Reboot EVAL_CTORS with the new wasm-ctor-eval #16011

kripken · 2022-01-13T20:51:45Z

This updates us to use Binaryen's new version of wasm-ctor-eval, which can now
do a lot more things, like eval just part of a function, eval to globals, etc. That plus
other changes on the emscripten side that move more things like sbrk into pure
wasm means that we can eval a lot more code.

Previously -Oz would enable EVAL_CTORS. That was pretty dangerous as
often it does not help code size. You really just need to run with the option and
then measure the code size change vs the startup speed improvement. So this
PR makes us no longer do anything automatically - you must manually build with
-s EVAL_CTORS.

A new mode EVAL_CTORS=2 is also added. This enables wasm-ctor-eval's
new --ignore-external-input flag, which ignores the environment, params to
main, etc. This is unsafe, and probably we should have separate options for
these things, but for now this seems useful for experimentation.

Tested by running all of wasm2 with EVAL_CTORS=2 enabled, and then
ignoring the failures that are expected (things reading from argv, for example).
Also I ran around 200,000 fuzzer iterations on binaryen.

Example results on ./emcc tests/hello_libcxx.cpp -O3:

mode	wasm size(bytes)
normal	136625
EVAL_CTORS=1	136616
EVAL_CTORS=2	133059

The output on the last one is:

trying to eval __wasm_call_ctors
  ...success on __wasm_call_ctors.
trying to eval main
  ...partial evalling successful, but stopping since could not eval: call import: wasi_snapshot_preview1.fd_write
  ...stopping

It completely evals the ctors, and in main it evals some stuff, until it reaches
a call to print to stdout.

Fixes #15402

sbc100 · 2022-01-13T21:02:41Z

Can you explain a little more about how that safe version (EVAL_CTORS=1) might not be desirable by default at some opt levels? How could it regress anything? I guess its could increase the size of your static data segment? Is that the only possible downside?

sbc100 · 2022-01-13T21:04:03Z

emcc.py

+    elif settings.ASYNCIFY:
+      # In Asyncify exports can be called more than once, and this seems to not
+      # work properly yet (see test_emscripten_scan_registers).
+      diagnostics.warning('emcc', 'disabling EVAL_CTORS due to asyncify')


I all these cases I think its better to just error our given contradictory settings. That is what we normally do.

kripken · 2022-01-13T21:05:38Z

Yes, that's right (unchanged from before this PR, but we didn't handle it well). It is just very possible that a small amount of code generates a large amount of changes to the data section, which ends up making the wasm potentially a lot bigger. However, that might still be useful in some cases that care more about startup time.

sbc100 · 2022-01-13T21:08:57Z

tests/test_other.py

+    second = test(3000, 1000, 2000, 0xf, 0x8f5) # noqa; 1 will succedd
+    third  = test(2000, 3000, 1000, 0xf, 0xf58) # noqa; 0 will succeed
+    print(first, second, third)
+    assert first < second and second < third, [first, second, third]


self.assertLess(first, second) self.assertLess(second, third)

sbc100 · 2022-01-13T21:10:46Z

tools/building.py

-  #   cmd += get_binaryen_feature_flags()
-  # check_call(cmd)
+def eval_ctors(js_file, wasm_file, debug_info=False): # noqa
+  WASM_CALL_CTORS = '__wasm_call_ctors'


We already have emscripten.py:WASM_INIT_FUNC can we unify these?

sbc100 · 2022-01-13T21:11:35Z

tools/building.py

+    args = ['--ctors=' + ctor, '--kept-exports=' + ctor]
+  if settings.EVAL_CTORS == 2:
+    args += ['--ignore-external-input']
+  logger.warning('ctor_evaller: trying to eval global ctors (' + ' '.join(args) + ')')


This isn't really a warning right? Maybe just print(..

