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  1. Erlang/OTP
  2. ERL-1350

Cannot produce compile:forms asm automatically, nor convert beam_disasm:file output to .S



    • Type: Bug
    • Status: Help Wanted
    • Priority: Minor
    • Resolution: Unresolved
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: compiler
    • Labels:


      The real problem here is that a few functions are not exported.

      The first problem is that compile:preprocess_asm_forms/1 is not exported publicly.  This prevents one from taking a .erl or .S file and automatically generating the asm tree variant.  Of course the code for compile:preprocess_asm_forms/1, compile:collect_asm/2, compile:collect_asm_function/2 can just be copied along with the asm_module record and then one can cleanly do this transformation.

      Use case: generating on the fly compiled functions via compile:forms without having any usage of external files.  Both the ASM source and BEAM can be provided and generated in memory.  And the ASM source is now in an erlang preprocessed format and requires no strings or parsing.  Sure you could unstick, modify a copy of the compile source and recompile it but thats even worse than a solution like:

      -record(asm_module, {module,-record(asm_module, {module,      exports,      labels,      functions=[],      attributes=[]}).
      preprocess_asm_forms(Forms) ->
          R = #asm_module{},
          R1 = collect_asm(Forms, R),
            R1#asm_module.labels}}.collect_asm([{module,M} | Rest], R) ->
          collect_asm(Rest, R#asm_module{module=M});
      collect_asm([{exports,M} | Rest], R) ->
          collect_asm(Rest, R#asm_module{exports=M});
      collect_asm([{labels,M} | Rest], R) ->
          collect_asm(Rest, R#asm_module{labels=M});
      collect_asm([{function,A,B,C} | Rest0], R0) ->
          {Code,Rest} = collect_asm_function(Rest0, []),
          Func = {function,A,B,C,Code},
          R = R0#asm_module{functions=[Func | R0#asm_module.functions]},
          collect_asm(Rest, R);
      collect_asm([{attributes, Attr} | Rest], R) ->
          collect_asm(Rest, R#asm_module{attributes=Attr});
      collect_asm([], R) -> R.collect_asm_function([{function,_,_,_}|_]=Is, Acc) ->
      collect_asm_function([I|Is], Acc) ->
          collect_asm_function(Is, [I|Acc]);
      collect_asm_function([], Acc) ->
      s_to_beamasm(Fname) ->
        {ok,Forms0} = file:consult(Fname),
      compile_beamasm(Fname) ->
        {ok, _, CplBin} = compile:forms(element(2, s_to_beamasm(Fname)), [binary, from_asm, [outdir, "ebin"]]).

      The second issue is that beam_disasm:file produces output that is effectively useless without beam_disasm:pp which are inconveniently only exported if the DEBUG_DISASM flag is set on compilation of beam_disasm.  So you again have to mess with library files to get the job done:

      -define(LibPath, code:root_dir() ++ "/lib/").
      disassembleToDis(Outfile, Fname) ->
        Filename = case is_atom(Fname) of true -> code:which(Fname); _ -> Fname end,
        code:unstick_mod(beam_disasm), c:c(?LibPath ++ "/compiler-7.6/src/beam_disasm.erl", [debug_info,{d,'DEBUG_DISASM'},{outdir, "ebin"}]),
        DisasmCode = element(6, beam_disasm:file(Filename)), beam_disasm:pp(Outfile, [{file, Filename}, {code, DisasmCode}]), ok.

      There are some reasons you might want to disassemble a BEAM file to a recompilable .S.  Namely to make patches to it and recompile, for example, when you do not have the source code.  Yes this is a debugging type of operation but why not allow it by default.

      I see no good reason why important functions like this are not exported which are the subjects of questions on the web now and again, and cause custom built, hacks to be provided as answers since most do not want to study the details of a large source base to find this stuff.

      See the attached guidance of the flow of transformations which are needed for going between all the variations of erl source, BEAM, BEAM assembly, and AST.  It is interesting because the epp module is not mentioned.  Of course some transformations are missing such as erl -> AST and the picture should be further flushed out.  In fact, something like this should be part of the erlang documentation...




            otp_team_vm Team VM
            gregorymorse GregoryMorse
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