Re: Stub group default for hppa elf

["John David Anglin" <dave at hiauly1 dot hia dot nrc dot ca>]

> This change doesn't directly address the stub overflow problem, but
> changes stub section placement to always be before the branches into
> the stubs.  ie. you've gone from
> 
> ======
> code group A
> ======
> stubs for A + B
> ======
> code group B
> ======
> 
> to
> 
> ======
> stubs for A
> ======
> code group A
> ======
> stubs for B
> ======
> code group B
> ======

Yes.

> Of course, the chances are that "stubs for A + B" is roughly the size
> of "stubs for A" plus "stubs for B" (may be smaller than the sum due
> to merging of duplicate stubs).  ie. a default of 1 tends to make stub
> sections twice as large as a default of -1, and that is why changing
> the default helps.

I think the problem when the default is 1 is that "stubs for A + B"
looks like

code for group A
=====
stubs for B
stubs for A
====
code for group B

Thus, each code group has to branch further than it has to.  However,
this is just speculation.

> I think the following patch is more appropriate, but will leave
> reverting your patch to you.  The only real reason to always place
> stubs before their associated code section is when you're doing
> conditional branches to a stub and you want to control branch
> prediction.
> 
> 	* elf32-hppa.c (elf32_hppa_size_stubs): Double the size left for
> 	stubs if !stubs_always_before_branch.

I'm not convinced that it's actually necessary to do this if the
stubs are placed correctly.

> Index: bfd/elf32-hppa.c
> ===================================================================
> RCS file: /cvs/src/src/bfd/elf32-hppa.c,v
> retrieving revision 1.93
> diff -u -p -r1.93 elf32-hppa.c
> --- bfd/elf32-hppa.c	8 Jan 2003 02:19:23 -0000	1.93
> +++ bfd/elf32-hppa.c	8 Feb 2003 00:53:20 -0000
> @@ -2901,11 +2901,22 @@ elf32_hppa_size_stubs (output_bfd, stub_
>    if (stub_group_size == 1)
>      {
>        /* Default values.  */
> -      stub_group_size = 7680000;
> -      if (htab->has_17bit_branch || htab->multi_subspace)
> -	stub_group_size = 240000;
> -      if (htab->has_12bit_branch)
> -	stub_group_size = 7500;
> +      if (stubs_always_before_branch)
> +	{
> +	  stub_group_size = 7680000;
> +	  if (htab->has_17bit_branch || htab->multi_subspace)
> +	    stub_group_size = 240000;
> +	  if (htab->has_12bit_branch)
> +	    stub_group_size = 7500;
> +	}
> +      else
> +	{
> +	  stub_group_size = 6971392;
> +	  if (htab->has_17bit_branch || htab->multi_subspace)
> +	    stub_group_size = 217856;
> +	  if (htab->has_12bit_branch)
> +	    stub_group_size = 6808;
> +	}
>      }
>  
>    group_sections (htab, stub_group_size, stubs_always_before_branch);

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6605)