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\startcomponent onandon-53

% copy-edited by Alan Braslau

\environment onandon-environment

\startchapter[title={From \LUA\ 5.2 to 5.3}]

When we started with \LUATEX\ we used \LUA\ 5.1 and then moved seamlessly to 5.2
when that became available. We didn't run into issues with this language version
change because there were no fundamental differences that could not be easily
dealt with. However, when \LUA\ 5.3 was announced in 2015 we were not sure if we
should make the move. The main reason was that we'd chosen \LUA\ because of its
clean design part of which meant that we had only one number type: double. In 5.3
on the other hand, deep down a number can be either an integer or a floating
point quantity.

Internally \TEX\ is mostly (up to) 32-bit integers so when we go from \LUA\ to
\TEX\ we are forced to round numbers. Nonetheless, or perhaps because of this,
one can expect some benefits in using integers in \LUA. Performance|-|wise we
didn't expect much, and memory consumption would be the same too. So the main
question then was: can we get the same output and not run into trouble due to
possible differences in serializing numbers? After all \TEX\ is about stability.
The serialization aspect is for instance important when we compare quantities
and|/|or use numbers in hashes, so one must be careful.

Apart from this change in the number model (which comes with a few extra
helpers), another interesting extension in 5.3 was that bit|-|wise operations are
now part of the language. However, the lpeg library is still not part of stock
\LUA. There is also added some minimal \UTF8 support, but less than we provide in
\LUATEX\ already. So, considering these changes, we were not in a big hurry to
update. Also, it made sense to wait until this important number|-|related change
became stable.

But, a few years later, we still had it on our agenda to test the new version of
\LUA, and after the \CONTEXT\ 2017 meeting we decided to give it a try; here are
some observations. A quick test involved just dropping in the new \LUA\ code and
seeing if with this we could still compile a \CONTEXT\ format. Indeed that was no
big deal but the test run failed because at some point a (for instance) \type {1}
became a \type {1.0}. It turned out that serializing has some side effects, and
with some ad hoc prints for tracing (in the \LUATEX\ source) I could figure out
what was going on. How numbers are seen can (to some extent) be deduced from the
\type {string.format} function, which is in \LUA\ a combination of parsing,
splitting and concatenation combined with piping to the \CCODE\ \type {sprintf}
function: \footnote {Actually, at some point I decided to write my own formatter
on top of \type {format} and I ended up with splitting as well. It's only now
that I realize why this is working out so well (in terms of performance): simple
format (single items) are passed more or less directly to \type {sprintf} and as
\LUA\ itself is fast, due to some caching, the overhead is small compared to the
built|-|in splitter method. An advantage is that the \CONTEXT\ formatter has many
more options and is also extensible.}

\starttyping
local a =  2   * (1/2) print(string.format("%s",  a),math.type(x))
local b =  2   * (1/2) print(string.format("%d",  b),math.type(x))
local c =  2           print(string.format("%d",  c),math.type(x))
local d = -2           print(string.format("%d",  d),math.type(x))
local e =  2   * (1/2) print(string.format("%i",  e),math.type(x))
local f =  2.1         print(string.format("%.0f",f),math.type(x))
local g =  2.0         print(string.format("%.0f",g),math.type(x))
local h =  2.1         print(string.format("%G",  h),math.type(x))
local i =  2.0         print(string.format("%G",  i),math.type(x))
local j =  2           print(string.format("%.0f",j),math.type(x))
local k = -2           print(string.format("%.0f",k),math.type(x))
\stoptyping

This gives the following results:

\starttabulate[|cBT|c|T|c|cT|]
\BC a \NC  2   * (1/2)\NC   s \NC 1.0 \NC float   \NC \NR
\BC b \NC  2   * (1/2)\NC   d \NC 1	  \NC float   \NC \NR
\BC c \NC  2          \NC   d \NC 2   \NC integer \NC \NR
\BC d \NC -2          \NC   d \NC 2	  \NC integer \NC \NR
\BC e \NC  2   * (1/2)\NC   i \NC 1	  \NC float   \NC \NR
\BC f \NC  2.1        \NC .0f \NC 2	  \NC float   \NC \NR
\BC g \NC  2.0        \NC .0f \NC 2	  \NC float   \NC \NR
\BC h \NC  2.1        \NC   G \NC 2.1 \NC float   \NC \NR
\BC i \NC  2.0        \NC   G \NC 2	  \NC float   \NC \NR
\BC j \NC  2          \NC .0f \NC 2	  \NC integer \NC \NR
\BC k \NC -2          \NC .0f \NC 2	  \NC integer \NC \NR
\stoptabulate

This demonstrates that we have to be careful when we need numbers represented as
strings. In \CONTEXT\ the places where we had to check for this was not that
many: in fact, only some hashing related to font sizes had to be done using
explicit rounding.

Another surprising side effect is the following. Instead of:

\starttyping
local n = 2^6
\stoptyping

we now need to use:

\starttyping
local n = 0x40
\stoptyping

or just:

\starttyping
local n = 64
\stoptyping

because we don't want this to be serialized to \type {64.0} which is due to the
fact that a power results in a float. One can wonder if this makes sense when we
apply it to an integer.

