On Fri, Sep 5, 2008 at 2:36 AM, Klaas-Jan Stol <[EMAIL PROTECTED]> wrote: > On Tue, Sep 2, 2008 at 12:43 PM, Allison Randal <[EMAIL PROTECTED]> wrote: > >> Klaas-Jan Stol wrote: >> >>> >>> This must make the following syntax rule illegal: >>> >>> target = null >>> >>> because if "null" is declared as a .local, you can't know whether you want >>> to nullify target, or want to set target's value to that of the .local >>> variable "null". >>> >>> I take it this is no problem; just stick to >>> >>> null target >>> >>> if you actually want to set target to 0/null. >>> >> >> Yes, that's reasonable. The syntactic sugar was confusing in that case >> anyway. (Seemed like you were assigning a null value to the destination >> register, rather than nullifying the PMC in the destination register.) >> >> This belongs in a general category of opcodes where the standard >> transformation of "call the opcode with the first argument stuck before an >> '=' sign" doesn't really make sense. >> >> Allison >> > > the problem seems to be a bit bigger than I had foreseen. The issue is that > ops with the first operand marked as 'OUT' may be rewritten as: > target = op [operand [, operand]*]? > > However, consider the following: > > .local pmc getstdin > > $P0 = getstdin > > How should this be resolved? is the opcode 'getstdin' meant here, or is it > the value of the .local getstdin. This problem occurrs with all ops, not > only with single-operand ops. > > kjs > the sugar for what can be on the left side of an equals sign needs to be changed. simply having a first parameter with OUT isn't enough. the same thing happens for $P0 = push $S1 which is legal pir syntax, but obscure at best.
ops must have some means of specifying (perhaps an attribute like C<:returns> or C<:rvalue>?) that allows them to be on the right side of the equals. only this class of ops allows the syntax described above. in the case of .local pmc getstdin ... $P0 = getstdin this can be resolved because the (now) compiler knows (either during parsing or semantic analysis) that the C<getstdin> op is not allowed to be an rvalue and lookup C<getstdin> in the list of C<.local>s as a fallback. ~jerry
