On Mon, Aug 5, 2013 at 9:33 AM, Xinliang David Li <davi...@google.com> wrote: > 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? >
-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. -- H.J.
