Re: PROPOSAL: Extend inline asm syntax with size spec

Nadav Amit
2018-10-10T16:31:49Z

at 7:53 AM, Segher Boessenkool <segher@kernel.crashing.org> wrote:

> On Mon, Oct 08, 2018 at 11:07:46AM +0200, Richard Biener wrote:
>> On Mon, 8 Oct 2018, Segher Boessenkool wrote:
>>> On Sun, Oct 07, 2018 at 03:53:26PM +0000, Michael Matz wrote:
>>>> On Sun, 7 Oct 2018, Segher Boessenkool wrote:
>>>>> On Sun, Oct 07, 2018 at 11:18:06AM +0200, Borislav Petkov wrote:
>>>>>> Now, Richard suggested doing something like:
>>>>>>
>>>>>> 1) inline asm ("...")
>>>>>
>>>>> What would the semantics of this be?
>>>>
>>>> The size of the inline asm wouldn't be counted towards the inliner size
>>>> limits (or be counted as "1").
>>>
>>> That sounds like a good option.
>>
>> Yes, I also like it for simplicity. It also avoids the requirement
>> of translating the number (in bytes?) given by the user to
>> "number of GIMPLE instructions" as needed by the inliner.
>
> This patch implements this, for C only so far. And the syntax is
> "asm inline", which is more in line with other syntax.
>
> How does this look?

It looks good to me in general. I have a couple of reservations, but I
suspect you will not want to address them:

1. It is not backward compatible, requiring a C macro to wrap it, as the
kernel might be built with different compilers.

2. It is specific to asm. I do not have in mind another use case (excluding
the __builtin_constant_p), but it would be nicer IMHO to have a builtin
saying “ignore the cost of this statement” for the matter of optimizations.

Regards,
Nadav