Periodic flashing in Safari (only)

[rtrussell]

This SDL2-based Newton's Cradle program was compiled with the latest version of Emscripten (3.1.31). When running in Safari, only, it suffers from 'flashing' as if the display is periodically being blanked for one field period - at least it does on my 'Apple Silicon' Mac Mini.

I don't see this effect in Chrome-based browsers or Firefox (whether desktop or Android); it's a multithreaded app so won't run at all in iOS. Even on slow platforms which don't manage to achieve the full frame rate there is no evidence of this effect.

The obvious conclusion to draw is that it is a defect in Safari, but I'm reasonably sure it didn't happen when compiled with a much earlier version of Emscripten. It's a long shot I know, but does anybody have a clue what the cause might be?

[kripken]

If no one has an idea, I'd recommend checking if there is a difference in an older version. If so then bisecting could give us an answer.

[rtrussell]

If no one has an idea, I'd recommend checking if there is a difference in an older version. If so then bisecting could give us an answer.

Sadly bisecting is not practical. As I explained in an earlier thread, the code does not run at all in a large range of Emscripten versions (something like 2.0.13 to 3.1.20 inclusive) because support for multithreading in SDL2 was broken in those versions.

It's possible that the problem arises not from Emscripten but from SDL2, which has itself undergone many changes in the same period (I've always used the version you get from -s USE_SDL=2).

As I said, asking here was a long shot. Have any Safari-specific issues ever been identified?

[kripken]

(I can't think of any rendering-specific ones, sorry. But maybe someone else will remember something.)

[rtrussell]

bisecting could give us an answer.

As it turns out that was a good suggestion, because the problem has seemingly been introduced quite recently and a bisection has allowed me to find when: Emscripten 3.1.27 does not exhibit the flashing in Safari whereas 3.1.28 does.

Also, fortunately, the bundled versions of SDL2, SDL2_ttf and SDL2_net did not change between these releases, so that presumably rules out that they are contributing to the issue.

So something changed between 3.1.27 and 3.1.28 which has resulted in the flashing in Safari (but not in any other browser I have tried, including other Mac browsers like Vivaldi).

If you want to try it yourself here are versions of the Newton's Cradle simulation built with 3.1.27 and 3.1.28. I am using Safari on an 'Apple Silicon' Mac Mini which shows the difference clearly,

[rtrussell]

So something changed between 3.1.27 and 3.1.28 which has resulted in the flashing in Safari (but not in any other browser I have tried, including other Mac browsers like Vivaldi).

Is this regression going to receive some attention? Can I provide any other information to help?

[kripken]

I think the best way to make progress here is to bisect into that range, to find the specific commit and not just the release. Once we have that commit, the problem might be obvious.

[rtrussell]

I think the best way to make progress here is to bisect into that range, to find the specific commit

I didn't realise that was possible. Can I type emsdk install <something> to install a particular commit?

I hoped that as the fault is so specific (black frames being displayed at a regular rate, which I would estimate as a little faster than 1Hz) it would be easy to guess which change was responsible. What happens inside Emscripten (i.e. the Java or Web Assembly) at that rate anyway?

If you have a Mac, have you been able to repro the fault using the two builds I linked to?

[kripken]

Yes, you can install builds to bisect. It does require large downloads in each step, but the number of steps is logarithmic so it's not that bad in practice. Here is the link again: https://emscripten.org/docs/contributing/developers_guide.html#bisecting

I really can't think of anything in Emscripten that could cause such a problem, so I don't really have any guess at this point... But maybe something in rendering code, or event loop timing..?

(I do not have a mac, sorry.)

[rtrussell]

Here is the link again: https://emscripten.org/docs/contributing/developers_guide.html#bisecting

Thanks, but the instructions there are not very clear. For example "If instead you only know Emscripten version numbers, use emscripten-releases-tags.json to find the hashes" , that means nothing to me at all! What is emscripten-releases-tags.json and how is it used? I am a retired hardware engineer, and only dabble in programming (in BASIC and a smattering of C).

