[PR38711] Use DF_LIVE in IRA if it available (for -O2 and higher)

[Vladimir Makarov]

On 12-10-13 9:40 AM, Steven Bosscher wrote:
> Hello,
>
> This fixes the long-standing enhancement request to use DF_LIVE in
> IRA. To quote the first comment in the PR:
>
> IRA should be using the DF-LIVE sets, which are smaller than the DF-LR
> sets when they are available (typically at O2 and above). The proper
> sets can be conveniently accessed using the df_get_live_[in,out]
> functions which use DF-LIVE if it is available and fall back to DF-LR
> if it is not.
>
> I thought that DF_LIVE wasn't available at -O1 in IRA, but
> interestingly ira.c:ira() adds it to the DF-problems list in that case
> already:
>
> $ grep -C3 -n df_live_add ira.c
> 4160-
> 4161-  if (optimize == 1)
> 4162-    {
> 4163:      df_live_add_problem ();
> 4164-      df_live_set_all_dirty ();
> 4165-    }
> 4166-#ifdef ENABLE_CHECKING
>
> So DF_LIVE is already being computed for IRA. It's just not being used
> because the DF_LR_{IN,OUT} macros are used instead of the
> df_get_live_{in,out} functions. With my patch, the DF_LIVE sets are
> used, which results in marginally faster compile times for my set of
> cc1-i files on power64 and on x86_64, as well as smaller code. There's
> also a good speedup for the PR54146 test case.
>
> Only reload.c still uses the DF_LR_OUT. The comment in
> find_dummy_reload() suggests this is intentional.
>
> I also fond a bug in IRA:
>
> 4392-     touch the artificial uses and defs.  */
> 4393-  df_finish_pass (true);
> 4394-  if (optimize > 1)
> 4395:    df_live_add_problem ();
> 4396-  df_scan_alloc (NULL);
> 4397-  df_scan_blocks ();
> 4398-
>
> At optimize>1 the DF_LIVE problem is already always computed. I think
> the idea here was to *remove* the DF_LIVE problem at -O1, which is
> also part of the attached patch.
> (Perhaps we should even simply not add the DF_LIVE problem at -O1, but
> that can be done in a follow-up patch if necessary.)
>
> Bootstrapped and tested (with default checking and with release
> checking) on powerpc64-unknown-linux-gnu and x86_64-unknown-linux-gnu.
> OK for trunk?
>
>
LIVE is not used on purpose.  I added LIVE usage to the old RA about 10 
years ago (it was before DF-infrastructure).  Everything was ok until, 
somebody reported compiler crash on semantically wrong program 
(something with usage of uninitialized variable).  After that I removed 
this code.

Probably, that is also a reason why reload uses LR not LIVE.  So I'd 
like to wait a few weeks.  When I have more time, I am going to 
reproduce this test suite and look how IRA behaves on it.

Thanks, Steven.