Re: JIT compilation speed

[David Malcolm <dmalcolm@redhat.com>]

On Tue, 2015-07-28 at 01:00 +0200, Basile Starynkevitch wrote:
> On 07/28/2015 00:52, Dibyendu Majumdar wrote:
> >
> >> So, for Dibyendu Majumdar: if the GCCJIT compilation time matters a lot to
> >> you in RAVI and the performance of the emitted machine code matters much
> >> less to you, I am afraid that GCCJIT is not the optimal library for your
> >> needs in RAVI (using GNU lightning or asmjit would make JIT-ing time much
> >> faster, at the expense of a produced machine code which runs 2 to 5 times
> >> slower than what GCCJIT can achieve with -O1).
> >>
> > Yes understand that - I think that libgccjit needs to be at least as
> > efficient as LLVM - that should be good enough.
> 
> 
> Given that today Clang/LLVM is significantly quicker to compile C code  
> than GCC is for -O1, that won't happen very soon. GCCJIT is GCC based, 
> so will continue to keep the heavy weight of GCC.

I don't know that -O1 in gcc vs -O1 in llvm is an "apples-to-apples"
comparison; different compilers may have a different idea of what
optimizations should be enabled at each numbered level.

> Some people within GCC are trying to make it work faster (as a C or C++ 
> compiler). When that would happen (and it might take years) GCCJIT will 
> also work faster. But don't hope much of that for this 2015 year (and 
> probably not even for 2016).
> 
> GCCJIT is GCC based, so cannot be faster than GCC is!

That said, we could tweak GCC for specific workloads.  For example, each
-O level expresses a particular combination of passes.  libgccjit could
potentially build an alternate pass_manager for a particular
gcc_jit_context, and build a specifically-tuned tree of passes,
eliminating passes that aren't wanted, and perhaps even having custom
passes.  I'm not sure how to express any of that in the API though.

> Between GCCJIT & LLVM, I guess that GCCJIT -when used with -O3- will 
> produce slightly faster machine code than what LLVM can give, at the 
> expense of a significantly slower JIT time.
> 
> Look at GCCJIT as something GCC based, without the inconvenience of 
> having to generate by yourself some C code. BTW, in my MELT experience, 
> generating C code and compiling and dlopen-ing it, can be fast enough 
> for some interactive usage.
> 
> Regards.
>