Re: [SPARC] Disable -fira-share-save-slots by default

[Jeff Law <law at redhat dot com>]

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On 05/24/11 16:30, Eric Botcazou wrote:
> The new save slot sharing algorithm has a documented limitation:
> 
>    Future work:
> 
>      In the fallback case we should iterate backwards across all possible
>      modes for the save, choosing the largest available one instead of
>      falling back to the smallest mode immediately.  (eg TF -> DF -> SF).
That's not new -- I wrote that circa 1992/1993.  The whole point behind
the changes from ~1992 was to try and use DFmode insns when
caller-saving FP regs on the sparc.  However, I didn't think it was
worth the effort to deal with that case (TF -> DF -> SF).  The code
tries to build a save/restore insn of MOVE_MAX_WORDS size and if that
fails, then it drops back to WORD_SIZE IIRC.






> that is annoying for the SPARC when it comes to floating-point code because the 
> floating-point registers are single (SF) but there is a fully-fledged support 
> for double (DF) arithmetics in the architecture.  So saving registers on an 
> individual basis really pessimizes here.  For example, the size of the object 
> generated for the Ada unit a-nlcefu.ads at -O2 decreases from 96080 to 95088 
> bytes when you pass -fno-ira-share-save-slots.
It's the new slot sharing code that doesn't have support for saving
larger hunks.  Having written the original code to handle larger saves
specifically to help sparc, I can certainly understand why the new code
is causing you grief :-)



> 
> Experiments have shown that the impact on integer code is null in terms of code 
> size and negligible in terms of stack usage (-fstack-usage reports 8/16 bytes 
> increase for most functions).  Therefore this patch disables the option by 
> default for the SPARC.  Boostrapped/regtested on SPARC/Solaris, applied on the 
> mainline and 4.6 branch.
> 
> 
> Jeff, I'd like to apply it to the 4.5 branch as well, but I need your patch:
> 
> 2011-01-21  Jeff Law  <law@redhat.com>
> 
> 	PR rtl-optimization/41619
> 	* caller-save.c (setup_save_areas): Break out code to determine
> 	which hard regs are live across calls by examining the reload chains
> 	so that it is always used.
> 	Eliminate code which checked REG_N_CALLS_CROSSED.
> 
> Do you have any objections to me backporting it to the branch?
No objections at all.  I don't believe there were any follow-up patches
and all the change did was make more of the paths through caller-save
consistent in how they determined what needed to be saved.

jeff
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