[ragel-users] MSVC-friendly -GT2 mode

[Victor Khimenko]

On Mon, May 14, 2012 at 9:12 PM, Adrian Thurston <thurston at complang.org>wrote:

> Hi Victor, I haven't looked at the patch yet, I'll hopefully do that
> tonight. It sounds similar to the 'split goto' mode I had worked on some
> years back but is now unmaintained. The machine was partitioned naively --
> psuedo optimally requires advanced heuristics. Between partitions, tables
> were used. Within a partition, directly executable was used. That sound the
> same?
>

Well, the implementations are kind-of similar, but the end goals are
different. In "split goto" mode you have few functions which call each
other which of course makes compiler happier, but does not help us because
we really want to convince compiler to keep some critical information on
registers (in our case actions fire once per 2-3 bytes, function calls or
lack of them really do a world of difference in such a case) - this is only
possible if we keep everything in one large functions (plus may be some
inlined functions which does not help the compiler at all).

My patch replaces large switches with table which contains the next state
and then does jump to _again label.This reduces number of basic blocks
about 5-7 times and makes compiler much happier and the cost is small: I
just add one table and one unconditional goto (which is easy for the CPU to
predict). Of course I remove a lot of conditional jumps which makes it
harder for the CPU to keep track of things - I think this is where 20%
slowdown comes from.

I can send you example of our .rl file and test-driver if you want to
experiment (it's open-source projects, there are nothing secret here), but
it's huge (about 3MB .rl file and 1MB test file) and the problem only
happens on Windows thus it's not very hack-friendly case.


> -----Original Message-----
> From: Victor Khimenko <khim at chromium.org>
> Sender: ragel-users-bounces at complang.org
> Date: Sun, 13 May 2012 16:35:35
> To: <ragel-users at complang.org>
> Reply-To: ragel-users at complang.org
> Subject: [ragel-users] MSVC-friendly -GT2 mode
>
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