586 vs 686

[Josh Boyer]

On Fri, Feb 20, 2009 at 09:14:31AM +0100, Jakub Jelinek wrote:
>On Fri, Feb 20, 2009 at 07:44:57AM +0100, Ralf Corsepius wrote:
>> Kevin Kofler wrote:
>>> Jesse Keating wrote:
>>>> No, they're being made to be able to produce those for those reasons.
>>>> It is yet to be seen if it will be successful, and using the x86_64
>>>> kernel on i586 installs hinges upon this functionality so I recall.
>>>
>>> AFAIK, Debian has been shipping i386 GCC with -m64 support for ages.  The
>>> drawback is that it's slower even when not using -m64 because some internal
>>> types (HOST_WIDE_INT and related stuff) are larger.
>>
>> Not true. -m64/-32 cause gcc to switch between multilibs, when using a  
>> multilibbed setup. Unless something is broken somewhere, the internal  
>> types, library search paths etc. will be set up correctly.
>
>This is actually true, not false.  If you support both -m32 and -m64,
>you need 64-bit HOST_WIDE_INT, while i?86-linux configured gcc normally uses 32-bit
>HOST_WIDE_INT.  On the extremely register starved i?86 using 64-bit
>HOST_WIDE_INT obviously slows things down (and increases compiler memory
>requirements).
>
>What I'm probably going to do is just some hack in the gcc driver, if -m64
>is passed, switch to a different cc1 backend, maybe even driver (similarly
>to how -V or -b switches work) and use a i586-redhat-linux ->
>x86_64-redhat-linux cross-compiler in that case.
>But before doing that, we'll need either a x86_64 cross binutils or i?86
>binutils supporting both i?86 and x86_64 in rawhide.

I'm slightly confused as to why this hasn't already been done.  IIRC, the
ppc toolchain works this way already (supports both -m32 and -m64 with the
same binutils), and it has for quite a while...

Is the register starvation issue the only reason i586/x86_64 don't do the
biarch support?

josh