On Mon, Aug 5, 2013 at 8:26 AM, H.J. Lu <hjl.to...@gmail.com> wrote: > 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.
Turn off all features or just toggle the features? What is the use case for -mno-default? thanks, David It can be used to turn on a group of > features without checking which features are on/off by default. > > > -- > H.J.
