C inlines that are also builtins.

[Marc Glisse]

On Tue, 10 Nov 2015, Sergey Organov wrote:

> Martin Sebor <msebor@gmail.com> writes:
>
>> On 11/09/2015 12:38 PM, Sergey Organov wrote:
>>> Martin Sebor <msebor@gmail.com> writes:
>>>
>>>> On 11/09/2015 05:57 AM, Sergey Organov wrote:
>>>>> Hello,
>>>>>
>>>>> GCC, when compiling C code, seems to always generate out-of-line copy of
>>>>> any [C99] inline function that also happens to be a GCC builtin,
>>>>> resulting in link errors (see a test-case below). According to C99
>>>>> standard, an out-of-line copy of a function should only be instantiated
>>>>> in those compilation unit(s) where the function is also declared
>>>>> 'extern'.
>>>>>
>>>>> Apparently, all builtin functions implicitly get 'extern' declaration that
>>>>> forces out-of-line copy of inline function in every compilation unit.
>>>>>
>>>>> Is it a bug of feature? If the latter, what is the way for a library to
>>>>> provide generic inline functions that might happen to be GCC builtins?
>>>>
>>>> Depending on the -std= option, GCC can generate a copy of an inline
>>>> function (regardless of whether or not the function also has a builtin
>>>> form) in each translation unit that defines it. To avoid multiple
>>>> definition errors, define inline functions in C headers as static.
>>>>
>>>> The following page explains how GCC treats the inline specifier
>>>> in each of the standard mode:
>>>>
>>>>    https://gcc.gnu.org/onlinedocs/gcc/Inline.html
>>>
>>> The point is that for builtin functions it apparently does it wrong.
>>
>> I see. Yes, that does look like a bug. symtab_node::needed_p()
>> returns false for an ordinary inline function but true for one
>> that has a builtin. I didn't spend enough time debugging this
>> to see what sets it and why, and I couldn't find any tests for
>> this to confirm that it's deliberate.
>>
>> On the other hand, speaking in the strict C sense, abs and most
>> (all?) such symbols that have corresponding builtins are reserved
>> in a hosted implementation so defining them is undefined. They are
>> only allowed to be defined in a freestanding environment.
>
> Yeah, freestanding environment is what I care about.

Then you should be using -ffreestanding (which implies -fno-builtin).

-- 
Marc Glisse