Re: Could __builtin_printf parameters be optimized when being compiled

[Jonny Grant <jg@jguk.org>]

Thank you for your quick reply Richard.

On 15/02/2023 14:30, Richard Earnshaw wrote:
> 
> 
> On 15/02/2023 14:18, Jonny Grant wrote:
>> Hi
>> Has GCC considered an improvement to "compile out" from the builtin printf the strings? That being to change it to just be something like puts("file /app/example.cpp:4")
>> I had a look, but couldn't find it being asked before.
>>
>> This is just a short example to demonstrate.
>> It would be useful to see the exact string in the debugger "file /app/example.cpp:4", also it saves a few lines of asm.
>>
>> https://godbolt.org/z/aKz3o6aPd
>>
>>
>> int main()
>> {
>>      __builtin_printf("file %s:%d", __FILE__, __LINE__);
>> }
>>
>>
>> .LC0:
>>          .string "/app/example.cpp"
>> .LC1:
>>          .string "file %s:%d"
>> main:
>>          subq    $8, %rsp
>>          movl    $4, %edx
>>          movl    $.LC0, %esi
>>          xorl    %eax, %eax
>>          movl    $.LC1, %edi
>>          call    printf
>>          xorl    %eax, %eax
>>          addq    $8, %rsp
>>          ret
> 
> We already do when the printf contains simply the format string and no additional arguments.
> 
> I guess it might be possible to handle cases where all the arguments are constant, but even that has its problems, eg:
> 
> - can we guarantee identical output to the platform printf?

That's a good question. I was using __builtin_printf so that should be GCC I expect.

> - does it cause string bloat (what if there were 30 or so such statements in your program all identical except for the line number)?

That's probably what I am expecting, to see those 30 different formatted strings.

> - does it even happen often enough to be worth adding (and maintaining) support?  Nothing comes for free in a compiler and the optimisations have to be worth-while in the real world.
> 
> R.

You're completely right, it could bloat the file with strings.

I can do some with multi-line literals, to get "file /app/example.cpp compiled Feb 15 2023"
__FILE__ and __DATE__ worked ok.

but it didn't like me putting __PRETTY_FUNCTION__ in the middle. Maybe I'm missing something obvious. Likewise I can't use __builtin_LINE() as that is a function rather than a string. Maybe __PRETTY_FUNCTION__ and __FUNCTION__ are calls to  __builtin_FUNCTION().

int main()
{
    const char * s = "file " \
        __FILE__ \
        " compiled " \
        __DATE__ \
        "\n";

    __builtin_printf("%s", s);
    __builtin_printf("%s\n", __PRETTY_FUNCTION__);  // didn't work when I put in the middle.


https://godbolt.org/z/xso9soWaf

Regards
Jonny