Tail-Call style for interp.c by timo · Pull Request #1999 · MoarVM/MoarVM

timo · 2026-04-24T17:00:56Z

The suggestion to give moarvm an interpreter in tail-call style with the musttail attribute (going to be called return goto in a future standard of C) which was a very nice performance boost for some interpreters out there. @MasterDuke17 pointed it out already in april of 2021 (see refs below)

This is the first stab at allowing interp.c to be compiled into that kind of interpreter, without changing a huge amount of the code inside it.

Refs:

timo · 2026-04-24T21:19:09Z

I'm not entirely happy with how the signature of the op functions isMVMuint8 **arg_cur_op, MVMuint8 **arg_bytecode_start, MVMRegister **arg_reg_base, MVMCompUnit **arg_cu, i.e. it's passing a pointer to these variables that live in the calling stack frame around between calls, so I think it keeps having to dereference over and over, which could explain that the performance doesn't seem so fantastic.

Here's a compiler explorer snippet where I tried to just YOLO it and just take the address of these variables on the stack inside the first frame that tail-calls, and asan immediately complains about accessing the data after the stack frame it's from has disappeared (I was hoping tail-call here means the arguments live on the stack and the frames are "compatible" in that way).

https://godbolt.org/z/P717413Gc

lizmat · 2026-04-25T12:01:41Z

For me on MacOS on Apple silicon, this is not an improvement YET:

Building rakudo before:

Stage start      :   0.000
Stage parse      :  23.644
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   3.177
Stage mast       :   4.813
Stage mbc        :   0.828
+++ Compiling	blib/CORE.d.setting.moarvm
The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :   0.144
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   0.009
Stage mast       :   0.017
Stage mbc        :   0.003
+++ Compiling	blib/CORE.e.setting.moarvm
The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :   0.964
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   0.122
Stage mast       :   0.203
Stage mbc        :   0.017

after:

The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :  26.049
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   3.475
Stage mast       :   5.242
Stage mbc        :   0.956
+++ Generating	gen/moar/BOOTSTRAP/v6d.nqp
+++ Compiling	blib/Perl6/BOOTSTRAP/v6d.moarvm
+++ Compiling	blib/CORE.d.setting.moarvm
The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :   0.147
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   0.009
Stage mast       :   0.018
Stage mbc        :   0.003
+++ Generating	gen/moar/BOOTSTRAP/v6e.nqp
+++ Compiling	blib/Perl6/BOOTSTRAP/v6e.moarvm
+++ Compiling	blib/CORE.e.setting.moarvm
The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :   1.023
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   0.136
Stage mast       :   0.215
Stage mbc        :   0.019

lizmat · 2026-04-27T23:12:53Z

It feels a little better than before, but still doesn't beat the old way just yet:

The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :  25.293
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   3.091
Stage mast       :   5.035
Stage mbc        :   0.930
+++ Compiling	blib/CORE.d.setting.moarvm
The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :   0.144
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   0.009
Stage mast       :   0.018
Stage mbc        :   0.003
+++ Compiling	blib/CORE.e.setting.moarvm
The following step can take a long time, please be patient.
Stage start      :   0.000
Stage parse      :   0.999
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   0.131
Stage mast       :   0.235
Stage mbc        :   0.019

And the spectest feels a lot slower as well:

Files=1361, Tests=120339, 75 wallclock secs ( 9.34 usr  3.40 sys + 893.59 cusr 71.28 csys = 977.61 CPU)

which is in the old way:

Files=1361, Tests=120339, 72 wallclock secs ( 8.22 usr  3.05 sys + 860.92 cusr 69.16 csys = 941.35 CPU)

MasterDuke17 · 2026-04-28T22:07:35Z

These are in Kubuntu Asahi on my MacBook Air M2, while plugged in.

With GCC 15.2.0.
main (gcc):

dan@athena:~/r/rakudo$ taskset -c 4 hyperfine --shell=none '/home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting'
Benchmark 1: /home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting
  Time (mean ± σ):      2.288 s ±  0.012 s    [User: 2.269 s, System: 0.015 s]
  Range (min … max):    2.266 s …  2.305 s    10 runs

branch (gcc):

dan@athena:~/r/rakudo$ taskset -c 4 hyperfine --shell=none '/home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting'
Benchmark 1: /home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting
  Time (mean ± σ):      2.377 s ±  0.051 s    [User: 2.356 s, System: 0.015 s]
  Range (min … max):    2.335 s …  2.498 s    10 runs

With Clang 20.1.8.
main (clang):

dan@athena:~/r/rakudo$ taskset -c 4 hyperfine --shell=none '/home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting'
Benchmark 1: /home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting
  Time (mean ± σ):      2.301 s ±  0.025 s    [User: 2.275 s, System: 0.014 s]
  Range (min … max):    2.262 s …  2.331 s    10 runs

branch (clang):

dan@athena:~/r/rakudo$ taskset -c 4 hyperfine --shell=none '/home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting'
Benchmark 1: /home/dan/r/install/bin/moar --libpath=/home/dan/Source/raku/rakudo/blib --libpath=/home/dan/r/install/share/nqp/lib rakudo.moarvm --setting=NULL.e --ll-exception --optimize=3 --target=mbc --stagestats --output=blib/CORE.e.setting.moarvm gen/moar/CORE.e.setting
  Time (mean ± σ):      2.382 s ±  0.021 s    [User: 2.365 s, System: 0.016 s]
  Range (min … max):    2.341 s …  2.417 s    10 runs

MasterDuke17 · 2026-04-28T22:09:46Z

But wow, it's cool seeing the interpreter functions in perf!

so that we can have interp.c as musttail and program.c as Computed Goto

timo force-pushed the tail_call_interpreter branch from 5d28616 to 48aebe7 Compare April 27, 2026 22:01

timo added 3 commits April 30, 2026 22:52

Tail-Call style for interp.c

2489c46

Split MVM_CGOTO into "has feature" and "use for interp"

f5bd647

so that we can have interp.c as musttail and program.c as Computed Goto

Fix probing for musttail and preserve_none

0c56bf5

timo force-pushed the tail_call_interpreter branch from 6e1c5a6 to 0c56bf5 Compare April 30, 2026 20:53

coke force-pushed the tail_call_interpreter branch from 90fc7e2 to 0c56bf5 Compare June 2, 2026 23:31

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Tail-Call style for interp.c#1999

Tail-Call style for interp.c#1999
timo wants to merge 3 commits into
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timo commented Apr 24, 2026

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lizmat commented Apr 27, 2026 •

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