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Re: New option to do fine grain control [on|off] of micro-arch specific features : -mtune-ctrl=....


On Sun, Aug 4, 2013 at 9:23 AM, Xinliang David Li <davidxl@google.com> wrote:
> On Sun, Aug 4, 2013 at 4:40 AM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> Xinliang David Li <davidxl@google.com> wrote:
>>>Hi, GCC/i386 currently has about 73 boolean parameters/knobs (defined
>>>in ix86_tune_features[], indexed by ix86_tune_indices) to perform
>>>micro-arch specific performance tuning. However such settings are hard
>>>coded (fixed with a given -mtune setting) and is very hard to do
>>>performance experiment.
>>>
>>>The attached patch fixes the problem. The patch introduces a new
>>>option -mtune-ctrl=. Its parameter is a comma separated list of
>>>feature names to turn on associated features. Feature name can be
>>>prefixed by ^ to do the opposite. For instance,
>>>
>>>  -mtune-ctrl=prologue_using_move,epilogue_using_move,^pad_returns
>>>
>>>tells the compiler to use move instructions in prologue/epilogue
>>>(instead of push/pop), and *not* pad return instructions.
>>>
>>>To facilitate the change, the feature tuning enums defined in i386.h
>>>are moved to a new file x86-tune.def and this file can be used to
>>>generate both the enums and names of the features.
>>>
>>>
>>>Ok for trunk?
>>>
>>
>> The patch fails to add documentation. And I am nervous about testing coverage - is this considered a development option only or are random combinations expected to work in all situations? I expect not, thus this looks like a dangerous option?
>>
>
> This option is intended to be used by developers -- otherwise we will
> have to document all possible feature knobs. I saw a couple of
> existing options in i386.opt marked as 'Undocumented' -- is this mark
> used for case like this?   Since this option is for experimental
> purpose, user certainly can shoot their foot with it :)
>
> If there is support of target specific --params which takes strings as
> args, it might be a better choice to use that.
>

I have a similar patch to turn on/off each feature and it is very
useful to fine tune x86 backend.  But mine is not automated.
Anothing I found useful is a command line switch to turn off all
features, like -mno-default. It can be used to turn on a group of
features without checking which features are on/off by default.


--
H.J.


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