Fix compilation for armv7 targets with vfp < v4 and gcc >= 8

[jolivain]

When using a armv7 gcc >= 8 toolchain (like [1]) with Highway configured with -DHWY_CMAKE_ARM7=OFF and HWY_ENABLE_CONTRIB=ON, compilation fails with error:

In file included from /build/highway-1.0.3/hwy/ops/arm_neon-inl.h:33,
                 from /build/highway-1.0.3/hwy/highway.h:358,
                 from /build/highway-1.0.3/hwy/contrib/sort/shared-inl.h:104,
                 from /build/highway-1.0.3/hwy/contrib/sort/traits128-inl.h:27,
                 from /build/highway-1.0.3/hwy/contrib/sort/vqsort_128d.cc:23,
                 from /build/highway-1.0.3/hwy/foreach_target.h:81,
                 from /build/highway-1.0.3/hwy/contrib/sort/vqsort_128d.cc:20:
/toolchain/lib/gcc/arm-buildroot-linux-gnueabihf/12.2.0/include/arm_neon.h: In function ‘void hwy::N_NEON::StoreU(Vec128<long long unsigned int, 2>, Full128<long long unsigned int>, uint64_t*)’:
/toolchain/lib/gcc/arm-buildroot-linux-gnueabihf/12.2.0/include/arm_neon.h:11052:1: error: inlining failed in call to ‘always_inline’ ‘void vst1q_u64(uint64_t*, uint64x2_t)’: target specific option mismatch
11052 | vst1q_u64 (uint64_t * __a, uint64x2_t __b)
      | ^~~~~~~~~
/build/highway-1.0.3/hwy/ops/arm_neon-inl.h:2786:12: note: called from here
 2786 |   vst1q_u64(unaligned, v.raw);
      |   ~~~~~~~~~^~~~~~~~~~~~~~~~~~

The same errors happen when configured with HWY_ENABLE_EXAMPLES=ON, or from client libraries like libjxl (at other places).

The issue is that Highway Arm NEON ops have a dependency on the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The SIMD (Neon) v1 and VFPv3 instructions are not supported.

There was several attempts to fix variants of this issues. See #834 and #1032.

HWY_NEON target is selected only if __ARM_NEON is defined. See: https://github.com/google/highway/blob/1.0.3/hwy/detect_targets.h#L251

This test is not sufficient since __ARM_NEON will be predefined in any cases when Neon is enabled (neon-vfpv3, neon-vfpv4).

The issue is that HWY_CMAKE_ARM7=ON implies VFPv4 / NEON SIMD v2. When setting HWY_CMAKE_ARM7=OFF, "neon-vfpv4" will not be forced, but the code is still using intrinsics assuming VFPv4. Gcc will fail with error because code cannot be generated for the selected architecture.

This issue can be avoided by adding "-DHWY_DISABLED_TARGETS=HWY_NEON" in CXXFLAGS. The problem with this solution is that every client program will also need to do the same. This goes against the very purpose of "hwy/detect_targets.h".

Technically, Armv7-a processors with VFPv4 can be detected using some ACLE (Arm C Language Extensions [2]) predefined macros:

Basically, we want Highway to define HWY_NEON only when the target supports SIMDv2/VFPv4 or higher. An older target with vfpv3 only (e.g. Cortex-A8, A9, ...) would NOT define HWY_NEON, and therefore would fallback on HWY_SCALAR implementation.

However, not all compiler completely support ACLE. There is also several versions too. So we cannot easily rely on macros like "ARM_VFPV4" (which clang predefine, but not gcc).

The alternative solution proposed in this patch, is to declare the HWY_NEON target architecture as broken, when we detect the target is Armv7-A, but mandatory features for vfpv4 (namely half-float, FMA) are missing. Half-floats are tested using the macro __ARM_NEON_FP, and the FMA with the macro __ARM_FEATURE_FMA. See ACLE [2]. The intent of declaring the target as broken, rather than selecting HWY_NEON only if vfpv4 features are detected is to remain a bit conservative, since the detection is slithly inaccurate.

For a given compiler/cflags, predefined macros for Arm/ACLE can be reviewed with commands like:

arm-linux-gnueabihf-gcc -mcpu=cortex-a9 -mfpu=neon-vfpv3 -Wp,-dM -E -c - < /dev/null | grep -Fi arm | sort
arm-linux-gnueabihf-gcc -mcpu=cortex-a7 -mfpu=neon-vfpv4 -Wp,-dM -E -c - < /dev/null | grep -Fi arm | sort
clang -target armv7a -mcpu=cortex-a9 -mfpu=neon-vfpv3 -mfloat-abi=hard -Wp,-dM -E -c - < /dev/null | grep -Fi arm | sort
clang -target armv7a -mcpu=cortex-a7 -mfpu=neon-vfpv4 -mfloat-abi=hard -Wp,-dM -E -c - < /dev/null | grep -Fi arm | sort

The different values of __ARM_NEON_FP can be seen, depending which "-mfpu" is passed. Same for __ARM_FEATURE_FMA.

[1] https://toolchains.bootlin.com/downloads/releases/toolchains/armv7-eabihf/tarballs/armv7-eabihf--glibc--bleeding-edge-2022.08-1.tar.bz2
[2] https://github.com/ARM-software/acle/

[jan-wassenberg]

Thank you @jolivain for this thorough analysis. Looks like you have found the root cause of this issue, that's great.
Also good idea to use HWY_BROKEN_TARGETS, someone can still opt-out of this check by predefining HWY_BROKEN_TARGETS=0 if required.

Should we then also revert !965 and allow runtime dispatch for ARMv7?