On Thu, Nov 14, 2013 at 11:12 AM, Xinliang David Li <davi...@google.com> wrote: > On Thu, Nov 14, 2013 at 10:17 AM, Andrew Pinski <pins...@gmail.com> wrote: >> On Thu, Nov 14, 2013 at 8:25 AM, Xinliang David Li <davi...@google.com> >> wrote: >>> Can we revisit the decision for this? Here are the reasons: >>> >>> 1) It seems that the motivation to make C++ consistent with c99 is to >>> avoid confusing users who build the C source with both C and C++ >>> compilers. Why should C++'s default behavior be tuned for this niche >>> case? >> >> It is not a niche case. It is confusing for people who write C++ code >> to rewrite their code to C99 > > Compared with people who just work on C++ or C but do not worry about > rewrite nor cross language comparison? > >>and find that C is much slower because of >> correctness? I think they have this backwards here. C++ should be >> consistent with C here. > > Correctness by what definition?
See also http://gcc.gnu.org/ml/gcc-patches/2005-02/msg00568.html which is one of the libstdc++ maintainers opinion on this subject. > >> >>> 2) It is very confusing for users who see huge performance difference >>> between compiler generated code for Complex multiplication vs manually >>> expanded code >> >> I don't see why this is an issue if they understand how complex >> multiplication works for correctness. I am sorry but correctness over >> speed is a good argument of why this should stay this way. >> >>> 3) The default setting can also block potential vectorization >>> opportunities for complex operations >> >> Yes so again this is about a correctness issue over a speed issue. >> >>> 4) GCC is about the only compiler which has this default -- very few >>> user knows about GCC's strict default, and will think GCC performs >>> poorly. >> >> >> Correctness over speed is better. I am sorry GCC is the only one >> which gets it correct here. If people don't like there is a flag to >> disable it. > > You can say the same thing that people who find C is slower can use > the flag to disable it. > > thanks, > > David > >> >> Thanks, >> Andrew Pinski >> >>> >>> thanks, >>> >>> David >>> >>> >>> On Wed, Nov 13, 2013 at 9:07 PM, Andrew Pinski <pins...@gmail.com> wrote: >>>> On Wed, Nov 13, 2013 at 5:26 PM, Cong Hou <co...@google.com> wrote: >>>>> This patch is for PR58963. >>>>> >>>>> In the patch http://gcc.gnu.org/ml/gcc-patches/2005-02/msg00560.html, >>>>> the builtin function is used to perform complex multiplication and >>>>> division. This is to comply with C99 standard, but I am wondering if >>>>> C++ also needs this. >>>>> >>>>> There is no complex keyword in C++, and no content in C++ standard >>>>> about the behavior of operations on complex types. The <complex> >>>>> header file is all written in source code, including complex >>>>> multiplication and division. GCC should not do too much for them by >>>>> using builtin calls by default (although we can set -fcx-limited-range >>>>> to prevent GCC doing this), which has a big impact on performance >>>>> (there may exist vectorization opportunities). >>>>> >>>>> In this patch flag_complex_method will not be set to 2 for C++. >>>>> Bootstraped and tested on an x86-64 machine. >>>> >>>> I think you need to look into this issue deeper as the original patch >>>> only enabled it for C99: >>>> http://gcc.gnu.org/ml/gcc-patches/2005-02/msg01483.html . >>>> >>>> Just a little deeper will find >>>> http://gcc.gnu.org/ml/gcc/2007-07/msg00124.html which says yes C++ >>>> needs this. >>>> >>>> Thanks, >>>> Andrew Pinski >>>> >>>>> >>>>> >>>>> thanks, >>>>> Cong >>>>> >>>>> >>>>> Index: gcc/c-family/c-opts.c >>>>> =================================================================== >>>>> --- gcc/c-family/c-opts.c (revision 204712) >>>>> +++ gcc/c-family/c-opts.c (working copy) >>>>> @@ -198,8 +198,10 @@ c_common_init_options_struct (struct gcc >>>>> opts->x_warn_write_strings = c_dialect_cxx (); >>>>> opts->x_flag_warn_unused_result = true; >>>>> >>>>> - /* By default, C99-like requirements for complex multiply and divide. >>>>> */ >>>>> - opts->x_flag_complex_method = 2; >>>>> + /* By default, C99-like requirements for complex multiply and divide. >>>>> + But for C++ this should not be required. */ >>>>> + if (c_language != clk_cxx && c_language != clk_objcxx) >>>>> + opts->x_flag_complex_method = 2; >>>>> } >>>>> >>>>> /* Common initialization before calling option handlers. */ >>>>> Index: gcc/c-family/ChangeLog >>>>> =================================================================== >>>>> --- gcc/c-family/ChangeLog (revision 204712) >>>>> +++ gcc/c-family/ChangeLog (working copy) >>>>> @@ -1,3 +1,8 @@ >>>>> +2013-11-13 Cong Hou <co...@google.com> >>>>> + >>>>> + * c-opts.c (c_common_init_options_struct): Don't let C++ comply with >>>>> + C99-like requirements for complex multiply and divide. >>>>> + >>>>> 2013-11-12 Joseph Myers <jos...@codesourcery.com> >>>>> >>>>> * c-common.c (c_common_reswords): Add _Thread_local.
