RE: [Caml-list] Why is some code compiled with 4.04.0 or 4.05.0 running 2.3 times slower than the same code compiled with 4.03.0?

[David Allsopp <dra-news@metastack.com>]

Berke Durak wrote:
> I compiled and ran a simple matrix-multiplication-style benchmark
> program written in imperative style with Bigarrays (see below) 
> using versions 4.01.0, 4.03.0, 4.03.0+flambda, 4.04.0+trunk+flambda
> (with -version 4.04.0+dev9-2015-09-05), 4.04.0+beta2+flambda 
> (4.04.0+beta2) and 4.05+trunk+flambda (4.05.0+dev0-2016-08-01).
>
> Execution time increased by a factor of 2.3 from 4.03+flambda to
> 4.04+trunk+flambda.  I tried a few of the newer optimization 
> switches (-rounds X, -unboxed-types, -unbox-closures, etc.) but
> that didn't make a significant difference. 
>
> Execution times, repeatable within ~5%:
>
> 4.01.0                5.45s
> 4.03.0                4.28s
> 4.03.0+flambda        4.44s
> 4.04.0+trunk+flambda  10.45s
> 4.04.0+beta2+flambda 10.72s
> 4.05.0+trunk+flambda  10.36s

Note that this is specifically an flambda problem - neither 4.04 nor trunk exhibit the slowdown with flambda disabled.

I've bisected and identified commit f7dcb as the problem (it was added to trunk after 4.03 was branched). The specific issue is the change in https://github.com/ocaml/ocaml/blob/trunk/bytecomp/simplif.ml#L473 disabling conversion of refs to mutable variables when flambda is enabled. If you remove the `&& Config.flambda = false` and rebuild ocamlopt, you should find your benchmark speed restored. flambda is supposed to be performing that optimisation itself, so something is clearly (very) wrong!

Could you open a Mantis PR for this, please?


David