How to use x86intrin.h

Question

In one of my applications, I need to efficiently de-interleave bits in a long stream of data. Ideally, I would like to use the BMI2 pext_u32() and/or pext_u64() x86_64 intrinsic instructions when available. I scoured the internet for doc on x86intrin.h (GCC), but couldn't find much on the subject; so, I am asking the gurus on StackOverflow to help me out.

1. Where can I find documentation about how to work with functions in x86intrin.h?
2. Does gcc's implementation of pext_*() already have code behind it to fall back on, or do I need to write the fallback code myself (for conditional compile)?
3. Is it possible to write a binary that automatically falls back to an alternate implementation if a target does not support the intrinsic? If so, how does one do so?
4. Is there a known programming pattern that will be recognized by GCC and automatically converted to pext_*() when compiling with optimization enabled and with -mbmi2?

Answer 1

Intel publishes the Intrinsics Guide, which also applies to GCC. You will have to write your own fallback code if you use these intrinsics.

You can achieve automatic switching of implementations by using IFUNC resolvers, but for non-library code, using conditionals or function pointers is probably simpler.

Looking at the gcc/config/i386/i386.md and gcc/config/i386/i386.c files, I don't see anything in GCC 8 which would automatically select the pext instruction without intrinsics in the source code.

Answer 2

The design philosophy of Intel's intrinsics is that you can only use them in functions that will run only on CPUs with the required extensions. Checking for support every instruction would add way too much overhead, and then there's have to be a fallback (there isn't).

Intel intrinsics are not like GNU C __builtin_popcountll (which does use a fallback if compiled without -mpopcnt, but not you can enable target options on a per-function basis with attributes.)