Are SVE/I8MM ever detected at runtime?

[AntoinePrv]

Related to working #1245, I am wondering if dynamic dispatch to SVE (to name one) is ever possible.

If I understand correctly, for a give function add we would like to dispatch, it would get compiled to

• add_neon.cpp (assuming we also dispatch it for simplicity here) with XSIMD_WITH_NEON (__ARM_NEON...)
• add_sve.cpp XSIMD_WITH_SVE (__ARM_FEATURE_SVE...)
• add_dispatch.cpp where we use xsimd::dispatch with no compilation options in particular (since it needs to run on all platforms).

However in the supported architecture code, getting sve value from getauxval is gated behind a check for compile-time activation of SVE.
Therefore in add_dispatch.cpp, SVE is never detected.

xsimd/include/xsimd/config/xsimd_cpuid.hpp

Lines 115 to 119 in 4d185a6

	#elif defined(__ARM_FEATURE_SVE) && defined(__ARM_FEATURE_SVE_BITS) && __ARM_FEATURE_SVE_BITS > 0
	#if defined(__linux__) && (!defined(__ANDROID_API__) || __ANDROID_API__ >= 18)
	sve = bool(getauxval(AT_HWCAP) & HWCAP_SVE);
	#endif

Am I understanding this correctly @serge-sans-paille @kou ?

[kou]

How did you confirm it?

I tried the following program on QEMU:

#include <stdio.h>
#include <sys/auxv.h>

int
main(void)
{
  unsigned long x = getauxval(AT_HWCAP);
#ifdef __ARM_FEATURE_SVE
  printf("__ARM_FEATURE_SVE: Defined\n");
#else
  printf("__ARM_FEATURE_SVE: Not defined\n");
#endif
  printf("%lu: %lu\n", x, x & HWCAP_SVE);
  return 0;
}

$ docker run --platform linux/arm64 -it --rm arm64v8/ubuntu:24.04
$ apt update
$ apt install -y gcc
$ gcc -o xxx -O3 -march=armv8-a xxx.c && ./xxx
__ARM_FEATURE_SVE: Not defined
5100273659: 4194304
$ gcc -o xxx -O3 -march=native /host/xxx.c && ./xxx
__ARM_FEATURE_SVE: Defined
5100273659: 4194304

It seems that we can detect SVE without enabling SVE in compiling time.

[AntoinePrv]

Thanks for the feedback!

It seems that we can detect SVE without enabling SVE in compiling time.

Exactly what is intended I believe. The issue however is that in xsimd the runtime detection is removed if we are not compiling with SVE.

[serge-sans-paille]

Fixed by #1253 . thanks!