[qvtd-dev] M4 commits

[Ed Willink <ed@xxxxxxxxxxxxx>]

Hi

My presentation at York had one major flaw. It assumed that a call graph 
built by considering producers and consumers of region heads was 
satisfactory for both control and data flow. It isn't.

If A provides something for B (data flow), it is convenient to call B 
from A (control flow). BUT if B has a further required value that A 
cannot satisfy, the call of B must be deferred until the further 
requirement of B is satisfied. A must therefore place its output in a 
temporary buffer within a suitable ancestor until all the directly 
referenced inputs of a B invocation are available. This problem for a 
single headed input is the same as occurs for multi-headed input. The 
solution can be therefore shared and is now on ewillink/480567, but one 
test fails intermittently; bug partially understood.

QVTi does not support a mapping that executes by assigning to or 
accumulating in a buffer. It is possible to fudge a "x : Set(X)" guard 
variable and then do "x := x->union(more);" but it's a but klunky. 
Better to introduce an updateable middle guard "where (x : Set(T))" that 
can be updated by a bottom assignment "x += more;". The new form of an 
assignment uses a new ConnectionAssignment AS class. The new form of 
guard variable is distinctive since the middle/where guard had no 
variables in QVTi.

The intermittency might be fixed today, but the general clean up and 
review certainly won't be, so I'll try to cherry pick all the 
ConnectionAssignment support changes for M4 so that M4 can be used as a 
stable development platform for the new scheduler.

    Regards

        Ed Willink