Re: [PATCH] Use __builtin_is_constant_evaluated in std::less etc. (PR tree-optimization/88775)

[Jonathan Wakely <jwakely at redhat dot com>]

On 10/01/19 10:02 +0100, Jakub Jelinek wrote:
> Hi!
>
> In Marc's testcase, we generate terrible code for std::string assignment,
> because the __builtin_constant_p is kept in the IL for way too long and the
> optimizers (jump threading?) create way too many copies of the
> memcpy/memmove calls that it is then hard to bring it back in sanitity.
> On the testcase in the PR, GCC 7 emits on x86_64 with -O2 99 bytes long
> function, GCC 9 unpatched 259 bytes long, with this patch it emits
> 139 bytes long, better but still not as good as before.  I guess we'll need
> to improve GIMPLE optimizers too, but having twice as small IL for these
> heavily used operators where e.g. _M_disjunct uses two of them and we wind
> up with twice as many branches because of that is IMHO very useful.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 1) I'm not really sure about proper formatting in libstdc++, I thought you
>   don't use space before ( in function calls, but then why is there a space
>   in __builtin_constant_p?

I guess I probably copy&pasted it from something you gave me :-)

The space shouldn't be there.

> 2) not really sure about that #if __cplusplus >= 201402L either, I think we
>   don't really want to use __builtin_is_constant_evaluated at least in
>   C++98 code, but even in C++11, if the operator isn't constexpr, is there
>   any point trying to help it do the right thing in constexpr contexts?

I think there's no point, so only doing it for C++14 and later looks
OK to me.

OK for trunk, thanks.