Or logger.info

sbc100 · 2022-01-13T21:12:11Z

tools/building.py

@@ -1481,7 +1518,7 @@ def run_binaryen_command(tool, infile, outfile=None, args=[], debug=False, stdou
  cmd += get_binaryen_feature_flags()
  # if we are emitting a source map, every time we load and save the wasm
  # we must tell binaryen to update it
-  if settings.GENERATE_SOURCE_MAP and outfile:
+  if settings.GENERATE_SOURCE_MAP and outfile and tool in ['wasm-opt', 'wasm-emscripten-finalize']:


Only those tools support source maps. wasm-ctor-eval for example does not. I'll add a TODO

sbc100 · 2022-01-13T21:13:29Z

site/source/docs/optimizing/Optimizing-Code.rst

+EVAL_CTORS
+==========
+
+Building with ``-s EVAL_CTORS`` will evaluate as much code as possible at


I think we should skip the space after -s these day in our docs. I know we are not consistent but its more concise and compatible (especially with cmake) so I think we should prefer it.

kripken · 2022-01-14T18:04:18Z

Thanks for the comments, I pushed commits to address them now.

kripken added 30 commits November 1, 2021 15:09

re-enble

1c1c911

Merge remote-tracking branch 'origin/main' into eval_ctors

af28037

test

95b67e7

Merge remote-tracking branch 'origin/main' into eval_ctors

9315ab0

Merge remote-tracking branch 'origin/main' into eval_ctors

7657c9a

Get basic logic working again

495db3e

Merge remote-tracking branch 'origin/main' into eval_ctors

72343db

don't eval ctors by default any more

efdf2aa

temp [ci skip]

c402873

force test

4b5447a

fornow

269a69a

Merge remote-tracking branch 'origin/main' into eval_ctors2

773dcc6

work [ci skip]

df56bb8

work

d6512b4

nicer [ci skip]

ec504ba

fix

a136023

fix

279a23a

work [ci skip]

86312e9

test [ci skip].

111bfc6

restore

dced5fb

better

af0358f

work

bfd2561

test [ci skip]

bb68e12

test [ci skip]

0cd741c

fix

9a5a3cf

fix

40cc46d

todo [ci skip]

025acaa

wip [ci skip]

3fa026e

Merge remote-tracking branch 'origin/main' into eval_ctors2

f6dea85

rebaseline [ci skip]

d17a184

kripken added 8 commits January 13, 2022 06:55

comments

ff71843

text

8f5c13d

unbench

3ab442b

bench

4fd2869

cleanup

0589371

Merge remote-tracking branch 'origin/main' into eval_ctors2

b99db6b

changelog [ci skip]

28b3945

comment [ci skip]

d80f2a4

kripken requested a review from sbc100 January 13, 2022 20:51

add PR #

cf66fc8

sbc100 reviewed Jan 13, 2022

View reviewed changes

sbc100 approved these changes Jan 13, 2022

View reviewed changes

This was referenced Jan 13, 2022

Support for fast startup by restoring pre-initialized memory #15402

Closed

Experiment with re-enabling EVAL_CTORS #15403

Closed

kripken added 7 commits January 14, 2022 09:58

warnings => errors

19a7caa

testfix

2251183

unify WASM_CALL_CTORS

ef875b4

loggingfix

ae7b9e2

comment

b1707c6

-sEVAL_CTORS, no space

facd964

Merge remote-tracking branch 'origin/main' into eval_ctors2

4e720af

sbc100 approved these changes Jan 14, 2022

View reviewed changes

kripken enabled auto-merge (squash) January 14, 2022 18:10

kripken merged commit 7240c3d into main Jan 14, 2022

kripken deleted the eval_ctors2 branch January 14, 2022 19:12

kg mentioned this pull request Jan 14, 2022

WASM startup time optimization tracking issue dotnet/runtime#63809

Open

emscripten-core deleted a comment Jan 26, 2022

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Reboot EVAL_CTORS with the new wasm-ctor-eval #16011

Reboot EVAL_CTORS with the new wasm-ctor-eval #16011

kripken commented Jan 13, 2022 •

edited

Loading

sbc100 commented Jan 13, 2022

sbc100 Jan 13, 2022

kripken commented Jan 13, 2022

sbc100 Jan 13, 2022

sbc100 Jan 13, 2022

sbc100 Jan 13, 2022

sbc100 Jan 13, 2022

sbc100 Jan 13, 2022

kripken Jan 14, 2022

sbc100 Jan 13, 2022

kripken commented Jan 14, 2022

Reboot EVAL_CTORS with the new wasm-ctor-eval #16011

Reboot EVAL_CTORS with the new wasm-ctor-eval #16011

Conversation

kripken commented Jan 13, 2022 • edited Loading

sbc100 commented Jan 13, 2022

Choose a reason for hiding this comment

kripken commented Jan 13, 2022

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

kripken commented Jan 14, 2022

kripken commented Jan 13, 2022 •

edited

Loading