Re: [PATCH] Optimize macro: make it more predictable

["Martin Liška" <mliska@suse.cz>]

On 11/3/20 2:34 PM, Jakub Jelinek wrote:
> On Tue, Nov 03, 2020 at 02:27:52PM +0100, Richard Biener wrote:
>> On Fri, Oct 23, 2020 at 1:47 PM Martin Liška <mliska@suse.cz> wrote:
>>> This is a follow-up of the discussion that happened in thread about no_stack_protector
>>> attribute: https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545916.html
>>>
>>> The current optimize attribute works in the following way:
>>> - 1) we take current global_options as base
>>> - 2) maybe_default_options is called for the currently selected optimization level, which
>>>        means all rules in default_options_table are executed
>>> - 3) attribute values are applied (via decode_options)
>>>
>>> So the step 2) is problematic: in case of -O2 -fno-omit-frame-pointer and __attribute__((optimize("-fno-stack-protector")))
>>> ends basically with -O2 -fno-stack-protector because -fno-omit-frame-pointer is default:
>>>       /* -O1 and -Og optimizations.  */
>>>       { OPT_LEVELS_1_PLUS, OPT_fomit_frame_pointer, NULL, 1 },
>>>
>>> My patch handled and the current optimize attribute really behaves that same as appending attribute value
>>> to the command line. So far so good. We should also reflect that in documentation entry which is quite
>>> vague right now:
>>>
>>> """
>>> The optimize attribute is used to specify that a function is to be compiled with different optimization options than specified on the command line.
>>> """
>>>
>>> and we may want to handle -Ox in the attribute in a special way. I guess many macro/pragma users expect that
>>>
>>> -O2 -ftree-vectorize and __attribute__((optimize(1))) will end with -O1 and not
>>> with -ftree-vectorize -O1 ?
>>
>> Hmm.  I guess the only two reasonable options are to append to the active set
>> and thus end up with -ftree-vectorize -O1 or to start from an empty set and thus
>> end up with -O1.
> 
> I'd say we always want to append, but only take into account explicit
> options.

Yes, I also prefer to always append and basically drop the "reset" functionality.

> So basically get the effect of
> take the command line, append to that options from the optimize/target
> pragmas in effect and append to that options from optimize/target
> attributes and only from that figure out the implicit options.

Few notes here:
- target and optimize attributes are separate so parsing happens independently; however
   they use global_options and global_options_set as a starting point
- you can have a series of wrapped optimize/pragma macros and again information is shared
in global_options/global_options_set
- target and optimize options interact, but in a controlled way with SET_OPTION_IF_UNSET

That said, I hope the biggest offender is right now the handling of -Olevel.

@Jakub: Do you see a situation with my patch where it breaks?

Thanks,
Martin

> 
> 	Jakub
>