Re: Could __builtin_printf parameters be optimized when being compiled

[Richard Earnshaw <Richard.Earnshaw@foss.arm.com>]

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.