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Re: Correct line/column numbers in byte compiler messages

From: Andrea Corallo
Subject: Re: Correct line/column numbers in byte compiler messages
Date: Sat, 21 Mar 2020 16:28:55 +0000
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)

Alan Mackenzie <address@hidden> writes:

> Hello, Andrea.
> On Sat, Mar 21, 2020 at 11:22:03 +0000, Andrea Corallo wrote:

>> The outcome as I see it is that total bootstrap time gets bigger 1.1x
>> while normal runtime appears not affected.
> Well, it looks like the normal runtime is around 2.x% slower for
> scratch/accurate-warning-pos.

Well I studied physics so for me 2% is pretty much zero :) :) Joking
apart I'm not sure this is really sufficient to conclude is noise or

>> For my quick understanding of how it works this is expected.  The
>> additional branch and compare against symbols_with_pos_enabled in `eq'
>> is a kind of branch that is very easily predictable by any modern CPU,
>> therefore when the feature is off (not compiling) it becomes transparent
>> (I'd see a compiler branch hit there too).
> In other words, the processor will test symbols_with_pos_enabled
> simultaneously with starting the continuation for the "not" case.

The processor will just speculate guessing the target branch without
having to wait for symbols_with_pos_enabled value to be loaded.  Given
this change rarely, speculation there should be pretty much always

I'd wrap symbols_with_pos_enabled into something like:

   __builtin_expect(symbols_with_pos_enabled, 0) 

To make sure we minimize instruction cache overhead too.

> This extra test in the EQ code was always the main thing in the slowdown
> occurring in this git branch.

Is the EQ overhead the main/only one?  Also GC seems marginally affected.

I think would be interesting to write a nano benchmark EQ focused to
test this accurately.

> When I timed things back in 2018, I got a slowdown of somewhat more than
> 2.x%.  May I ask what sort of processor you're using?  Mine (unchanged
> since then) is an AMD Ryzen.

I did the test on a "Xeon E5-1660 v3".  I think we can classify it as a
good system from few (6?) years ago.  Not very fast by today's standards
but still quite beefy in terms of caches.




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