Chained numeric assignments

[Stone, Joshua I]

Mike Mason wrote:
> Should the following chained assignment:
>     writes[execname()] = total_io[execname()] += $return
> be equivalent to:
>        writes[execname()] += $return        total_io[execname()] += $return

Not if we mimic C's operator associativity, which is right-to-left for 
assignment operators.  So your statement is effectively:

     writes[execname()] = ( total_io[execname()] += $return )

Or equivalent to:

     total_io[execname()] += $return
     writes[execname()] = total_io[execname()]

Also, if you look at the -p1 output, it will show you the explicit 
associativity that was parsed.  e.g.

     $ stap -ue 'probe begin { a = b += c }' -p1
     # parse tree dump
     # file <input>
     probe begin{
     (a) = ((b) += (c))
     }

> They yield different results in a couple scripts I tried (see below). 
> For example, it appears that writes[] gets assigned total_io[] instead 
> of += $return.

That's correct.  But if you're seeing inconsistent results, please file 
a bug and commit a testcase.

Josh