[LLVMdev] A faster instruction selector?

[Mark Shannon]

> On Dec 17, 2008, at 4:53 AM, Mark Shannon wrote:
>> Choosing the fast selector does speed code-generation by almost  
>> double,
>> when using llc, but the reduction in final code speed is obviously a
>> downside.
> [...]
>> Since my toolkit generates an interpreter, I am able to just compile
>> hotspots, so final speed of compiled code is quite important.
> 
> I recommend doing some measurements here rather than
> guessing.  I note that you've posted several numbers comparing
> compile times with lcc, but no numbers comparing the quality of
> the generated code yet :-).

Of course, llvm generated code is faster :)
But lcc doesn't do many optimisations and uses a fairly simple local 
register allocator.
However, the lburg instruction selector is optimal (for trees, not for 
DAGs) and very fast. It was a fast instruction selector for llvm
that I was hoping for, the optimisation passes and register allocator 
are already quite fast.

>>> Do you have time to do more detailed profiling? It might be  
>>> interesting
>>> to see which parts of codegen are hot in your use cases.
>> Unfortunately I could get any meaningful information from valgrind  
>> as to
>> where llc is spending its time (Unknown code: 45%), but I suspect the
>> relative slowness is caused by the number of intermediate data
>> structures required and the time spent in creating them.
> 
> On some hosts LLVM defaults to being built with -fomit-frame-pointer,
> and this interferes with some performance tools, so that's something
> worth checking.
I'll rebuild llvm with symbols and frame-pointer and try to get some 
useful profile data. I get back to you when I've done so.
Probably in a week or two.

> 
> This would also be a good time to sanity check that you're using an
> optimized build of LLVM, which can make a tremendous difference.
> FWIW, the default mode for LLVM Makefiles is non-optimized.
> 
Yes, I am running the optimised version

>> Finally, it must be possible to select the register allocator for the
>> JIT using the API, but I am unable to find out how to do this, any  
>> ideas?
> 
> See RegAllocRegistry.h and RegisterRegAlloc::setDefault.
Thanks

> 
> Dan
>  

Seasons Greetings,
Mark.