VB Decompiler

drk||Raziel · New User Joined: 25 Aug 2004 Posts: 8 Location: Athens

I didn't found any info on the net so i decided to load IDA and a hexeditor..

This is what i found:

--Visual Basic 6 Compiler Options(c2) List--

-nowarn4786:?? disable warning 4786=??
-nowarn4715:?? disable warning 4715=??
-cap:??
-nodlp:??
-dlp:??
-ZX:??? needs an input file,seems to need a Object File , then complier error (internal.. error)
-ZI:/ZI enable Edit and Continue debug info(in c), seems to work
-S:?? it seems to be Display listing on screen
-dos:?? from 16 bit VB?? it is used from VB6 , not needed to complie
-Fo# : object file (-Fo"C:\myobj.obj")
-H#: external name length restriction (default 31) (c2 c++v6)
-Zl:/Zl omit default library name in .OBJ(in C) ,it seems to work
-Za:/Za disable extensions (implies /Op)(in c) , seems to have no effect...
-Zd:/Zd line number debugging info only ,seems to work
-Zi:/Zi enable debugging information (in c)
-Brepro:?? it seems to have no effect
-Bd: it seems to have no effect
-Fs#: generate a source file listing with .LST extension (c2 c++v6)
-Fa#:/Fa[file] name assembly listing file
-FA#:/FA[sc] configure assembly listing s= source , c= code bytes
-pr:??
-QIWMTemu:??
-QIfist:/QIfist[-] use FIST instead of ftol()
-QI0f:/QI0f[-] enable Pentium 0x0f fix
-QIfdiv:/QIfdiv[-] enable Pentium FDIV fix
-QIf : ?? possibly enable QIfist and QIfdiv ?? [RE needed]
-p6gj:??
-noblend:?? possibly forcode generation ,No optimize for blended model (default)
-nolock :???
-noehopt :??? possibly some optimization, off
-ehopt :??? possibly some optimization, on
-bzalign :??align mode perhaps??
-basic :??possibly compile VB code.. but his compiler complies VB code so what's this ?? , compiler freeses if removed
-nogen :?? hmm possibly do not generate output (syntax checking olny..) it seems to have no effect

These are REALY strange... possibly left from C compiler.. [RE needed]
-MT :/MT link with LIBCMT.LIB
-MD :/MD link with MSVCRT.LIB
-ML :/ML link with LIBC.LIB , used by vb6 , not needed to compile
-MTd :/MTd link with LIBCMTD.LIB debug lib
-MDd :/MDd link with MSVCRTD.LIB debug lib
-MLd :/MLd link with LIBCD.LIB debug lib
-EHa : /EHa enable C++ EH (w/ SEH exceptions)
-EHs : /EHs enable C++ EH (no SEH exceptions)

-loopopt : ?? Posibly loop expanding optimization
-Loop# : ?? Possibly how much to expand.. (the param..)

-vol# :?? takes one par...
-il# :Complier Intermediate File (???) input , must exist , needed for compilation
-isize# :?? takes one par..
-ide : ?? runing from ide?? never used from VB6 ide tho...

-Fd# :/Fd[file] name .PDB file
-f# :File to Compile , -f "filename" /-f"Filename"
-ef# :?? takes one par,seems to have no effect
-off#:?? takes one par, seems to have no effect
-db# :?? taes one par , debug maby? , seems to have no effect
-W# :possibly warning level...
-O# :/O[opts] where "opts" is a string containing the following:
a relax alias checking
g enable global optimizations and global common expressions
p improve consistency in floating-point calculations
s optimize for size
t optimize for speed (default)
w assume no aliasing except across function calls

-A# : All these are legal
/AT Tiny Memory Model; combined code and data limited to 64K
/AS Small Memory Model; code and data limited to 64K each
/AM Medium Memory Model; data is limited to 64K
/AC Compact Memory Model; code is limited to 64K
/AL Large Memory Model; unlimited code and data but arrays are limited to 64K
/AH Huge Memory Model; unlimited code, data and array size

-G# :All these are legal
/G3 optimize for 80386
/Gh enable _penter function call
/G4 optimize for 80486
/G5 optimize for Pentium
/G6 optimize for PPro, P-II, P-III
/GX[-] enable C++ EH (same as /EHsc)
/GA optimize for Windows Application : ok too
/Gf enable string pooling
/Gy separate functions for linker :This is used by Vb6
/GZ Enable stack checks (/RTCs)
/Gs[num] control stack checking calls

-fastcap:??? it has effect on the ouput
-stkfill#:???
-Gt# : /Gt[num] places data items larger than "num" bytes into different
segments. The default value for "num is 256 bytes.

-Gs# :/Gs[num] control stack checking calls , Gs4096 by defualt
-cbstring

I hope you like it :)

Dr Memory · Expert Joined: 16 Aug 2004 Posts: 147 Location: Surrey, UK

I hadn't spotted "ehopt" before - sounds like ExceptHandling maybe? ... sounds intriguing ...

You have to remember many of these options are not really changeable - C2 was a real C compiler, the original MVC (?) compiler maybe, which they adopted a copy of and performed some sort of surgery

-Z options

Suggest don't mess with them - your OBJ files will probably not link properly, as they can affect naming conventions

-basic, -il

Crucial, that tells C2 it's VB who is calling!

The IL files contain the data tables needed for C2 to embed the correct table data - COM, DB links, Line numbering for RESUME support, etc etc

Again, unwise to touch

However, C programmers might be able to tell us (or somebody can try), whether C2 can still compile C if we leave off the -basic option ???

/G options

A 1-1 correspondence with the control panel, so that all makes sense, you can control all the GUI options basically, from the command line, which is the ideal situation.

/AH ????

Now, have I been asleep? Are they hiding the "Huge" model from us for a reason? Or did it not go according to plan?

Dr Memory · Expert Joined: 16 Aug 2004 Posts: 147 Location: Surrey, UK

Tsk,... by "VB calling", I mean "VB executing", of course ....

drk||Raziel · New User Joined: 25 Aug 2004 Posts: 8 Location: Athens

Calling Vb's C2 without -basic makes it go on an infinitive loop...

VB's C2 is not so similar with C C2.dll...
The compiler backend is the same
[Code generation , Asm level Optimizarion,Register managment]

The compiler Frontend is a lot diferent
[Language Syntax/Gramar , Higth Level opts (Very important) , Type conrvetions , OOP handing , exeptions ect..]

That's the reson that vb suffers from performance... Bad front end that can make *bad* code run realy slow + Slow runtime dll's

On C the first pass is made by cl.exe , c1.dll, c1xx.dll and the code generation is made by c2.dll [as far as i know , mauby wrong..]

VB's c2 is a subset of C's c2.dll + a buggy, fast made parser/fisrt pass analyser... This causes the problems with the listings too..

If i have time i will do some real RE on this .. but i think that it is not worth... it would be better to write a VB frontend for GCC and a new runtime Lib... waht do you think??

*Edit*
The Zx seems to work well .. at least , the exe compiles...
i don't have IDA rigth now , i'l test it later

Dr Memory · Expert Joined: 16 Aug 2004 Posts: 147 Location: Surrey, UK

VB/GCC, now that's exotic!

That's one form of C I do actually have somewhere ...

Dr Memory · Expert Joined: 16 Aug 2004 Posts: 147 Location: Surrey, UK

Yes, it's not hard to write good code - you can emulate RtlMoveMemory with conventional code and get a 6 to 4 performance ratio, not bad for a HLL, especially with all its other talents

I wonder what % of the millions of compilations done every day are using all default options?