Re: fftw3: Patch for powerpc build failure

["Steven G. Johnson" <stevenj@alum.mit.edu>]

Steven G. Johnson wrote:
> Can you tell us whether -mabi=altivec is really required for programs 
> calling FFTW?  I never did figure out exactly what this flag does on 
> Linux.  (It is not needed for 16-byte stack alignment, apparently, since 
> I think that is guaranteed by the SVR4 ABI.)

I checked with my co-author on this, and there is an additional problem:

Currently, FFTW detects Altivec by reading an altivec register that says 
whether it is enabled...on other machines, e.g. a G3, this causes a 
SIGILL and we use setjmp/longjmp to recover from the trap.  However, 
last he checked, gcc -mabi=altivec caused setjmp/longjmp to try to save 
altivec registers, failing on the G3.  There are several possible 
workarounds, but since the ABI seemed incompatible we gave it up as a 
lost cause.

So, with the current version of FFTW, the runtime Altivec detection will 
not work under Linux on non-Altivec machines (last we checked), and 
should therefore be disabled in the standard Debian ppc package.

If you can assure us that the ABIs are compatible on Debian, and that 
-mabi=altivec won't cause FFTW to fail on e.g. a G3 for other reasons, 
we can revisit this issue and try to come up with a way to detect 
Altivec support properly at runtime.  (There is also still the question 
of whether altivec-FFTW-calling programs need -mabi=altivec, and whether 
this will break them on non-altivec architectures.)

Steven