yronglin added a comment. In D150528#4343102 <https://reviews.llvm.org/D150528#4343102>, @aaron.ballman wrote:
> In D150528#4342646 <https://reviews.llvm.org/D150528#4342646>, @yronglin > wrote: > >> In D150528#4342600 <https://reviews.llvm.org/D150528#4342600>, >> @aaron.ballman wrote: >> >>> In D150528#4342535 <https://reviews.llvm.org/D150528#4342535>, @yronglin >>> wrote: >>> >>>> Thanks a lot for your review @aaron.ballman ! BTW, does the most good >>>> approach is that we just add a new `ParsedAttribute` with `TypeSourceInfo` >>>> but not an `UnaryExprOrTypeTraitExpr`, and only do a sema check same as >>>> `alignof(type-id)`? >>> >>> Because you've already got a `ParsedAttr`, I think you'd use this >>> constructor: >>> https://github.com/llvm/llvm-project/blob/main/clang/include/clang/Sema/ParsedAttr.h#L274 >>> and move the call to `GetTypeFromParser()` until later, but it should >>> still be modelled as a `UnaryExprOrTypeTraitExpr`. >> >> Many thanks for the tips, I think the next step is to build >> `UnaryExprOrTypeTraitExpr ` in this place >> (https://github.com/llvm/llvm-project/blob/9715af434579022b5ef31781be40b722d7e63bee/clang/lib/Sema/SemaDeclAttr.cpp#L4335-L4358) >> and then calculate the alignment? If this approach is correct, I'm glad to >> investigate it. > > Hmm, not quite. Roping in @erichkeane in case he's got other ideas, but I > think this will be more involved. What I think needs to happen is: > > - We need to add another argument to `Aligned` in Attr.td so that the > attribute takes either an expression argument or a type argument. > - We need to update SemaDeclAttr.cpp and other places accordingly > - We need to add an overload of `Sema::AddAlignedAttr()` that takes a > `ParsedType` and call this overload when given `alignas(type)` > - We need to add code to TreeTransform.h to instantiate a dependent type so > we can support `template <typename Ty> alignas(Ty) int foo = 12;` > > Your approach of creating a `UnaryExprOrTypeTraitExpr` would still end up > having us model as `alignas(alignof(type))` because the argument to the > `alignas` attribute would be the type trait. Sorry for the late reply and thanks for your tips! @aaron.ballman I followed your advice to update this patch, and below AST generated by clang: class alignas(void *) Foo {}; _Alignas(void*) char align32array[128]; template <typename T> class alignas(T) Bar {}; template <unsigned N> class alignas(N) FooBar {}; int main() { Bar<int> bar; FooBar<4> fooBar; return 0; } ➜ test ../rel/bin/clang++ -cc1 -triple x86_64-pc-linux -ast-dump ./alignas.cpp -std=c++11 alignas _Alignas alignas alignasTranslationUnitDecl 0x14b043208 <<invalid sloc>> <invalid sloc> |-TypedefDecl 0x14b043a70 <<invalid sloc>> <invalid sloc> implicit __int128_t '__int128' | `-BuiltinType 0x14b0437d0 '__int128' |-TypedefDecl 0x14b043ae0 <<invalid sloc>> <invalid sloc> implicit __uint128_t 'unsigned __int128' | `-BuiltinType 0x14b0437f0 'unsigned __int128' |-TypedefDecl 0x14b043e58 <<invalid sloc>> <invalid sloc> implicit __NSConstantString '__NSConstantString_tag' | `-RecordType 0x14b043bd0 '__NSConstantString_tag' | `-CXXRecord 0x14b043b38 '__NSConstantString_tag' |-TypedefDecl 0x14b043ef0 <<invalid sloc>> <invalid sloc> implicit __builtin_ms_va_list 'char *' | `-PointerType 0x14b043eb0 'char *' | `-BuiltinType 0x14b0432b0 'char' |-TypedefDecl 0x14b087858 <<invalid sloc>> <invalid sloc> implicit __builtin_va_list '__va_list_tag[1]' | `-ConstantArrayType 0x14b087800 '__va_list_tag[1]' 1 | `-RecordType 0x14b043fe0 '__va_list_tag' | `-CXXRecord 0x14b043f48 '__va_list_tag' |-CXXRecordDecl 0x14b0878e8 <./alignas.cpp:1:1, col:28> col:23 class Foo definition | |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init | | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr | | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param | | |-MoveConstructor exists simple trivial needs_implicit | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param | | |-MoveAssignment exists simple trivial needs_implicit | | `-Destructor simple irrelevant trivial needs_implicit | |-AlignedAttr 0x14b087a08 <col:7> alignas 'void *' | `-CXXRecordDecl 0x14b087a68 <col:1, col:23> col:23 implicit class Foo |-VarDecl 0x14b087be8 <line:2:1, col:38> col:22 align32array 'char[128]' | `-AlignedAttr 0x14b087c50 <col:1> _Alignas 'void *' |-ClassTemplateDecl 0x14b087e80 <line:4:1, col:45> col:40 Bar | |-TemplateTypeParmDecl 0x14b087cf8 <col:11, col:20> col:20 referenced typename depth 0 index 0 T | |-CXXRecordDecl 0x14b087df0 <col:23, col:45> col:40 class Bar definition | | |-DefinitionData empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init | | | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr | | | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param | | | |-MoveConstructor exists simple trivial needs_implicit | | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param | | | |-MoveAssignment exists simple trivial needs_implicit | | | `-Destructor simple irrelevant trivial needs_implicit | | |-AlignedAttr 0x14b0880d0 <col:29> alignas 'T' | | `-CXXRecordDecl 0x14b088130 <col:23, col:40> col:40 implicit class Bar | `-ClassTemplateSpecializationDecl 0x14b09e200 <col:1, col:45> col:40 class Bar definition | |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init | | |-DefaultConstructor exists trivial constexpr defaulted_is_constexpr | | |-CopyConstructor simple trivial has_const_param implicit_has_const_param | | |-MoveConstructor exists simple trivial | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param | | |-MoveAssignment exists simple trivial needs_implicit | | `-Destructor simple irrelevant trivial needs_implicit | |-TemplateArgument type 'int' | | `-BuiltinType 0x14b043310 'int' | |-AlignedAttr 0x14b09e458 <col:29> alignas 'int':'int' | |-CXXRecordDecl 0x14b09e520 <col:23, col:40> col:40 implicit class Bar | |-CXXConstructorDecl 0x14b09e5e0 <col:40> col:40 implicit used constexpr Bar 'void () noexcept' inline default trivial | | `-CompoundStmt 0x14b09eb48 <col:40> | |-CXXConstructorDecl 0x14b09e778 <col:40> col:40 implicit constexpr Bar 'void (const Bar<int> &)' inline default trivial noexcept-unevaluated 0x14b09e778 | | `-ParmVarDecl 0x14b09e898 <col:40> col:40 'const Bar<int> &' | `-CXXConstructorDecl 0x14b09e978 <col:40> col:40 implicit constexpr Bar 'void (Bar<int> &&)' inline default trivial noexcept-unevaluated 0x14b09e978 | `-ParmVarDecl 0x14b09ea98 <col:40> col:40 'Bar<int> &&' |-ClassTemplateDecl 0x14b088310 <line:5:1, col:48> col:40 FooBar | |-NonTypeTemplateParmDecl 0x14b0881f0 <col:11, col:20> col:20 referenced 'unsigned int' depth 0 index 0 N | |-CXXRecordDecl 0x14b088280 <col:23, col:48> col:40 class FooBar definition | | |-DefinitionData empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init | | | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr | | | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param | | | |-MoveConstructor exists simple trivial needs_implicit | | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param | | | |-MoveAssignment exists simple trivial needs_implicit | | | `-Destructor simple irrelevant trivial needs_implicit | | |-AlignedAttr 0x14b088580 <col:29> alignas | | | `-DeclRefExpr 0x14b088260 <col:37> 'unsigned int' NonTypeTemplateParm 0x14b0881f0 'N' 'unsigned int' | | `-CXXRecordDecl 0x14b0885e0 <col:23, col:40> col:40 implicit class FooBar | `-ClassTemplateSpecializationDecl 0x14b09ed10 <col:1, col:48> col:40 class FooBar definition | |-DefinitionData pass_in_registers empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init | | |-DefaultConstructor exists trivial constexpr defaulted_is_constexpr | | |-CopyConstructor simple trivial has_const_param implicit_has_const_param | | |-MoveConstructor exists simple trivial | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param | | |-MoveAssignment exists simple trivial needs_implicit | | `-Destructor simple irrelevant trivial needs_implicit | |-TemplateArgument integral 4 | |-AlignedAttr 0x14b09ef90 <col:29> alignas | | `-ConstantExpr 0x14b09ef70 <col:37> 'unsigned int' | | |-value: Int 4 | | `-SubstNonTypeTemplateParmExpr 0x14b09ef48 <col:37> 'unsigned int' | | |-NonTypeTemplateParmDecl 0x14b0881f0 <col:11, col:20> col:20 referenced 'unsigned int' depth 0 index 0 N | | `-IntegerLiteral 0x14b09ef28 <col:37> 'unsigned int' 4 | |-CXXRecordDecl 0x14b09f058 <col:23, col:40> col:40 implicit class FooBar | |-CXXConstructorDecl 0x14980fc00 <col:40> col:40 implicit used constexpr FooBar 'void () noexcept' inline default trivial | | `-CompoundStmt 0x149810108 <col:40> | |-CXXConstructorDecl 0x14980fd68 <col:40> col:40 implicit constexpr FooBar 'void (const FooBar<4> &)' inline default trivial noexcept-unevaluated 0x14980fd68 | | `-ParmVarDecl 0x14980fe88 <col:40> col:40 'const FooBar<4> &' | `-CXXConstructorDecl 0x14980ff68 <col:40> col:40 implicit constexpr FooBar 'void (FooBar<4> &&)' inline default trivial noexcept-unevaluated 0x14980ff68 | `-ParmVarDecl 0x149810088 <col:40> col:40 'FooBar<4> &&' `-FunctionDecl 0x14b0886e0 <line:7:1, line:11:1> line:7:5 main 'int ()' `-CompoundStmt 0x1498101e8 <col:12, line:11:1> |-DeclStmt 0x14b09eca0 <line:8:3, col:15> | `-VarDecl 0x14b09e3b0 <col:3, col:12> col:12 bar 'Bar<int>':'Bar<int>' callinit | `-CXXConstructExpr 0x14b09ec78 <col:12> 'Bar<int>':'Bar<int>' 'void () noexcept' |-DeclStmt 0x1498101a0 <line:9:3, col:19> | `-VarDecl 0x14b09eec0 <col:3, col:13> col:13 fooBar 'FooBar<4>':'FooBar<4>' callinit | `-CXXConstructExpr 0x149810178 <col:13> 'FooBar<4>':'FooBar<4>' 'void () noexcept' `-ReturnStmt 0x1498101d8 <line:10:3, col:10> `-IntegerLiteral 0x1498101b8 <col:10> 'int' 0 This AST dose not have a `UnaryExprOrTypeTraitExpr` in `alignas`, What do you think? 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