At any rate, once we were able to process a file, two standard documents were
chosen for a performance test. Some experiments with loops and casts had
demonstrated that we could expect a small performance hit and indeed, this was
the case. Processing the \LUATEX\ manual takes 10.7 seconds with 5.2 on my
5-year-old laptop and 11.6 seconds with 5.3. If we consider that \CONTEXT\ spends
about 50\% of its time in \LUA, then we find here a 20\% performance penalty
using the later version of \LUA. Processing the \METAFUN\ manual (which has lots
of \METAPOST\ images) went from less than 20 seconds (and \LUAJITTEX\ does it in
16 seconds) to up to more than 27 seconds. So there we lose more than 50\% on the
\LUA\ end. When we observed these kinds of differences, Luigi and I immediately
got into debugging mode, partly out of curiosity but also because consistent
performance is always important to us.

As these results made no sense, we traced different sub-mechanisms and eventually
it became clear that the reason behind the speed penalty was in fact that the
core \typ {string.format} function was behaving quite badly in the \type {mingw}
cross|-|compiled binary, as can be seen by this test:

\starttyping
local t = os.clock()
for i=1,1000*1000 do
 -- local a = string.format("%.3f",1.23)
 -- local b = string.format("%i",123)
    local c = string.format("%s",123)
end
print(os.clock()-t)
\stoptyping

\starttabulate[|c|c|c|c|c|]
\BC   \BC lua 5.3 \BC lua 5.2 \BC texlua 5.3  \BC texlua 5.2 \BC \NR
\BC a \NC 0.43    \NC 0.54    \NC 3.71 (0.47) \NC 0.53       \NC \NR
\BC b \NC 0.18    \NC 0.24    \NC 3.78 (0.17) \NC 0.22       \NC \NR
\BC c \NC 0.26    \NC 0.68    \NC 3.67 (0.29) \NC 0.66       \NC \NR
\stoptabulate

Both 5.2 binaries perform the same but the 5.3 \LUA\ binary greatly outperforms
the \LUATEX binary so we had to figure out why. After all, the integer
optimization should bring some gain! It took us a while to figure out what was
going wrong, and the numbers in parentheses are the results after fixing \LUATEX.

Because font internals are specified in integers one would expect a gain
in running the command:

\starttyping
mtxrun --script font --reload force
\stoptyping

and indeed that is the case. On my machine a scan results in 2561 registered
fonts from 4906 read files and with 5.2 that takes 9.1 seconds while 5.3 needs a
bit less: 8.6 seconds (with the bad cross|-|compiled format performance) and even
less once that was fixed.

For a test:

\starttyping
\setupbodyfont[modern]     \tf \bf \it \bs
\setupbodyfont[pagella]    \tf \bf \it \bs
\setupbodyfont[dejavu]     \tf \bf \it \bs
\setupbodyfont[termes]     \tf \bf \it \bs
\setupbodyfont[cambria]    \tf \bf \it \bs
\starttext \stoptext
\stoptyping

This code needs 30\% more runtime using the newer version of \LUA\ so the
question is: how often do we call \type {string.format} there? A first run (when
we wipe the font cache) needs some 715\,000 calls while successive runs need
115\,000 calls so the slow down definitely comes from the bad handling of \type
{string.format}.

When we drop in a \LUA\ or whatever other dependency update we don't want this
kind of impact. In fact, when one uses external libraries that are or can be
compiled under the \TEX\ Live infrastructure and the impact would be so dramatic,
this would be very bad advertising, especially when one considers the occasional
complaint about \LUATEX\ being slower than other engines.

The good news is that eventually Luigi was able to nail down this issue and we
got a binary that performed well. It looks like \LUA\ 5.3.4 (cross|)|compiles
badly under both \GCC\ 5.3.0 and 6.3.0.

So in the end loading the fonts takes:

\starttabulate[||c|c|]
\BC            \BC caching   \BC running \NC \NR
\BC 5.2 stock  \NC  8.3      \NC 1.2     \NC \NR
\BC 5.3 bugged \NC 12.6      \NC 2.1     \NC \NR
\BC 5.3 fixed  \NC  6.3      \NC 1.0     \NC \NR
\stoptabulate

So indeed after an initial scare it looks like 5.3 is able to speed up \LUATEX\ a
bit, given that one integrates it in the right way! The use of a recent compiler
is needed here, although one can wonder when another bad case will show up again.
One can also wonder why such a slow down can mostly go unnoticed, because for
sure \LUATEX\ is not the only compiled program integrating the \LUA\ language.
\footnote{We can only speculate that others do not pay such close attention to
performance.}

The next examples are some edge cases that show you need to be aware
that
\startitemize[n,text,nostopper]
    \startitem an integer has its limits, \stopitem
    \startitem that hexadecimal numbers are integers, and \stopitem
    \startitem that \LUA\ 5.2 and \LUAJIT\ can differ in small details: \stopitem
\stopitemize

\starttabulate[||T|T|]
\NC        \NC \tx print(0xFFFFFFFFFFFFFFFF) \NC \tx print(0x7FFFFFFFFFFFFFFF) \NC \NR
\HL
\BC lua 52 \NC 1.844674407371e+019 \NC 9.2233720368548e+018 \NC \NR
\BC luajit \NC 1.844674407371e+19  \NC 9.2233720368548e+18  \NC \NR
\BC lua 53 \NC -1                  \NC 9223372036854775807  \NC \NR
\stoptabulate

We see here that \LUA\ 5.3 clearly represents some progress.

