Re: readability improvements to i386-opc.tbl

["Jan Beulich" <JBeulich at suse dot com>]

>>> On 15.11.17 at 13:43, <hjl.tools@gmail.com> wrote:
> On Wed, Nov 15, 2017 at 1:33 AM, Jan Beulich <JBeulich@suse.com> wrote:
>> H.J.,
>>
>> for a while I've been thinking about how to shrink some of the overly
>> long lines in that file, as that's imo severely hampering readability
>> (and hence maintainability). There are two pretty simple
>> adjustments I'd like to propose as at least a first step, but which I
>> wouldn't want to carry out without some prior agreement (as the
>> resulting patches would obviously be huge):
>>
>> 1) BaseIndex implies Disp{8,16,32,32S} (with a few MPX insn
>> exceptions, which aren't necessary as 16-bit addressing not being
>> legal for them is being dealt with in tc-i386.c explicitly anyway, in
>> md_assemble()). i386-gen could easily set those implied bits.
>>
>> 2) Many instructions allow no suffixes at all (in particular most of
>> the myriad of SIMD ones). Many other instructions allow just a
>> single suffix. I'd like to propose No_Suf to accompany No_*Suf
>> and {b,w,l,s,q,ld}Suf as the inverse of No_*Suf, to be used when
>> the list of permitted suffixes is shorter than the list of forbidden
>> ones. Again this could be dealt with in i386-gen relatively easily.
>>
>>
> 
> They sound good.  Can you take a look at
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=20320 
> 
> as the first step?

I've looked some more into what exceptions exist that would
complicate achieving that goal:

- register only insns (push, pop, inc, etc): Reg8 => Reg|Byte etc?
  (could then be extended to also eliminate FloatReg, FloatAcc, RegXMM, RegYMM,
  and RegZMM; in turn FirstXmm0 could become Acc|Xmmword)
- lgdt etc, lds etc, far call/jmp: size != meaning of suffix
- movs/movz: final suffix describes (register) destination
- bound's memory operand is double the size of the register one
- floating point: alternative sizes implied from 'l'
- {,v}pinsrw allow l/q suffixes (but memory operand is 16 bits); oddly enough
  the AVX512BW form as well as {,v]pinsr{b,d} and {,v}pextr{b,d} don't allow any
  suffix, while the 0fc5 {,v}pextrw form again does (and again except for the
  AVX512BW form).
- {,v}cvt{,t}ss2si allow l/q suffixes (but memory operand is 32 bits)
- {,v}cvt{,t}sd2si allow l/q suffixes (but memory operand is 64 bits)

This is not necessarily an exhaustive list. I think that addressing
the first item separately up front would be a useful thing anyway
(with the general goal - as per the title of this thread - in mind), so
unless you have a clear reason to object to such an approach I'll
see to look into this. I would want to keep Reg* as shorthands for
the new Reg|<width> construct, to further improve readability as
well as to reduce the amount of change needed / wanted to
i386-opc.tbl.

Independently I'd like to have your opinion on said pinsrw/pextrw
suffix inconsistencies: Is this really intended? If so, why? If not,
which of the two possible ways should this be corrected (drop
all suffix permissions, or add missing ones)?

Jan