Re: Could __builtin_printf parameters be optimized when being compiled

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



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