The case for more macro-instructions?

[Ludovic Courtès]

Hi Andy and all,

Looking at the disassembly of -O1 code in a quest for more concise
bytecode¹ (a quest that’s not necessarily always relevant but probably
is at -O1), I noticed a few things:

  1. Code for free variable lookup, emitted by
     ‘emit-cached-toplevel-box’, is too large (~7 instructions per
     variable) for little in return.

  2. The ‘.data’ section is surprisingly large: for each symbol in the
     source, we end up in that section with a string, a stringbuf
     (pointing to contents in the ‘.rodata’ section), and a symbol.
     More on that below.

  3. ‘*lcm-page-size*’ is set to 64 KiB for the purposes of reducing the
     number of .go variants needed under prebuilt/.

     Should we default to sysconf(_SC_PAGESIZE) and use that common
     denominator only when building .go files under prebuilt/ (this
     requires adding a compiler flag to choose a different alignment)?

     (In the meantime, I changed the linker to create sparse files in
     commit 112b617f5921c67b4b2c45aae39f54cccd34d7ef.)

Regarding ‘.data’, look:

--8<---------------cut here---------------start------------->8---
$ echo sym > /tmp/t.scm
$ ./meta/uninstalled-env guild compile /tmp/t.scm -o /tmp/t.go
wrote `/tmp/t.go'
$ readelf -a /tmp/t.go |grep -A10 "^Section Headers"
readelf: Warning: [ 5]: Link field (0) should index a string section.
readelf: Warning: local symbol 0 found at index >= .symtab's sh_info value of 0
readelf: Warning: local symbol 1 found at index >= .symtab's sh_info value of 0
Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .guile.procprops  PROGBITS         0000000000000000  00010588
       0000000000000000  0000000000000000           0     0     8
  [ 2] .rodata           PROGBITS         0000000000000158  00000158
       0000000000000014  0000000000000000   A       0     0     8
  [ 3] .data             PROGBITS         0000000000010000  00010000
       0000000000000058  0000000000000000  WA       0     0     8
--8<---------------cut here---------------end--------------->8---

That’s 88 bytes for ‘.data’.

If ‘lookup-bound’ would take a string (instead of a symbol) like
‘lookup-bound-private’ and ‘lookup-bound-public’ do, we’d save
relocations and space.

Perhaps these instructions (or rather variants thereof) could even take
a raw UTF-8 buffer instead?

As for #1, I’m not sure what the best option is.  I initially thought
about adding a new macro-instruction, but then we’d lose on cache-hit
path, which is not good.

Thoughts?

Ludo’.

¹ https://issues.guix.gnu.org/70398