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Usenet Posting #4 - Trigeminal Software, Inc. (English)


Subject: INFO: The real deal on the /Decompile switch
(Originally posted 5/22/99)
This post is to give a little historical background and info on this undocumented command line switch in msaccess.exe. To use it, you simply run:

msaccess /decompile <your database name>

and thats all there is to it. But what exactly does it do?????

VBA AND THE 11 STATES OF COMPILATION

Thats right, internally there are 11 different compilation levels between decompiled and fully compiled like you would find in an MDE. As you make objects dirty, you will decompile the project, but dirtying Module1 does not remove all the "compiled" info about Module2 or Form1, for example. The exact levels are impossible to even detect unless you have source and debugging symbols for VBA, and insanely difficult even then.... so lets just leave it as read that the yes/no question of "is it compiled?" has many subcategories under the NO answer that essentially mean its not compiled but some parts of it are kind of.

P-CODE VERSUS CANONICAL TEXT

Your code is stored in two forms, each one of which is a Stream object in the project's storage(s). One form is the canonical text that you look at and edit and such, the other is the compiled version of the code that runs. VBA must always compile before it runs, so in an app that runs you will always find p-code. And unless you are running in an MDE (where the canonical code is stripped out) you will always have the canonical text too. Any time VBA thinks that the compiled code is invalid (such as when you make a change or the binary format changes, which is so far only during beta cycles), it will "decompile" the module and then compile it again from the canonical text.

ACCESS BETAS: BINARY FORMATS, ETC.

People who were on the betas for Access 95, 97, and/or 2000 will remember the binary format issues. From build to build, changes would be made in VBA or in the Access OM which would make old compiled code invalid. Usually a crash is the best you could expect. To help fix this, some work was done to have a global way to decompile *ALL* code that is present in a project so that you do not risk having any invalid code that might crash.

**********
This is the reason the flag is there and the only reason. The command line switch is undocumented because there is never a binary format change except during betas and in internal builds.... so there is no reason to document something never intended to be used.

**********

NOW there are some positive benefits that are side effects that people have made use of:

1) SIDE EFFECT: CORRUPTED PROJECTS

Now, as a side effect, you have a way to deal with corrupt projects! You see, the canonical text is never what is corrupted, it is always some compiled portion of a project, like a module or most commonly the typeinfo of a form or report. By globally telling Access that the compiled portion should be thrown away, you get rid of whatever the bogus piece of code is. Now this kind of fix is what would have taken care of the old Access 95 vba232 page faults and other problems where Access was walking off the end of a vtable and crashing, as well as a zillion other such little problems. This is what made PSS first expose this flag for.... if a project is corrupted, this is the best way to uncorrupt it.

2) SIDE EFFECT: PROJECT SIZE

It was found that there are times where an object would be decompiled and while the Stream object from the storage would be properly invalidated, it would be orphaned and left in the storage, and then would not be cleaned  up later. There are many applications that use structured storage that have such a problem in their garbage collection... VBA/Access is just one of them, thats all. Over time, these orphaned streams will contribute some bloat to the project. People noticed that a fully compiled app would take up more space than the same app fully compiled with all objects imported to a new database..... and that is the very issue being discussed here. As you may have guessed, the /decompile switch, which invalidates *all* streams that contain compiled code, does an effective job of garbage collection and removes these orphaned streams. Thus, a /decompile /compact will make for the smallest possible size of as database.

RISKS TO DECOMPILE: WHY YOU SHOULD NOT USE IT CONSTANTLY

If you think about the mechanism, you are relying on the canonical text always being completely valid, and you are relying on the ability to globally invalidate a compiled state. If there is ever a problem in either area, /decompile will take a project that was working fine and turn it into cottage cheese. And while such bugs should not happen.... it is impossible to make a /decompile bug happen without using /decompile. They simply did not extensively test a command line switch that was never meant to be used.... nor should they have to, really.

SO, PLEASE REMEMBER that this is a very powerful technique that was added for reasons having nothing to do with any of the reasons that you may want to use it now. It may help you save an otherwise hopelessly corrupted project. But use it sparingly as you may end up in a worse situation than you started by just globally using the switch on projects that do not need it.

IF IT AIN'T BROKE, DON'T FIX IT.

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