Teaching expression() to treat some operations specially

[lkcl]

alan thank you for the insights. i am using an unusual mailer which only topposts: rather than subject everyone to that i'm trimming all context.

some background on svp64 predication (which is not part of Power ISA): it is the same concept as RVV and NEC SX Aurora (Cray-style) predicate masks except a bit more sophisticated. basic concept of Predicated Cray Vectors: elements may be dynamically skipped at runtime based on VL bits read from the regiater file:

for i in range(VL):
   if get_mask()&(1<<i) == 0: continue
   // do operation

get_mask() above may be one of the following sources:

* all ones (disables predication)
* the contents of r3, r10, r31 or their bit-inverses
* 1<<r3 i.e. one single unary bit
* the concatenation (and inversion) of a selected CR Field
   from CR0, CR1, ..... CRn
   sm=eq means concatenate CR0.eq CR1.eq CR2.eq as the mask

i particularly like 1<<r3 because it is

for i in range(VL):
   if (1<<GPR(r3))&(1<<i) == 0: continue
   GPR(RT+i) = GPR(RA+i) + GPR(RB+i) # vector add

i.e. just

   GPR(RT+GPR(r3)) = GPR(RA+GPR(r3)) + GPR(RB+GPR(r3))


this should help clear up some potential confusion on the meaning of sm=~r3, it is not the inversion of a static constant "3" (to create 0xfff....fffc) but a mnemonic to put a bitpattern into the 24-bit prefix, "at *instruction execute* time please read then use contents of register r3 as the predicate mask, invert all its bits before passing it on".

we could have used "sm=0b0111" or other numeric code and simply dropped that directly into the 24-bit prefix. however this loses the opportunity to perform macro substitution.

another idea we discussed was to move the "~" to the lhs:

    ~sm=r3 / !sm=r3

or

     sm ~= r3

on the basis that only the rhs gets passed to expression().  in english:

      inverted source mask is r3

readability of 1<<r3 is compromised however by trying something aimilar.  we thought perhaps "sm=ur3" or such, for "source mask is unary from r3" but it is pushing things.

adapting 0_constant to recognise 1<<r3 as a special pattern would help preserve readability greatly. i take it you mean that after the lexer has recognised "1" "<<" "r3" that there is a tree-walk done by cr_operand() that *replaces* the patterns?

l.