| Message ID | 20220724043902.1777378-2-tom@honermann.net |
|---|---|
| State | New |
| Headers | show |
| Series | c++/106423: Fix pragma suppression of -Wc++20-compat diagnostics | expand |
On Sun, 24 Jul 2022, Tom Honermann via Gcc-patches wrote: > Gcc's '#pragma GCC diagnostic' directives are processed in "early mode" > (see handle_pragma_diagnostic_early) for the C++ frontend and, as such, > require that the target diagnostic option be enabled for the preprocessor > (see c_option_is_from_cpp_diagnostics). This change modifies the > -Wc++20-compat option definition to register it as a preprocessor option > so that its associated diagnostics can be suppressed. The changes also There are lots of C++ warning options, all of which should support pragma suppression regardless of whether they are relevant to the preprocessor or not. Do they all need this kind of handling, or is it only -Wc++20-compat that has some kind of problem?
On 7/27/22 7:09 PM, Joseph Myers wrote: > On Sun, 24 Jul 2022, Tom Honermann via Gcc-patches wrote: > >> Gcc's '#pragma GCC diagnostic' directives are processed in "early mode" >> (see handle_pragma_diagnostic_early) for the C++ frontend and, as such, >> require that the target diagnostic option be enabled for the preprocessor >> (see c_option_is_from_cpp_diagnostics). This change modifies the >> -Wc++20-compat option definition to register it as a preprocessor option >> so that its associated diagnostics can be suppressed. The changes also > There are lots of C++ warning options, all of which should support pragma > suppression regardless of whether they are relevant to the preprocessor or > not. Do they all need this kind of handling, or is it only -Wc++20-compat > that has some kind of problem? I had only checked -Wc++20-compat when working on the patch. I did some spot checking now and confirmed that suppression works as expected for C++ for at least the following warnings: -Wuninitialized -Warray-compare -Wbool-compare -Wtautological-compare -Wterminate I don't know the diagnostic framework well. As best I can tell, this issue is specific to the -Wc++20-compat option and when the particular diagnostic is issued (e.g., during lexing as opposed to during parsing). The following call chains appear to be relevant. cp_lexer_new_main -> cp_lexer_handle_early_pragma -> c_invoke_early_pragma_handler cp_parser_* -> cp_parser_pragma -> c_invoke_pragma_handler (where * might be "declaration", "toplevel_declaration", "class_head", "objc_interstitial_code", ...) The -Wc++20-compat enabled warning regarding new keywords in C++20 is issued from cp_lexer_get_preprocessor_token. Tom.
On Sat, Jul 30, 2022 at 7:06 PM Tom Honermann via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > On 7/27/22 7:09 PM, Joseph Myers wrote: > > On Sun, 24 Jul 2022, Tom Honermann via Gcc-patches wrote: > > > >> Gcc's '#pragma GCC diagnostic' directives are processed in "early mode" > >> (see handle_pragma_diagnostic_early) for the C++ frontend and, as such, > >> require that the target diagnostic option be enabled for the preprocessor > >> (see c_option_is_from_cpp_diagnostics). This change modifies the > >> -Wc++20-compat option definition to register it as a preprocessor option > >> so that its associated diagnostics can be suppressed. The changes also > > There are lots of C++ warning options, all of which should support pragma > > suppression regardless of whether they are relevant to the preprocessor or > > not. Do they all need this kind of handling, or is it only -Wc++20-compat > > that has some kind of problem? > > I had only checked -Wc++20-compat when working on the patch. > > I did some spot checking now and confirmed that suppression works as > expected for C++ for at least the following warnings: > -Wuninitialized > -Warray-compare > -Wbool-compare > -Wtautological-compare > -Wterminate > > I don't know the diagnostic framework well. As best I can tell, this > issue is specific to the -Wc++20-compat option and when the particular > diagnostic is issued (e.g., during lexing as opposed to during parsing). > The following call chains appear to be relevant. > cp_lexer_new_main -> cp_lexer_handle_early_pragma -> > c_invoke_early_pragma_handler > cp_parser_* -> cp_parser_pragma -> c_invoke_pragma_handler > (where * might be "declaration", "toplevel_declaration", > "class_head", "objc_interstitial_code", ...) > > The -Wc++20-compat enabled warning regarding new keywords in C++20 is > issued from cp_lexer_get_preprocessor_token. > > Tom. > I have been working on improving the handling of "#pragma GCC diagnostic" lately. The behavior for C++ changed since r13-1544 (https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=e46f4d7430c5210465791603735ab219ef263c51). I have some more comments about the patch's approach on the PR (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431#c44). "#pragma GCC diagnostic" formerly did not work in C++ at all, for diagnostics generated by libcpp, because C++ obtains all the tokens from libcpp first (including deferred pragmas), and then processes them afterward, too late to take effect for diagnostics that libcpp has already emitted. r13-1544 fixed this up by adding an early pragma