On 15/02/2023 15:10, Jonny Grant wrote:
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.
No, __builtin_printf is really just an internal hook that is used to
handle optimisations of printf - if you look at your assembly output
you'll see it is translated back to printf for the C library to handle.
- 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.
That's because __func__, __FUNCTION__ and __PRETTY_FUNCTION__ are not
string literals but implicitly declared identifies containing a constant
string literal as the initializer - you can't therefore combine them
with string literals by simple concatenation. See
https://gcc.gnu.org/onlinedocs/gcc/Function-Names.html
which clearly states this.
R.
https://godbolt.org/z/xso9soWaf
Regards
Jonny
