llvm dev - Aug 2014 - [LLVMdev] Advantages of LLVM Optimization

Hi,

I am planning to use LLVM in my compiler dev project. I would like to know
what power LLVM code optimization phase provides to my code ?

I had a look at http://llvm.org/docs/Passes.html which describes the
optimization passes.
>From a newbie standpoint can you please explain the power of LLVM
optimization?

Thank you,
Prakash
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Adding to my previous question.
Can you also kindly tell me the list of languages which purely use LLVM as
its backend instead of its own ?

I would like to know the languages which does not have its own backend
optimizer and code generator and bets it on LLVM only.

Thanks for your help.

Thank you,
Prakash


On Thu, Aug 7, 2014 at 11:56 AM, Prakash Premkumar <prakash.prax at
gmail.com>
wrote:
> Hi,
>
> I am planning to use LLVM in my compiler dev project. I would like to know
> what power LLVM code optimization phase provides to my code ?
>
> I had a look at http://llvm.org/docs/Passes.html which describes the
> optimization passes.
> From a newbie standpoint can you please explain the power of LLVM
> optimization?
>
> Thank you,
> Prakash
>
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Hi,

I had a look at http://llvm.org/docs/Passes.html which describes
the
> optimization passes.
> From a newbie standpoint can you please explain the power of LLVM
> optimization?
>
There are different levels of answers to your question. I will give it a
try, but it's certainly no substitute for reading the docs. LLVM is a
complex system...

- What effect do compiler optimizations have?
In general, they make your program faster. To get a feeling for that, I
recommend you take your favorite C/C++ program, compile it with the
'-O0',
'-O1', '-O2' and '-O3' flags, and run each of the
versions.

- What do compiler optimizations do?
Many optimizations in LLVM are classical, and found in similar form in
other compilers. Examples are:
* Instcombine: knows about many arithmetic simplifications, e.g., n*1 => n
* DCE: Dead code elimination removes values that are computed, but never
needed
* SimplifyCFG: Removes unreachable code, and converts things such as if (a)
x; if (a) y; => if (a) { x; y; }
Other instrumentations are more specific to the way the LLVM intermediate
representation works. For example, SROA tries to put values in registers
instead of having them in memory, thereby reducing the number of memory
accesses.

- What does LLVM actually do to optimize programs?
I recommend you to have a look at the PassManagerBuilder
<http://llvm.org/docs/doxygen/html/PassManagerBuilder_8cpp_source.html#l00140>.
It select the optimization passes that LLVM actually uses, and determines
their order. This will give you a good feeling for the inner workings of
LLVM.


Can you also kindly tell me the list of languages which purely use LLVM
as
> its backend instead of its own ?
>
Personally, I've used LLVM with C/C++. The rust compiler also translates to
LLVM
<http://tomlee.co/2014/04/03/a-more-detailed-tour-of-the-rust-compiler/>.
There is a frontend for the Go language <https://github.com/go-llvm/llgo>
as well, are certainly others.

Hope this helps,
Jonas
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