[LLVMdev] runtime optimizations in LLVM

[Gordon Henriksen]

On 2008-07-07, at 01:12, Kamal R. Prasad wrote:

> On Thu, 1/3/08, Gordon Henriksen <gordonhenriksen at mac.com> wrote:
>>
>
>> On Jan 2, 2008, at 05:57, Kamal R. Prasad wrote:
>>
>>> Can someone provide info on what runtime optimizations are done in  
>>> LLVM?
>>
>> You can use any of LLVM's optimizing transformations in a JIT  
>> context. There's a list here:
>>
>> http://llvm.org/docs/Passes.html
>>
>> Unlike Java, you're in the driver's seat. You can determine which  
>> optimizations are applied to your code, and when.
>
> yes -but can you tell me which optimizations have been proven to be  
> more effective when applied at run time i.e. when input data is  
> available?

LLVM's 'opt' tool supports a -std-compile-opts flag which runs a  
generic list of optimizations; this is a good starting point for  
aggressive optimizations, and you can access the same list  
programmatically. The problem of pass ordering is a nontrivial one,  
however, and only you can decide what is the best tradeoff for your  
application. Luckily, experimentation with passes is not especially  
difficult.

>>> do you have something along lines of JVM's hotspot feature?
>>
>> You can recompile a function in the JIT at any time.
>>
>
> I am interested in optimizing code that was compiled using a C/C++  
> compiler. So, I will have binaries but probably not the source code.

You must have LLVM IR (possibly in the form of bitcode files) to  
optimize using LLVM; LLVM cannot re-optimize native binaries. Search  
the archives for llvm-qemu to find information on the topic of binary  
translation/optimization using LLVM.

However, you can certainly reoptimize bitcode files without access to  
the source code.

>>> How efficient is the optimized code vis-a-vis native code?
>>
>> It is native code! :) LLVM is also an excellent static compiler  
>> (see llvm-gcc, clang).
>>
>
> can I pull out the backend part which is used for runtime  
> optimization?


These are the exact same components used for static optimization.  
Certainly, however, you can link with subsets of LLVM in order to  
reduce the size of your binary.

— Gordon