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Re: New option to do fine grain control [on|off] of micro-arch specific features : -mtune-ctrl=....
- From: Xinliang David Li <davidxl at google dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Jan Hubicka <hubicka at ucw dot cz>, Teresa Johnson <tejohnson at google dot com>
- Date: Sun, 4 Aug 2013 09:23:07 -0700
- Subject: Re: New option to do fine grain control [on|off] of micro-arch specific features : -mtune-ctrl=....
- References: <CAAkRFZLZh1q+p55O6kA3QSgyNERcPpAOW=tpWYye40Dzs+0KcA at mail dot gmail dot com> <f577b371-1d7e-4fd1-b26e-c8541aa54838 at email dot android dot com>
On Sun, Aug 4, 2013 at 4:40 AM, Richard Biener
> Xinliang David Li <firstname.lastname@example.org> 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
>>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,
>>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.
>>2013-08-03 Xinliang David Li <email@example.com>
>> * config/i386/i386.opt: New option -mtune-ctrl=.
>> * config/i386/x86-tune.def: New file.
>> * config/i386/i386.h: include x86-tune.def.
>> * config/i386/i386.c (ix86_option_override_internal):
>> Parsing -mtune-ctrl= option and set tune features.