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3.3 Binary PrologThe key idea of binary Prolog is the transformation ofclauses to binary clauses using a continuation passingstyle. BinProlog, an efficient emulator for binary Prologhas been developed by Paul Tarau [Tar91][Tar92].The implementation is based on the WAM which canbe greatly simplified in that case.In binary Prolog a clause has at most one subgoal.A clause can be transformed to a binary clause byrepresenting the call of subgoals explicitely using continuations[App92]. For that purpose the first subgoalis given an additional argument containing the successcontinuation. The success continuation is the listof subgoals to be executed if the first subgoal is executedsuccessfully. The head is given an additionalargument which passes on the continuation. A factis transformed to a clause, whose subgoal executes ameta-call of the continuation. For example, the followingclausesnrev( , []).nrev([111T] ,R) :-nrev(T,L) , append (L, [H] ,R) .are transformed intonrev( , D ,cont) (Cont).nrev( [HIT] ,R,Cont) : -nr ev (T , L , append (L , [H] , R , Cont ) )Compiling binary Prolog to the WAM it appearsthat the environment stack is superfluous since all variablesare temporary. Therefore, all instruction dealingwith local variables or managing the stack can beeliminated. So a small and efficient interpreter can beimplemented. But this simplification has a big problem.The continuation, which contains also the variablespreviously contained in the stack frame, is storedon the copy stack. This means that there is no lastcalloptimization. So for a working BinWAM an efficientgarbage collector is crucial. In some sense theBinWAM can be seen as mixture of a clause stackingmodel with the WAM.4 The Vienna Abstract Machine4.1 IntroductionThe VAM has been developed at the TU Wien as analternative to the WAM. The WAM divides the unificationprocess into two steps. During the first step thearguments of the calling goal are copied into argumentregisters and during the second step the values in theargument registers are unified with the arguments ofthe head of the called predicate. The VAM eliminatesthe register interface by unifying goal and head argumentsin one step. The VAM can be seen as a partialevaluation of the call. There are two variants of theVAM, the VAM 1 p and the VAM2P-A complete description of the VAM 2p can be foundin [KN90]. Here we give a short introduction to theVAM 2p which helps to understand the VAM ip andthe compilation method. The VAIN/1 2p (VAM with twoinstruction pointers) is well suited for an intermediatecode interpreter implemented in C or in assemblylanguage using direct threaded code [Be1731. The goalinstruction pointer points to the instructions of thecalling goal, the head instruction pointer points to theinstructions of the head of the called clause. During aninference the VAM2p fetches one instruction from thegoal, one instruction from the head, combines themand executes the combined instruction. Because informationabout the calling goal and the called head isavailable at the same time, more optimizations thanin the WAM are possible. The VAM features cheapbacktracking, needs less dereferencing and trailing, hassmaller stack sizes and implements a faster cut.The VAM i p (VAM with one instruction pointer)uses one instruction pointer and is well suited for nativecode compilation. It combines instructions atcompile time and supports additional optimizationslike instruction elimination, resolving temporary variablesduring compile time, extended clause indexing,fast last-call optimization, and loop optimization.4.2 The VAM2PLike the WAM, the VAM 2p uses three stacks. Stackframes and choice points are allocated on the environmentstack, structures and unbound variables arestored on the copy stack, and bindings of variablesare marked on the trail. The intermediate code of theclauses is held in the code area. The machine registersare the goalptr and headptr (pointer to the code of thecalling goal and of the called clause respectively), thegoalframeptr and the headframeptr (frame pointer ofthe clause containing the calling goal and of the calledclause respectively), the top of the environment stack,the top of the copy stack, the top of the trail, and thepointer to the last choice point.Values are stored together with a tag in one machineword. We distinguish integers, atoms, nil, lists, structures,unbound variables and references. Unboundvariables are allocated on the copy stack to avoid danglingreferences and the unsafe variables of the WAM.Furthermore it simplifies the check for the trailing ofbindings. Structure copying is used for the representationof structures.8
