VORTEXM4: s2_usable() runs RDSVL before the SME2 check (SIGILL without SME) #6150

s2_usable() in kernel/arm64/sme2_gemm_detect.h computes ok = sme2 && svcntsw() == 16;. Clang treats svcntsw() (RDSVL) as free of side effects and if-converts the expression, so RDSVL runs before the FEAT_SME2 result is tested. Disassembly of sme_sgemm_kernel.o (Apple clang 21):

bl    _sysctlbyname
ldr   w8, [sp, #0x8fc]      ; FEAT_SME2 value
rdsvl x9, #0x1              ; runs whether or not SME is present
lsr   x9, x9, #2
cmp   x9, #0x10
cset  w9, eq
cmp   w8, #0x0
csel  w8, wzr, w9, eq

On a CPU without SME this is SIGILL. A VORTEXM4 build is not meant to run on M1-M3, but before #6074 its level-3 routines still worked there on NEON kernels; now they reach the SME2 kernels' s2_usable() and crash. #6142 contains a fix (the vector length read in a separate noinline inline-asm function, called only when SME2 is present). That fix could also go in on its own, ahead of #6142.