The keyword I use to search the SWI-Prolog source code for where the Prolog predicates are implemented in C is PRED_IMPL
The way I think of the code is that what can be done with Prolog is done in Prolog, the low level actions such as unification are done in C. Then if a predicate needs to be faster it is done with calls straight into C.
Sometimes, as Jan notes, the code is so tricky that it is better to implement in C than Prolog.
That is why I say AFAIK it does not use a Virtual Machine such as WAM or an Intermediate Language such as JVM or CIL.
I know Jan will correct me if this is wrong and then I will have learned something.
Related questions:
How can a foreign language predicate also be a built-in predicate?
“decompiled prolog”
Personal Notes
?- vm_list(append).
========================================================================
append/2
========================================================================
0 s_trustme(clause(638140))
----------------------------------------
clause 1 (<clause>(000001707FC6F2F0)):
----------------------------------------
0 i_enter
1 b_atom(list)
3 b_var0
4 i_call(error:must_be/2)
6 b_var0
7 b_var1
8 i_depart(lists:append_/2)
10 i_exit
========================================================================
append/3
========================================================================
0 s_list(clause(735160),clause(641508))
----------------------------------------
clause 1 (<clause>(000001707FCCDEE0)):
----------------------------------------
0 h_nil
1 h_void
2 h_var(1)
4 i_exitfact
----------------------------------------
clause 2 (<clause>(000001707FC72790)):
----------------------------------------
0 h_list_ff(3,4)
3 h_void
4 h_list
5 h_var(3)
7 h_firstvar(5)
9 h_pop
10 i_enter
11 b_var(4)
13 b_var1
14 b_var(5)
16 i_depart(lists:append/3)
18 i_exit
========================================================================
append/1
========================================================================
0 i_fopen
1 i_fcalldetva(-4611686413639185926)
3 i_fexitdet
true.
Source code:
pl-wam.c
pl-vmi.c
pl-supervisor.c
pl-comp.c
pl-export
From: pl-vmi.c
Virtual machine instruction names. Prefixes:
| I_ | General instructions |
| B_ | Body specific version |
| H_ | Head specific version |
| A_ | Arithmetic compilation specific |
| C_ | Control (compilation of ;/2, etc.) |
| S_ | Supervisor instructions. See pl-supervisor.c |
References:
Logic Programming Implementation - Part I: The WAM
Limits on memory areas - Notes trail stack which is also part of WAM.
Warren’s Abstract Machine - A Tutorial Reconstruction - List the basic instructions of WAM such as put_structure, set variable, unify_value, unify_variable.
An Abstract Prolog Instruction Set
From pl-comp.c
This module (pl-comp.c) forms together with the module ‘pl-wam.c’ the complete
kernel of SWI-Prolog.
Excerpts from: _ Bowen et al. , 1983 D. L. Bowen, L. M. Byrd, and WF. Clocksin. A portable Prolog compiler.
We have opted for the structure-copying method of [Mellish 80] and [Bryunooghe 80], rather than the structure-sharing [Warren 77].
Our storage management strategy is basically that of [Warren 77], i.e. there is a heap containing the program, a “local” stack for control information and variable bindings, a “global” stack for structures, and a "trail stack which keeps track of when variables are bound so that they can be reset to “uninstantiated” at the appropriate time on backtracking. One change is that a reference count is maintained for each clause so that pointers to clauses can safely be included in asserted terms.
As our run-time system is based on previously published work [Warren 77] [warren 80]
EDIT: After responses (1) (2) by Jan W.
References:
SWI-Prolog Implementation history
Bowen et al. , 1983
D. L. Bowen, L. M. Byrd, and WF. Clocksin. A portable Prolog compiler. In L. M. Pereira, editor, Proceedings of the Logic Programming Workshop 1983 , Lisabon, Portugal, 1983. Universidade nova de Lisboa. - Explains low level concepts such as control instructions: enter, call, and exit.
Neumerkel, 1993
Ulrich Neumerkel. The binary WAM, a simplified Prolog engine. Technical report, Technische Universität Wien , 1993 - The binary WAM, a simplified Prolog engine - Notes design concepts of Prolog abstract machines (as opposed to virtual machine VM) at time, including ZIP.
EDIT:
This reminds me of minProlog and specifically the execution engine (solve.ml)
This also reminds me of the MIT open couseware lecture of advanced data structures (videos) . There was one in particular but I can’t remember the exact one.