: [PATCH] GCC: Can't use alignas() together with __attribute__() It's because GCC has problems when mixing the alignas() together with __attribute__() (that is used to export the symbols). The best solution is to use ALIGNAS() macro from //base/compiler_specific.h together with alignof() to have the equal functionality that compiles on GCC as well as on clang. Bug: 819294 Change-Id: Ieb169592a2965f17a18bfc88d28418eb723a4e5a Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1806735 Auto-Submit: Tom Popela Commit-Queue: Alex Clarke Reviewed-by: Alex Clarke Cr-Commit-Position: refs/heads/master@{#697330} --- diff --git a/base/task/promise/dependent_list.h b/base/task/promise/dependent_list.h index 020bdbfc..3245c1c 100644 --- a/base/task/promise/dependent_list.h +++ b/base/task/promise/dependent_list.h @@ -59,7 +59,7 @@ // Align Node on an 8-byte boundary to ensure the first 3 bits are 0 and can // be used to store additional state (see static_asserts below). - class BASE_EXPORT alignas(8) Node { + class BASE_EXPORT ALIGNAS(8) Node { public: Node(); explicit Node(Node&& other) noexcept; Re-use chromium alignas workaround in protobuf. --- diff --git a/third_party/protobuf/src/google/protobuf/port_def.inc b/third_party/protobuf/src/google/protobuf/port_def.inc index f1bd85d..9c204a1 100644 --- a/third_party/protobuf/src/google/protobuf/port_def.inc +++ b/third_party/protobuf/src/google/protobuf/port_def.inc @@ -528,6 +528,35 @@ PROTOBUF_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport)); #undef IN #endif // _MSC_VER +// Specify memory alignment for structs, classes, etc. +// Use like: +// class PROTOBUF_ALIGNAS(16) MyClass { ... } +// PROTOBUF_ALIGNAS(16) int array[4]; +// +// In most places you can use the C++11 keyword "alignas", which is preferred. +// +// But compilers have trouble mixing __attribute__((...)) syntax with +// alignas(...) syntax. +// +// Doesn't work in clang or gcc: +// struct alignas(16) __attribute__((packed)) S { char c; }; +// Works in clang but not gcc: +// struct __attribute__((packed)) alignas(16) S2 { char c; }; +// Works in clang and gcc: +// struct alignas(16) S3 { char c; } __attribute__((packed)); +// +// There are also some attributes that must be specified *before* a class +// definition: visibility (used for exporting functions/classes) is one of +// these attributes. This means that it is not possible to use alignas() with a +// class that is marked as exported. +#if defined(_MSC_VER) +#define PROTOBUF_ALIGNAS(byte_alignment) __declspec(align(byte_alignment)) +#elif defined(__GNUC__) +#define PROTOBUF_ALIGNAS(byte_alignment) __attribute__((aligned(byte_alignment))) +#else +#define PROTOBUF_ALIGNAS(byte_alignment) alignas(byte_alignment) +#endif + #if defined(__clang__) #pragma clang diagnostic push // TODO(gerbens) ideally we cleanup the code. But a cursory try shows many diff --git a/third_party/protobuf/src/google/protobuf/arena.h b/third_party/protobuf/src/google/protobuf/arena.h index dedc221..a8515ce 100644 --- a/third_party/protobuf/src/google/protobuf/arena.h +++ b/third_party/protobuf/src/google/protobuf/arena.h @@ -245,7 +245,7 @@ struct ArenaOptions { // well as protobuf container types like RepeatedPtrField and Map. The protocol // is internal to protobuf and is not guaranteed to be stable. Non-proto types // should not rely on this protocol. -class PROTOBUF_EXPORT alignas(8) Arena final { +class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8) Arena final { public: // Arena constructor taking custom options. See ArenaOptions below for // descriptions of the options available. diff --git a/third_party/protobuf/src/google/protobuf/port_def.inc b/third_party/protobuf/src/google/protobuf/port_def.inc index f1bd85d..6d02b53 100644 --- a/third_party/protobuf/src/google/protobuf/port_def.inc +++ b/third_party/protobuf/src/google/protobuf/port_def.inc @@ -528,6 +528,35 @@ PROTOBUF_EXPORT_TEMPLATE_TEST(DEFAULT, __declspec(dllimport)); #undef IN #endif // _MSC_VER +// Specify memory alignment for structs, classes, etc. +// Use like: +// class PROTOBUF_ALIGNAS(16) MyClass { ... } +// PROTOBUF_ALIGNAS(16) int array[4]; +// +// In most places you can use the C++11 keyword "alignas", which is preferred. +// +// But compilers have trouble mixing __attribute__((...)) syntax with +// alignas(...) syntax. +// +// Doesn't work in clang or gcc: +// struct alignas(16) __attribute__((packed)) S { char c; }; +// Works in clang but not gcc: +// struct __attribute__((packed)) alignas(16) S2 { char c; }; +// Works in clang and gcc: +// struct alignas(16) S3 { char c; } __attribute__((packed)); +// +// There are also some attributes that must be specified *before* a class +// definition: visibility (used for exporting functions/classes) is one of +// these attributes. This means that it is not possible to use alignas() with a +// class that is marked as exported. +#if defined(_MSC_VER) +#define PROTOBUF_ALIGNAS(byte_alignment) __declspec(align(byte_alignment)) +#elif defined(__GNUC__) +#define PROTOBUF_ALIGNAS(byte_alignment) __attribute__((aligned(byte_alignment))) +#else +#define PROTOBUF_ALIGNAS(byte_alignment) alignas(byte_alignment) +#endif + #if defined(__clang__) #pragma clang diagnostic push // TODO(gerbens) ideally we cleanup the code. But a cursory try shows many diff --git a/third_party/protobuf/src/google/protobuf/port_undef.inc b/third_party/protobuf/src/google/protobuf/port_undef.inc index b7e67fe..ba1fffc 100644 --- a/third_party/protobuf/src/google/protobuf/port_undef.inc +++ b/third_party/protobuf/src/google/protobuf/port_undef.inc @@ -80,6 +80,7 @@ #undef PROTOBUF_EXPORT_TEMPLATE_STYLE_MATCH_foj3FJo5StF0OvIzl7oMxA__declspec #undef PROTOBUF_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllexport #undef PROTOBUF_EXPORT_TEMPLATE_STYLE_MATCH_DECLSPEC_dllimport +#undef PROTOBUF_ALIGNAS