Re: The size of ‘.go’ files

["Ludovic Courtès" <ludo@gnu.org>]

Hello!

Andy Wingo <wingo@igalia.com> skribis:

> A few points of information :)

Much appreciated!

> The guile.arities section starts with a sorted array of fixed-size
> headers, then is followed by a sequence of ULEB128 references to local
> variable names, including non-arguments.  The size is a bit perplexing,
> I agree.  I can think of a number of ways to encode that section
> differently but we'd need to understand a bit more about it and why the
> baseline compiler is significantly different.

‘.guile.arities’ size should be proportional to the number of
procedures, right?  Additionally, if there are only/mostly thunks, the
string table for argument names should be small if not empty.  For N
thunks, I would expect roughly N 28-byte headers + NxM UL128, say 100
bytes per thunk; there’s 1000 of them, so we should be ~100,000 bytes.
This is roughly what we get observe with the baseline compiler.

>> “.rtl-text” is 38% smaller and “.guile.arities” is almost a tenth of
>> what it was.
>
> The difference in the text are the new baseline intrinsics,
> e.g. $vector-ref.  It goes in the opposite direction from instruction
> explosion, which sought to (1) make the JIT compiler easier by
> decomposing compound operations into their atomic parts, (2) make the
> optimizer learn more information from flow rather than type-checking
> side effects, and (3) allow the optimizer to eliminate / hoist / move
> the component pieces of macro-operations.
>
> However in the baseline compiler (2) and (3) aren't possible because
> there is no optimizer on that level, and therefore the result is
> actually a lose -- 10 micro-ops cost more than 1 macro-op because of
> stack traffic overhead, which isn't currently mitigated by the JIT (1).
>
> So instruction explosion is residual code explosion, which should pay
> off in theory, but not for the baseline compiler.  So I added new
> intrinsics for e.g. $vector-ref et al.  Thus the smaller code size.

Yes, that makes a lot of sense.  In particular, this file must use the
struct intrinsics a lot.

> There are more possibilities for making code size smaller, e.g. having
> two equivalent encodings for bytecode, where one is smaller:
>
>   https://webkit.org/blog/9329/a-new-bytecode-format-for-javascriptcore/

Like THUMB, but for bytecode.  :-)

I guess we could first analyze the generated code more closely and see
if there are opportunities there.

Thanks for the explanations!

Ludo’.