On Sun, Aug 4, 2013 at 9:23 AM, Xinliang David Li <davi...@google.com> wrote: > On Sun, Aug 4, 2013 at 4:40 AM, Richard Biener > <richard.guent...@gmail.com> wrote: >> Xinliang David Li <davi...@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.
