[LLVMdev] Disable optimization

Eli Friedman eli.friedman at gmail.com
Fri Nov 28 01:31:33 PST 2008

On Fri, Nov 28, 2008 at 12:36 AM, RAAD B <raad_7007 at yahoo.com> wrote:
> Hi Eli,
> I want to disable optimizations because, i'm going to implement a framework
> in JAVA for educational purposes.
> I have planned to do followings:
>     1) Create LLVM-IR and export it as Assembly (without optimizing the
> source-code)
>     2) Transform the LLVM-Assembly to a data-structure, similar to LLVM data
> structure (Module, Function, BB ...) but implemented in Java
>     3) Optimizing the code  (should be performed by students :-)  )
>     4) Transform my data-structure to LLVM-Assembly.
>     5) Generate machine code using "llvm-as"

Ah, so roughly, Java bindings to LLVM?  Interesting.  Although, IMO,
for a topic as advanced as compilers, I don't think it would be a huge
burden to require using C++.

> Could you list me the other optimizations, which are performed by front-end?

I don't know the gcc front-end too well, so this is a rough list.  The
obvious optimizations are constant folding and dead code elimination.
The front-end will also do local optimizations on expressions, like
optimizing "a+b-b" to "a".  There might also be some other small stuff
I'm not remembering, but there aren't any other big optimizations
happening at -O0.

If you really want the dumbest possible conversion into bitcode, you
can probably hack up clang (http://clang.llvm.org) to disable all
optimizations... clang generally does less optimization, and if
there's some particular optimization you want to disable, finding it
should be easier.  That said, it's probably not worth bothering; if
the point is to teach the students to write optimizers, it doesn't
really make a difference whether the code starts off slightly


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