So, to summarize the migration, a quick test was relatively easy: move 5.3 into
the code base, make slight adaptations to the internals (there were a few
\LUATEX\ interfacing bits where explicit rounding was needed), run tests, and
eventually fix some issues related to the Makefile (compatibility) and \CCODE\
obscurities (the very slow \type {sprintf}). \footnote{This demonstrates the
importance of compilers, or rather how one writes code with respect to each
compiler.}

Adapting \CONTEXT\ was also not much work, but the test suite uncovered some
nasty side effects. For instance, the valid 5.2 solution:

\starttyping
local s = string.format("02X",u/1024)
local s = string.char        (u/1024)
\stoptyping

now has to become (works with both 5.2 and 5.3):

\starttyping
local s = string.format("02X",math.floor(u/1024))
local s = string.char        (math.floor(u/1024))
\stoptyping

or (with 5.2 and emulated or real 5.3):

\starttyping
local s = string.format("02X",bit32.rshift(u,10))
local s = string.char        (bit32.rshift(u,10))
\stoptyping

or (5.3 only):

\starttyping
local s = string.format("02X",u >> 10))
local s = string.char        (u >> 10)
\stoptyping

or (5.3 only):

\starttyping
local s = string.format("02X",u//1024)
local s = string.char        (u//1024)
\stoptyping

Unfortunately, adapting a conditional section like:

\starttyping
if LUAVERSION >= 5.3 then
    local s = string.format("02X",u >> 10))
    local s = string.char        (u >> 10)
else
    local s = string.format("02X",bit32.rshift(u,10))
    local s = string.char        (bit32.rshift(u,10))
end
\stoptyping

will fail because (of course) the 5.2 parser doesn't like the 5.3 syntax
elements. In \CONTEXT\ we have some experimental solutions for this but it is
beyond the scope of this summary.

In the process of this update a few \UTF\ helpers were added to the string
library so that we have a common set for both \LUAJIT\ and \LUA\ (the \type
{utf8} library that was added to 5.3 is not very useful for \LUATEX). For now we
also keep the \type {bit32} library on board, of course, we'll not mention all
the details here.

When we consider a gain in speed of 5–10\% with 5.3 that also means that the gain
obtained using \LUAJITTEX\ compared to \LUA\ 5.2 becomes less important. For
instance in font processing both engines (\LUA\ 5.3 and \LUAJIT) now perform
roughly to the same.

As I write this, we've just entered 2018 and after a few months of testing
\LUATEX\ with \LUA\ 5.3 we're confident that we can move the code to the
experimental branch. This means that we will use this version in the \CONTEXT\
distribution and likely will ship this as 1.10 in 2019 where \LUA\ 5.3 becomes
the default. The 2018 version of \TEX~Live will have 1.07 with \LUA\ 5.2 while
intermediate versions of the \LUA\ 5.3 binary will end up on the \CONTEXT\
garden, probably with number 1.08 and 1.09 (who knows what else we will add or
change in the meantime).

\subsubject{Addendum}

Around the 2018 meeting I also started what is to become the next major upgrade
of \CONTEXT, this time using a new engine \LUAMETATEX. In working on that I
decided to try \LUA\ 5.4 to see what consequences this new version would have for
us. There are no real conceptual changes as were found with the number model in
5.3, so the tests didn't reveal any real issues. But as an additional step
towards a bit cleaner distinction between strings and numbers, I decided to
disable the automatic casting so that mixing strings and numbers in expression
for instance is no longer permitted. If I remember correctly, there was only in
one place I had to adapt the source (and we're talking about a pretty large \LUA\
code base).

There is a new mechanism in \LUA\ for freezing constants but I'm not yet sure if
it makes much sense to use it, although one of the intentions is to produce more
efficient bytecode. \footnote {Mid July 2019 some quick tests indeed show a
performance boost with the experimental code base, but if we want to benefit from
using constants, the \CONTEXT\ codebase has to be adapted, which means that those
parts no longer will work with stock \LUATEX.} It's use goes along with some
other restrictions, like the possibility to adapt loop counters inside the loop.
Inside the body of a loop one could always adapt such a variable, which (I can
imagine) can come in handy. I haven't checked the source code for that, but
probably I don't do this anywhere.

Another new feature is an alternative garbage collector which seems to perform
better when there are many variables with a short life spans. At least for now I
have decided to default to this variant in future releases.

Overall the performance of \LUA\ 5.4 is better than its predecessors which means
that the gap between \LUATEX\ and \LUAJITTEX\ is closed or is closing. This is
good because I have chosen not to support \LUAJIT\ in \LUAMETATEX.

\stopcomponent

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