Re: JIT compilation speed

[Basile Starynkevitch <basile@starynkevitch.net>]

On 07/27/2015 22:29, David Malcolm wrote:
> On Sat, 2015-07-25 at 11:28 +0100, Dibyendu Majumdar wrote:
>> Hi
>>
>> Is there a timeframe for the performance improvements to the JIT
>> compilation speed?
> The patch kit I posted here:
>    https://gcc.gnu.org/ml/gcc-patches/2015-06/msg00116.html
> gives about a 5x speedup (from about 50ms down to about 10ms on my box),


Of course, I'll be happy if that patch was eventually incorporated in 
GCC. I guess that the speedup would be that big only for JIT-ing of 
*small* gcc_jit_context-s with few (e.g. only hundreds) of 
gcc_jit_object-s and using -O0 or -O1. I guess that for JIT-ing or 
larger contexts with -O2, the time for generating assembler code won't 
dominate (since it does not dominate in GCC itself), so the effect of 
the above patch would be much smaller...

However, I don't think that GCCJIT compilation time (I mean the time 
needed for GCCJIT to emit machine code from GCCJIT objects) should 
matter that much.

The point is that, by design, GCCJIT is GCC based. And GCC is known to 
compile quite slowly but to produce fast machine code. So GCCJIT also 
will continue to compile a bit slowly but can be configured -by passing 
-O2 using GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL, to produce fast machine 
code.

In that respect, GCCJIT is a bit like LLVM (the library). It won't be 
able to produce quickly some machine code, but it will be able to 
produce, rather slowly, efficient machine code.

If the generation time matters much more than the performance of the 
generated JIT-ed machine code, GCCJIT is not the right tool (and 
probably would never be, in particular because -O0 is giving code which 
is worse than what many other JIT-ing libraries do, and the GCCJIT 
infrastructure won't compete with them).

There are several JIT libraries. On one hand, you have libraries like 
GNU lightning and GNU libjit and asmjit. they all are able to emit 
*quickly* some machine code (whose runtime performance is poor, but 
which can be quickly generated). On the other hand, you have 
compiler-related libraries like LLVM & GCCJIT. they can emit quite 
performant machine code, at the cost of some quite slow JIT generation.

Ultimately I would rather hope that eventually GCCJIT would be able to 
even do, with some even more flags à la GCC_JIT_STRING_OPTION*, 
link-time-optimization by doing the equivalent of what GCC is doing 
today with -flto -O3. I cannot help on that (because LTO internals are 
unfamiliar to me) and for my interests in GCCJIT today this is not a big 
priority.

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).

If several JIT libraries (GCCJIT, LLVM, libjit, asmjit, GNU lightning) 
are co-existing, it is because they have different position in the JIT 
spectrum. On one side (GNU lightning, asmjit, libjit, ...) you have 
libraries able to emit very quickly some slow machine code. On the other 
side (LLVM, GCCJIT) you have compiler-based libraries which are emitting 
slowly some quickly running machine code. Different needs have different 
libraries! But we cannot realistically hope for a JIT library (or a C 
compiler) which emits quickly some fast running machine code. There is 
no such thing as a free lunch.

I praise David Malcolm for his good work on GCCJIT, which provides a 
GCC-based JIT-ing library able to emit (a bit slowly, since based upon 
GCC) some quickly running code, thanks to and leveraging above all the 
optimizing infrastructure of GCC. See also this (mine) explanation 
http://programmers.stackexchange.com/a/273711/40065 about why there are 
so few C compilers!

GCCJIT has also (eventually) a very good side: it enables people 
experimenting new language design to profit of GCC infrastructure 
without having to generate C code.

Regards.

-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
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