Subject: Re: [PATCH, RFC] Fix build problems related to profile-directed optimization

From: Ted Ts'o <[email-masked]>
Date: Thu, 2 Feb 2012 15:12:26 -0500

On Thu, Feb 02, 2012 at 12:02:27PM -0800, Junio C Hamano wrote:
> 
> Thanks for a patch.  How does this compare with what was discussed in the
> other thread?
> 
>   http://thread.gmane.org/gmane.comp.version-control.git/188992/focus=189172

I wasn't aware of this other approach when I created this patch (I
must have missed the e-mail thread, sorry).

One of the reasons why I did it this way was for more flexibility.  I
wanted to be able to do:

$ make PROFILE_GEN=YesPlease PROFILE_DIR=/var/cache/FDO all
# make PROFILE_GEN=YesPlease PROFILE_DIR=/var/cache/FDO install

run a bunch of git commands on various git repositories to get
real-life usage...

Then do...

$ make PROFILE_USE=YesPlease PROFILE_DIR=/var/cache/FDO all
# make PROFILE_GEN=YesPlease PROFILE_DIR=/var/cache/FDO install

But for many people they would probably be satisfied with something
that builds git using a single magic recipe, even if they give up some
fractional performance improvement (keep in mind that the feedback
directed optimization seems to buy you only a single digit percentage
improvement according according to Andi's original experiment; I just
got interested in this more for amusement value than any thought that
it would save me serious amounts of time).

> I would wish a solution ideally would support
> 
> 	make PROFILE_BUILD=YesPlease
>         make PROFILE_BUILD=YesPlease install

At least in theory, it should be possible to have something which
supports both PROFILE_GEN/PROFILE_USE as well as a combined
PROFILE_BUILD.

The hard part is that PROFILE_BUILD requires a multi-pass process; you
need to build with one set of CFLAGS, then run the sample workload to
get the data for your feedback directed optimizations, and then re-run
the build with another set of CFLAGS.  I think what we could to check
for PROFILE_BUILD, and if it is set, do the first PROFILE_GEN / make
test commands as part of the top-level Makefile's all: rule, and then
do the normal build after that.

It's a little kludgy, but does that sound acceptable to you?

       	      	      	       	    	  - Ted