Re: Checking for built-in functions from build systems

[Luca Bacci <luca.bacci982@gmail.com>]

Thank you very much, Jonathan. That's very helpful!

This test program:

int main()
{
  void *a = alloca(10);
  return 0;
}

Compiles fine with:
 gcc -o test test.c
But then fails with:
 gcc -o test test.c -fno-builtin

That's on an Arch Linux system (based on glibc)

The issue is that what happens when built-ins are not inlined is not well
defined. The libc may not implement the routine, or it's not clear which
dependency should provide it.

Thanks,
Luca Bacci

Il giorno mer 10 ago 2022 alle ore 23:27 Jonathan Wakely <
jwakely.gcc@gmail.com> ha scritto:

>
>
> On Wed, 10 Aug 2022, 23:12 Luca Bacci via Gcc, <gcc@gcc.gnu.org> wrote:
>
>>
>> 1. Is inlining of built-ins dependant only on the target architecture and
>> command-line arguments?
>>
>
> No, I think it can depend on the arguments to the built-in as well.
>
>
> 2. If the answer to 1 is yes, could a __is_builtin_inlined (func) macro be
>> added to GCC? It should tell whether func is going to be substituted
>> inline
>> for the given compiler invocation
>
>
> I don't think that can work for some built-ins, e.g. in a single
> translation unit memcmp can be inlined for constant arguments of small
> size, and not inlined for other calls.
>
>
>
>