[Bug c++/58005] New: missed optimization printf constant string

[Bug c++/58005] New: missed optimization printf constant string

[dushistov at mail dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58005

            Bug ID: 58005
           Summary: missed optimization printf constant string
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dushistov at mail dot ru

Simple code:

#include <cstdio>

int main()
{
    printf("%s: test1\n", __PRETTY_FUNCTION__);//1
    printf("test2\n");//2
    return 0;
}

compiled to:

callq  4005a0 <__printf_chk@plt> (1)
and to
callq  400590 <puts@plt> for (2)

I think that, because of __PRETTY_FUNCTION__ is known during compile time, it
is also possible converting (1) to "puts" call.

This optimization can help speedup loging functionality.

[Bug tree-optimization/58005] missed optimization printf constant string

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58005

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org
          Component|c++                         |tree-optimization

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Something for Jakub, I think.

[Bug tree-optimization/58005] missed optimization printf constant string

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58005

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-07-27
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed also happens in C (s/cstdio/stdio.h/).

[Bug tree-optimization/58005] missed optimization printf constant string

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58005

--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
Such an optimization can increase code size (well, the total size of 
string constants in the program) if the same format string is used with 
many different arguments, so it may not always be a good idea (at least 
with -Os).

[Bug tree-optimization/58005] missed optimization printf constant string

[dushistov at mail dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58005

--- Comment #4 from Evgeniy Dushistov <dushistov at mail dot ru> ---
>Such an optimization can increase code size
>if the same format string is used with 
>many different arguments,

may be then two fputs calls?

fputs(__PRETTY_FUNCTION__, stdout);
fputs("%s: test1\n" + 2/*skip format*/, stdout);

yeah, still we have two calls vs one(bad for -Os),
but we not introduce new string constants,
so it is suitable optimization for -Ofast.

In such test:
       for (int i = 0; i < 100000; ++i) {
        #ifdef OPTIMIZATION
                fputs(__PRETTY_FUNCTION__, stdout);
                fputs("%s: test1\n" + 2, stdout);
        #else
                printf("%s: test1\n", __PRETTY_FUNCTION__);
        #endif
        }

fputs win with 
real    0m0.005s
user    0m0.000s
sys     0m0.000s

vs printf
real    0m0.011s
user    0m0.010s
sys     0m0.000s

[Bug tree-optimization/58005] missed optimization printf constant string

[daniel.santos at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58005

Daniel Santos <daniel.santos at pobox dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.santos at pobox dot com

--- Comment #5 from Daniel Santos <daniel.santos at pobox dot com> ---
(In reply to joseph@codesourcery.com from comment #3)
> Such an optimization can increase code size (well, the total size of 
> string constants in the program) if the same format string is used with 
> many different arguments, so it may not always be a good idea (at least 
> with -Os).

Yeah, I agree.  Such a subtle optimization would be bad for many people who
intentionally reuse a format string to reduce the data size and then only to
have that thwarted by the compiler.  Typically, printfs don't reside in
performance sensitive sections of code.