SIMDUTF_CAN_ALWAYS_RUN_* macros broken

[jakubjelinek]

As mentioned in https://bugzilla.redhat.com/show_bug.cgi?id=2273542#c14 the SIMDUTF_CAN_ALWAYS_RUN_{HASWELL,ICELAKE,WESTMERE} macros are broken at least for GCC.
This is because GCC (since ~4.7 when not using -save-temps, since 14 when preprocessing separately from compilation) on #pragma GCC target ISA changes undefines/defines the predefined preprocessor macros like __AVX2__, so that they reflect the current set of the enabled/disabled ISAs in the region.
When you

#define SIMDUTF_CAN_ALWAYS_RUN_AVX2 __AVX2__
#if SIMDUTF_CAN_ALWAYS_RUN_AVX2
#else
#pragma GCC push_options
#pragma GCC target ("avx2")
#endif
...
#if SIMDUTF_CAN_ALWAYS_RUN_AVX2
#else
#pragma GCC pop_options
#endif

and compile this with -mno-avx2, the first SIMDUTF_CAN_ALWAYS_RUN_AVX2 use will evaluate to 0 because __AVX2__ is not defined, but then #pragma GCC target ("avx2") defines it, on the second use it will evaluate to 1 because __AVX2__ is defined to 1 there and finally #pragma GCC pop_options will undefine it again.
The code clearly expects it to evaluate the same in both cases, otherwise there is a misbalance, which in the rhbz case causes unsupported_implementation to be compiled with avx2,bmi,lzcnt,popcnt isas.

I'd suggest either change macros like:

#define SIMDUTF_CAN_ALWAYS_RUN_HASWELL ((SIMDUTF_IMPLEMENTATION_HASWELL) && (SIMDUTF_IS_X86_64) && (__AVX2__))

to

#if ((SIMDUTF_IMPLEMENTATION_HASWELL) && (SIMDUTF_IS_X86_64) && (__AVX2__))
#define SIMDUTF_CAN_ALWAYS_RUN_HASWELL 1
#else
#define SIMDUTF_CAN_ALWAYS_RUN_HASWELL 0
#endif

Then it will have a constant value through the whole compilation unit.

[jakubjelinek]

What GCC normally uses in its intrinsic headers is:

#ifndef __AVX2__
#pragma GCC push_options
#pragma GCC target("avx2")
#define __DISABLE_AVX2__
#endif /* __AVX2__ */

at the start and

#ifdef __DISABLE_AVX2__
#undef __DISABLE_AVX2__
#pragma GCC pop_options
#endif /* __DISABLE_AVX2__ */

at the end.

[lemire]

Solved with last release. Thanks.