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

On Mon, Aug 5, 2013 at 9:33 AM, Xinliang David Li <> wrote:
> On Mon, Aug 5, 2013 at 8:26 AM, H.J. Lu <> wrote:
>> On Sun, Aug 4, 2013 at 9:23 AM, Xinliang David Li <> wrote:
>>> On Sun, Aug 4, 2013 at 4:40 AM, Richard Biener
>>> <> wrote:
>>>> Xinliang David Li <> 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.
> Turn off all features or just toggle the features? What is the use
> case for -mno-default?

-mno-default tunes off all features.  To turn on only features: f1,
f2, f3, --,fN.
we can do

-mno-default -mtune-ctrl=f1,f2,,..,fN

We don't need to check if other features are off since they are
turned off by -mno-default.


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