How do I specify different compiler flags for just one Python/C extension source file?

Question

I have a Python extension which uses CPU-specific features, if available. This is done through a run-time check. If the hardware supports the POPCNT instruction then it selects one implementation of my inner loop, if SSSE3 is available then it selects another, otherwise it falls back to generic versions of my performance critical kernel. (Some 95%+ of the time is spent in this kernel.)

Unfortunately, there's a failure mode I didn't expect. I use -mssse3 and -O3 to compile all of the C code, even though only one file needs that -mssse3 option. As a result, the other files are compiled with the expectation that SSSE3 will exist. This causes a segfault for the line:

start_target_popcount = (int)(query_popcount * threshold);

because the compiler used fisttpl, which is an SSSE3 instruction. After all, I told it to assume that SSSE3 exists.

The Debian packager for my package recently ran into this problem, because the test machine has a GCC which understands -mssse3 and generates code with that in mind, but the machine itself has an older CPU without those instructions.

I want a solution where the same binary can work on older machines and on newer ones, that the Debian maintainer can use for that distro.

Ideally, I would like to say that only one file is compiled with the -mssse3 option. Since my CPU-specific selector code isn't part of this file, no SSSE3 code will ever be executed unless the CPU supports it.

However, I can't figure out any way to tell distutils that a set of compiler options are specific to a single file.
Is that even possible?

Answer 1

A very ugly solution would be to create two (or more Extension) classes, one to hold the SSSE3 code and the other for everything else. You could then tidy the interface up in the python layer.

c_src = [f for f in my_files if f != 'ssse3_file.c']

c_gen = Extension('c_general', sources=c_src,
                 libraries=[], extra_compile_args=['-O3'])

c_ssse3 = Extension('c_ssse_three', sources=['ssse3_file.c'],
                 libraries=[], extra_compile_args=['-O3', '-mssse3'])

and in an __init__.py somewhere

from c_general import *
from c_ssse_three import *

Of course you don't need me to write out that code! And I know this isn't DRY, I look forward to reading a better answer!