Provide way to debug delta relations and expose iteration counter

[julienhenry]

Introducing new way to debug fix point computation on recursive relations: .debug_delta.

Example:

.decl R(x:number)
.output R()
.decl D = debug_delta(R)

This produces a new output by storing each tuples from the @delta_R relation after each main fix point loop iteration, adding the iteration number where this tuple was produced.

To obtain the iteration number, I introduce some new RAM nodes (Assign and Variable) to be able to talk about a variable and modify its value. I used these new nodes to introduce a variable storing the iteration number for each LOOP node.

Now that we have an iteration counter for main iteration loops, we can expose this to the user with the recursive_iteration_cnt() intrinsic functor. This has some use cases e.g. when someone wants to control the convergence of the fix point loop :

R(x + 1) :-
    R(x),
    recursive_iteration_cnt() < 100.

[codecov]

Codecov Report

Merging #2414 (eb41198) into master (2914f7b) will increase coverage by 0.00%.
The diff coverage is 77.93%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2414    +/-   ##
========================================
  Coverage   77.75%   77.75%            
========================================
  Files         474      479     +5     
  Lines       31164    31364   +200     
========================================
+ Hits        24232    24388   +156     
- Misses       6932     6976    +44     

Impacted Files	Coverage Δ
src/ast/analysis/typesystem/Type.cpp	47.37% <0.00%> (-0.47%)	⬇️
src/ast/analysis/typesystem/Type.h	50.00% <ø> (ø)
...c/ast/analysis/typesystem/TypeConstrainsAnalysis.h	100.00% <ø> (ø)
src/ast2ram/seminaive/ValueTranslator.h	100.00% <ø> (ø)
src/ast2ram/utility/TranslatorContext.h	100.00% <ø> (ø)
src/include/souffle/utility/SubProcess.h	55.00% <0.00%> (ø)
src/interpreter/Node.h	96.95% <ø> (ø)
src/ram/Variable.h	45.45% <45.45%> (ø)
src/ram/Assign.h	50.00% <50.00%> (ø)
src/ast/transform/SemanticChecker.cpp	95.96% <53.84%> (-1.08%)	⬇️
... and 22 more

... and 2 files with indirect coverage changes

[quentin]

As discussed offline, an alternative syntax would be: .decl IDENT = debug_delta(RELATION).

.decl R(x:number)
R(x) :- ... x ... R(y).

// declare a relation as being the "debug_delta" of another.
// the new relation has the signature of the original one 
// with one additional tail element that is the iteration counter. 
.decl R_debug = debug_delta(R)
.output R_debug()

.decl use_R_debug(x:number, min_iter:unsigned)
use_R_debug(x,min_iter) :-
  R_debug(x, _),
  min_iter = min iter : R_debug(x, iter).

That would be equivalent to:

.decl R_debug(x:number, iter:unsigned)
R_debug(x, relation_iteration_cnt()), R(x) :- ... x ... R(y).

For a non-recursive rule R, the debug_delta(R) would contains all facts of R and the iteration counter element value 0?

[quentin]

Interesting debug feature.
Shall we document the new syntax on Souffle's web page?