copy stack1trailenvironment stack1t4icode areacopyptrtrailptrchoicepntptrgoalframeptrheadframeptrgoalptrheadptrvariablesgoalptr'goalframeptr'local variablesI continuation code pointerI continuation frame pointerFigu re 15: stack frametrailptr' copy of top of trailcopyptr' copy of top of copy stackheadptr' alternative clausesgoalptr' restart code pointer (VAM2P)goalframeptr' restart frame pointerchoicepntptr' 1 previous choice pointFigure 16: choice pointFigure 13: VAM data areasVariables are classified into void, temporary and localvariables. Void variables occur only once in a clauseand need neither storage nor unification instructions.Different to the WAM, temporary variables occur onlyin the head or in one subgoal, counting a group ofbuiltin predicates as one goal. The builtin predicatesfollowing the head are treated as belonging to thehead. Temporary variables need storage only duringone inference and can be held in registers. All othervariables are local and are allocated on the environmentstack. During an inference the variables of thehead are held in registers. Prior to the call of the firstsubgoal the registers are stored in the stack frame. Toavoid initialisation of variables we distinguish betweentheir first occurrence and further occurrences.The clauses are translated to the VAM 2p abstractmachine code (see fig. 14). This translation is simpledue to the direct mapping between source codeand VAM2p code. During run time a goal and a headinstruction are fetched and the two instructions arecombined. Unification instructions are combined withunification instructions and resolution instructions arecombined with termination instructions. A differentencoding is used for goal unification instructions andhead unification instructions. To enable fast encodingthe instruction combination is solved by addingthe instruction codes and, therefore, the sum of twoinstruction codes must be unique.4.3 The VAMITThe VAM i p has been designed for native code compilation.A complete description can be found in [KB92].The main difference to the VAM2p is that instructioncombination is done during compile time instead ofrun time. The representation of data, the stacks andstack frames (see fig. 15) are identical to the VAM 2p.The VAM ip has one machine register less than theVAM2p. The two instruction pointers goalptr andheadptr are replaced by one instruction pointer calledcodeptr. Therefore, the choice point (see fig. 16) isalso smaller by one element since there is only oneinstruction pointer. The pointer to the alternativeclauses now directly points to the code of the remainingmatching clauses.Due to instruction combination during compile timeit is possible to eliminate instructions, to eliminate alltemporary variables and to use an extended clause indexing,a fast last-call optimization and loop optimization.In WAM based compilers abstract interpretationis used to derive information about mode, type andreference chain length. Some of this information is locallyavailable in the VAM i p due to the availability ofthe information of the calling goal.All constants and functors are combined and evaluatedto true or false. For a true result no code isemitted. All clauses which have an argument evaluatedto false are removed from the list of alternatives.In general no code is emitted for a combination with avoid variable. In a combination of a void variable withthe first occurrence of a local variable the next occurrenceof this variable is treated as the first occurrence.Temporary variables are eliminated completely. Theunification partner of the first occurrence of a temporaryvariable is unified directly with the unificationpartners of the further occurrences of the temporaryvariable. If the unification partners are constants, nocode is emitted at all. Flattened code is generated forstructures. The paths for unifying and copying structuresis split and different code is generated for eachpath. This makes it possible to reference each argumentof a structure as offset from the top of the copystack or as offset from the base pointer of the struc-9
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The PowerPC 604 RISC Microprocessor
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local ..verieble_table_lengthThis f
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1dc2wPush long or double from const
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I 3.4Storing Stack Values into Loca
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anewarrayAllocate new array of refe
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PastoreStore into single float arra
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laddLong integer addfSubSingle floa
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dreminegInegInegdnegDouble float re
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IliLong integer to integerSyntax:L
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ifleBranch if less than or equal to
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dcmpgDouble float compare (1 on NaN
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Appendix A: An OptimizationA.2 Push
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