handler, which runs as soon as a deferred pragma token is seen and handles diagnostic pragmas if they pertain to libcpp-controlled diagnostics. Non-libcpp diagnostics still need to be handled later, during parsing, or else they get processed too early and it leads to other problems. Basically, now each diagnostic pragma is handled as close in time as possible to the time the associated diagnostics might be generated. The early pragma handler determines that an option comes from libcpp, and so should be subject to early processing, if it was marked as such in the options definition file. Tom's patch points out that -Wc++20-compat needs to be handled early, and so marking it as a libcpp diagnostic in c-family/c.opt arranges for that to work as intended. Now one potential objection here is that -Wc++20-compat warnings are not technically generated by libcpp. They are generated by the C++ frontend immediately after lexing an identifier token from libcpp (cp_lexer_get_preprocessor_token()). But the distinction between these two steps is rather blurry and it seems logical to me, to denote this as a libcpp-related option. Also, the same is already done for -Wc++11-compat. Otherwise, we would need to add some new option property to indicate which ones need to be handled for pragmas at lexing time rather than parsing time. At the moment I don't see any other diagnostics issued from cp_lexer_get_preprocessor_token() that would need similar adjustments. Assuming the approach is OK, it might be nice to add a comment to that function, indicating that any diagnostics emitted there should be annotated as libcpp options in the .opt file? -Lewis
On 7/31/22 11:05 AM, Lewis Hyatt wrote: > On Sat, Jul 30, 2022 at 7:06 PM Tom Honermann via Gcc-patches > <gcc-patches@gcc.gnu.org> wrote: >> On 7/27/22 7:09 PM, Joseph Myers wrote: >>> On Sun, 24 Jul 2022, Tom Honermann via Gcc-patches wrote: >>> >>>> Gcc's '#pragma GCC diagnostic' directives are processed in "early mode" >>>> (see handle_pragma_diagnostic_early) for the C++ frontend and, as such, >>>> require that the target diagnostic option be enabled for the preprocessor >>>> (see c_option_is_from_cpp_diagnostics). This change modifies the >>>> -Wc++20-compat option definition to register it as a preprocessor option >>>> so that its associated diagnostics can be suppressed. The changes also >>> There are lots of C++ warning options, all of which should support pragma >>> suppression regardless of whether they are relevant to the preprocessor or >>> not. Do they all need this kind of handling, or is it only -Wc++20-compat >>> that has some kind of problem? >> I had only checked -Wc++20-compat when working on the patch. >> >> I did some spot checking now and confirmed that suppression works as >> expected for C++ for at least the following warnings: >> -Wuninitialized >> -Warray-compare >> -Wbool-compare >> -Wtautological-compare >> -Wterminate >> >> I don't know the diagnostic framework well. As best I can tell, this >> issue is specific to the -Wc++20-compat option and when the particular >> diagnostic is issued (e.g., during lexing as opposed to during parsing). >> The following call chains appear to be relevant. >> cp_lexer_new_main -> cp_lexer_handle_early_pragma -> >> c_invoke_early_pragma_handler >> cp_parser_* -> cp_parser_pragma -> c_invoke_pragma_handler >> (where * might be "declaration", "toplevel_declaration", >> "class_head", "objc_interstitial_code", ...) >> >> The -Wc++20-compat enabled warning regarding new keywords in C++20 is >> issued from cp_lexer_get_preprocessor_token. >> >> Tom. >> > I have been working on improving the handling of "#pragma GCC > diagnostic" lately. The behavior for C++ changed since r13-1544 > (https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=e46f4d7430c5210465791603735ab219ef263c51). > I have some more comments about the patch's approach on the PR > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431#c44). > > "#pragma GCC diagnostic" formerly did not work in C++ at all, for > diagnostics generated by libcpp, because C++ obtains all the tokens > from libcpp first (including deferred pragmas), and then processes > them afterward, too late to take effect for diagnostics that libcpp > has already emitted. r13-1544 fixed this up by adding an early pragma > handler, which runs as soon as a deferred pragma token is seen and > handles diagnostic pragmas if they pertain to libcpp-controlled > diagnostics. Non-libcpp diagnostics still need to be handled later, > during parsing, or else they get processed too early and it leads to > other problems. Basically, now each diagnostic pragma is handled as > close in time as possible to the time the associated diagnostics might > be generated. > > The early pragma handler determines that an option comes from libcpp, > and so should be subject to early processing, if it was marked as such > in the options definition file. Tom's patch points out that > -Wc++20-compat needs to be handled early, and so marking it as a > libcpp diagnostic in c-family/c.opt arranges for that to work as > intended. Now one potential objection here is that -Wc++20-compat > warnings are not technically generated by libcpp. They are