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

["Martin Liška" <mliska@suse.cz>]

PING^2

On 11/26/20 2:56 PM, Martin Liška wrote:
> PING^1
> 
> On 11/9/20 11:35 AM, Martin Liška wrote:
>> 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
>>>
>>
>