Re: Compiling kernel with -std=gnu11

Den 2025-01-05 kl. 18:44, skrev Taylor R Campbell:
>> Date: Sun, 5 Jan 2025 17:03:23 +0000
>> From: Taylor R Campbell<[email protected]>
>>
>> I skimmed through
>> https://gcc.gnu.org/onlinedocs/gcc-10.5.0/gcc/C-Extensions.html
>> for other things whose semantics might change with -std=gnu11 vs
>> -std=c11, and the only case I came up with was the `\e' escape in
>> strings, of which we have one use: [...]
> I should note that I do know of at least one other semantic difference
> between -std=gnu11 and -std=c11 which may be significant even though
> it doesn't affect whether something builds or not:
>
> float
> foo(float x, float y, float c)
> {
> 	float p = x*y;
>
> 	return p + c;
> }
>
> Under -std=c11, this rounds the product to float precision first, and
> then computes the sum, as required by the C spec.  Under -std=gnu11,
> this may compute everything in double or extended precision, just like
> if it had been `return x*y + c' with no intermediate `float' variable
> assignment.  (The finer-grained option here is -fexcess-precision:
> under -std=c11 it's set to `standard'; under -std=gnu11, to `fast'.)
Hm, interesting optimization.  I understand why they have done this,
but I would not have expected it to be the default behaviour.
Writing code like this is not uncommon to get rounding as expected.

-- Ragge