generated > by the C++ frontend immediately after lexing an identifier token from > libcpp (cp_lexer_get_preprocessor_token()). But the distinction > between these two steps is rather blurry and it seems logical to me, > to denote this as a libcpp-related option. Also, the same is already > done for -Wc++11-compat. Otherwise, we would need to add some new > option property to indicate which ones need to be handled for pragmas > at lexing time rather than parsing time. > > At the moment I don't see any other diagnostics issued from > cp_lexer_get_preprocessor_token() that would need similar adjustments. > Assuming the approach is OK, it might be nice to add a comment to that > function, indicating that any diagnostics emitted there should be > annotated as libcpp options in the .opt file? Thank you for those details; I wasn't aware of that history. If I'm interpreting your response correctly, it sounds like you agree with the direction of the patch. If you like, I can add a comment as you suggested and re-post the patch. Perhaps: diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc index 4f67441eeb1..c3584446827 100644 --- a/gcc/cp/parser.cc +++ b/gcc/cp/parser.cc @@ -924,7 +924,10 @@cp_lexer_saving_tokens (const cp_lexer* lexer) /* Store the next token from the preprocessor in *TOKEN. Return true if we reach EOF. If LEXER is NULL, assume we are handling an initial #pragma pch_preprocess, and thus want the lexer to return - processed strings. */ + processed strings. + + Diagnostics issued from this function must have their controlling option (if + any) in c.opt annotated as a libcpp option via the CppReason property. */ static void cp_lexer_get_preprocessor_token (unsigned flags, cp_token *token) Tom. > -Lewis
diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc index b9f01a65ed7..1ea37ba9742 100644 --- a/gcc/c-family/c-opts.cc +++ b/gcc/c-family/c-opts.cc @@ -1046,6 +1046,13 @@ c_common_post_options (const char **pfilename) else if (warn_narrowing == -1) warn_narrowing = 0; + if (cxx_dialect >= cxx20) + { + /* Don't warn about C++20 compatibility changes in C++20 or later. */ + warn_cxx20_compat = 0; + cpp_opts->cpp_warn_cxx20_compat = 0; + } + /* C++17 has stricter evaluation order requirements; let's use some of them for earlier C++ as well, so chaining works as expected. */ if (c_dialect_cxx () diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt index 44e1a60ce24..dfdebd596ef 100644 --- a/gcc/c-family/c.opt +++ b/gcc/c-family/c.opt @@ -455,7 +455,7 @@ Wc++2a-compat C++ ObjC++ Warning Alias(Wc++20-compat) Undocumented Wc++20-compat -C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ ObjC++,Wall) +C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ ObjC++,Wall) Init(0) CPP(cpp_warn_cxx20_compat) CppReason(CPP_W_CXX20_COMPAT) Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Wc++11-extensions diff --git a/gcc/testsuite/g++.dg/cpp0x/keywords2.C b/gcc/testsuite/g++.dg/cpp0x/keywords2.C new file mode 100644 index 00000000000..d67d01e31ed --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/keywords2.C @@ -0,0 +1,16 @@ +// { dg-do compile { target c++98_only } } +// { dg-options "-Wc++11-compat" } + +// Validate suppression of -Wc++11-compat diagnostics. +#pragma GCC diagnostic ignored "-Wc++11-compat" +int alignof; +int alignas; +int constexpr; +int decltype; +int noexcept; +int nullptr; +int static_assert; +int thread_local; +int _Alignas; +int _Alignof; +int _Thread_local; diff --git a/gcc/testsuite/g++.dg/cpp2a/keywords2.C b/gcc/testsuite/g++.dg/cpp2a/keywords2.C new file mode 100644 index 00000000000..8714a7b26b7 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/keywords2.C @@ -0,0 +1,13 @@ +// { dg-do compile { target c++17_down } } +// { dg-options "-Wc++20-compat" } + +// Validate suppression of -Wc++20-compat diagnostics. +#pragma GCC diagnostic ignored "-Wc++20-compat" +int constinit; +int consteval; +int requires; +int concept; +int co_await; +int co_yield; +int co_return; +int char8_t; diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h index 3eba6f74b57..9d90c18e4f2 100644 --- a/libcpp/include/cpplib.h +++ b/libcpp/include/cpplib.h @@ -547,6 +547,9 @@ struct cpp_options /* True if warn about differences between C++98 and C++11. */ bool cpp_warn_cxx11_compat; + /* True if warn about differences between C++17 and C++20. */ + bool cpp_warn_cxx20_compat; + /* Nonzero if bidirectional control characters checking is on. See enum cpp_bidirectional_level. */ unsigned char cpp_warn_bidirectional; @@ -655,6 +658,7 @@ enum cpp_warning_reason { CPP_W_C90_C99_COMPAT, CPP_W_C11_C2X_COMPAT, CPP_W_CXX11_COMPAT, + CPP_W_CXX20_COMPAT, CPP_W_EXPANSION_TO_DEFINED, CPP_W_BIDIRECTIONAL }; diff --git a/libcpp/init.cc b/libcpp/init.cc index f4ab83d2145..cca3c1dc1e7 100644 --- a/libcpp/init.cc +++ b/libcpp/init.cc @@ -202,6 +202,7 @@ cpp_create_reader (enum c_lang lang, cpp_hash_table *table, CPP_OPTION (pfile, cpp_warn_c90_c99_compat) = -1; CPP_OPTION (pfile, cpp_warn_c11_c2x_compat) = -1; CPP_OPTION (pfile, cpp_warn_cxx11_compat) = 0; + CPP_OPTION (pfile, cpp_warn_cxx20_compat) = 0; CPP_OPTION (pfile, cpp_warn_deprecated) = 1; CPP_OPTION (pfile, cpp_warn_long_long) = 0; CPP_OPTION (pfile, dollars_in_ident) = 1;