Rethinking GMP's configure

[Richard Guenther]

On Tue, 6 Dec 2011, Torbjorn Granlund wrote:

> nisse at lysator.liu.se (Niels Möller) writes:
> 
>   Makes sense to me. Either --with-cpu=ALL or --with-cpu=HOST would be a
>   sensible default.
>   
> My thinking is to make --with-cpu=ALL become the default.
> 
>   > First, let's admit there will be some overhead.  At some level, there
>   > will be a need to jump through a table, initiated for the run-time CPU.
>   > This indirection costs a few cycles each time.  But note that in a
>   > shared library, we typically access data through a GOT (global offset
>   > table) and call functions indirectly through a PLT (procedure linkage
>   > table).  I think we could stay within the overhead of shared library
>   > calls, if we do things right.  (For ELF, that is, where we can control
>   > such things.)
>   
>   I think I suggested, some time ago, that one could overwrite the table
>   entries in the PLT, with no *additional* indirections. But that may not
>   work. If I understand things correctly, there's no *the* PLT, there's a
>   PLT in each loadable object (executable or shared library file). And
>   then I have already forgotten how that is supposed to work with function
>   pointers.
>   
> Ian Lance Taylor has written some excellent texts about all this.  (They
> should be next to your search engine.)
> 
> This is a messy area, even if one ignores the worls outside of ELF.
> 
> (The PLT is, as you say, part of the process, and not to be manipulated
> by the shared library.  Perhaps it is *possible* to manipulate it using
> some dll calls on *some* systems, but I don't think this would be the
> right path.)

For GNU ELF systems you have the possibility to use IFUNC relocations
(basically the relocation depends on the outcome of a selector function
you implement).  GLIBC now internally uses this to select between
different implementations of for example memcpy for the CPU a
program is executed on.

That is basically "PLT modification", but at relocation time
(thus not really "modification").

The IFUNC relocations are a GNU ELF extension (and require recent
dynamic loader and binutils support, of course).

Richard.

-- 
Richard Guenther <rguenther at suse.de>
SUSE / SUSE Labs
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