as allows too many registers for vldm/vstm (ARMv7/8)

[Pekka Seppänen]

Hi.

It seems that as currently allows too many doubleword (or quadword 
alias) registers to be given for vldm/vstm (vpop/vpush) instructions on 
NEON MPE enabled platforms. Both ARMv7 and ARMv8 architecture reference 
manuals clearly state that vldm/vstm instructions must not use more than 
16 doubleword registers (while using more than 16 singleword registers 
would be fine). I didn't try on ARMv8, but at least on ARMv7 spec 
hardware vldm/vstm with 17 or more doubleword registers raise an 
undefined instruction exception (as pretty much expected based on the 
ARM ARM).

I guess the related code is around config/tc-arm.c, parse_vfp_reg_list. 
In this case the naming in the code has become is a bit ambiguous -- I'm 
assuming the code dates to pre-v6 ages when fldm/fstm either didn't have 
such a restriction or it simply wasn't documented. While max_regs / 
count ensures that used registers fall within the d0 .. d31 or d0 .. d15 
(VFPv3-D16) range it doesn't check that the size of the register list 
itself doesn't exceed 16 registers when using doubleword registers and a 
vldm/vstm instruction. (Note that this issue has nothing to with the 
said VFPv3-D16 register limitation, you may use d16 .. d31 registers as 
you will but you simply must operate with 16 or less doubleword 
registers at a time.)

So, while as properly rejects e.g. vpop {d16-d32} (invalid d32 register 
within the range) it doesn't reject vpop {d0-d16} or vpop {q0-q8} alias 
(too many d<N> registers within the range), for which the generated 
opcode will eventually fail if executed. I'm not sure how trivial the 
possible fix is, i.e. are there older platforms on which you could've 
used fldm/fstm with more than 16 doubleword registers etc.


-- Pekka