arm: incomplete support for ARMv6 systems

[carenas]

as shown when building in Debian 12 using the gcc-12-arm-linux-gnueabi cross compiler and testing with qemu-user-static (not a Debian issue, but attempting to reflect what is likely happenning with at least Rasperry PI):

$ make -j CC='arm-linux-gnueabi-gcc-12 -static -march=armv6k'
$ qemu-arm-static -cpu arm1176 bin/sljit_test -v
Run executable allocator test
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Illegal instruction (core dumped)

Partially addressed by #187 (note that RBIT is optional in armv6, so there might be still issues yet to be reported), but still missing fixes for test93 (which requires VFP3[1] for VMOV immediate) and with an implicit base of armv6k for atomics[2]

[1] https://developer.arm.com/documentation/ddi0406/c/Application-Level-Architecture/Advanced-SIMD-and-Floating-point-Instruction-Encoding/Floating-point-data-processing-instructions
[2] https://developer.arm.com/documentation/ddi0301/h/summary-of-arm1136jf-s-and-arm1176jzf-s-processor-differences/summary-of-differences/armv6k-extensions-support

[carenas]

RBIT seems to be available for ARMv7 and ARMv6 with Thumb2, but test79 case 21 failed with soft-float ABI in Thumb2, althought that might be an independent issue as it is likely also affecting armv7 and might be triggered by -mfloat-abi=soft

[zherczeg]

Is this still incomplete?

[carenas]

Is this still incomplete?

yes; armv6k is the minimum (so not all armv6 systems will work), and specifically haven't been able to either confirm or deny if the original Raspberry PI is no longer supported (by PCRE2) because of that.

there is also the issue of NEON not being mandatory for all armv7 systems and that could be considered also a related issue, although only if you require FPU or SIMD support. For PCRE2, this is likely a non-issue since only SIMD is needed and is only supported for armv8 (really aarch64 only) where NEON support is mandatory.

[carenas]

FWIW; thanks[1] to the NetBSD community, we know next release of PCRE2 is most likely going to work fine (at least in NetBSD 10) for the most common RaspberryPI that use armv6 CPUs.

Still. it might be a good idea to keep this open (as there are other systems than RaspberryPI that are also likely affected) and of course make sure to mention the deprecation of older than Armv6 CPUs with the possibility it might affect Armv6 and a link here in the release notes of PCRE2 10.43.

[1] https://mail-index.netbsd.org/port-arm/2023/11/29/msg008444.html