Re: Default 32 bit output

[Darryl Miles <darryl-mailinglists at netbauds dot net>]

Domen Vrankar wrote:
> > --target=i386-elf
> > regards
> > kanishk
>
> What about if I want to make target=i686 to output 32 bit executables?
> Is this even possible? Are there any benefits if you use target i686
> instead of i386?


I'm sure I use:

CFLAGS="-m32" LDFLAGS="-m32" ./configure


There are two issues here.  The issue of ABI, there is one 32bit ABI and 
one 64bit ABI on linux Intel IA32.  The above flags will change that and 
maybe the suggested --target=i386-elf does too.


The i386 and i686 issue is more to do with code generation and 
optimization.  Affecting load/store ordering, alignment, etc...  but the 
ABI is the same.

I don't think there are any new instructions used when compiling for 
i686 on GCC, it just optimizes the code and generation for the execution 
flow and pipelines for that CPU better than a 386/486.

Which means the code won't SIGABORT on you and fail, it will just 
perform a little bit faster or slower.



AFAIK there is no way to mark the resulting executable by way of ELF 
magic numbers to only be usable on i686 onwards because the code makes 
use of new instructions.

Most applications which use say MMX perform an auto-detect on the CPU to 
decide the best routines to use and falling back to non-MMX versions.


Darryl