I really can't think of anything in Emscripten that could cause such a problem, so I don't really have any guess at this point... But maybe something in rendering code, or event loop timing..?

Please remember that I am compiling code which runs faultlessly in Windows, MacOS, Linux (both 32-bit and 64-bit), Android and iOS. It has been run on a wide range of platforms, from very slow (e.g. a Raspberry Pi 3) to very fast. On none of those platforms has it ever exhibited anything similar to the 'flashing' I see in Safari with Emscripten 3.1.28 and later.

[kripken]

What is emscripten-releases-tags.json and how is it used?

Here is that file emscripten-releases-tags.json:

https://github.com/emscripten-core/emsdk/blob/main/emscripten-releases-tags.json

In it, if you look for 3.1.30 for example you'll see

"3.1.30": "dc1fdcfd3f5b9d29cb1ebdf15e6e845bef9b0cc1",

That means that the build hash of release 3.1.30 is dc1fdcfd3f5b9d29cb1ebdf15e6e845bef9b0cc1. Then you can do ./emsdk install dc1fdcfd3f5b9d29cb1ebdf15e6e845bef9b0cc1 to install it (you can also do ./emsdk install 3.1.30 directly, but using hashes you can install and use even intermediate builds, which is necessary for bisecting between releases).

Please remember that I am compiling code which runs faultlessly in Windows, MacOS, Linux (both 32-bit and 64-bit), Android and iOS. It has been run on a wide range of platforms, from very slow (e.g. a Raspberry Pi 3) to very fast. On none of those platforms has it ever exhibited anything similar to the 'flashing' I see in Safari with Emscripten 3.1.28 and later.

Understood, yeah, the web platform can be weird in some ways. The rendering loop ties into the JS event loop for example. Could be something there perhaps.

[rtrussell]

That means that the build hash of release 3.1.30 is dc1fdcfd3f5b9d29cb1ebdf15e6e845bef9b0cc1. Then you can do ./emsdk install dc1fdcfd3f5b9d29cb1ebdf15e6e845bef9b0cc1 to install it (you can also do ./emsdk install 3.1.30 directly, but using hashes you can install and use even intermediate builds, which is necessary for bisecting between releases).

OK, I will try a finer-grained bisection, but it may take a little while because it's quite a slow process to install/activate the new commit, do a clean build, upload to a web server and then test, even though it is only log2 of the total!

I have 'downgraded' to 3.1.27 for my release version so that my end-users are not inconvenienced by the bug. I know I could look at the release notes to see what has changed in 3.1.31, but is there anything specific I should worry about in doing that?

[kripken]

I can't think of a specific reason to worry about downgrading, unless you are using the experimental pthreads + dynamic linking support, which has seen some critical bugfixes recently.

[rtrussell]

OK, I will try a finer-grained bisection,

Sorry, I just can't figure it out. I don't have (or understand) git, so I can't use git bisect as suggested at the Developer's Guide. I was hoping to find a document which lists all the commit hashes between 3.1.27 and 3.1.28 which I could manually bisect, but I've failed to find such a document. Is there one?

[kripken]

The releases repo has the commit hashes for every build, including releases,

https://chromium.googlesource.com/emscripten-releases

For example, from before we mentioned

"3.1.30": "dc1fdcfd3f5b9d29cb1ebdf15e6e845bef9b0cc1",

Then that commit dc1fdcfd3f5b9d29cb1ebdf15e6e845bef9b0cc1 will appear in that repo. You can use git log to find the range between two releases there.

[sbc100]

Since you don't have git here are relevant commits:

3.1.27 is 48ce0b44015d0182fc8c27aa9fbc0a4474b55982 (https://github.com/emscripten-core/emsdk/blob/main/emscripten-releases-tags.json#L2)
3.1.28 is 30b9e46ddcea66e91530559379089002d8b692cf (https://github.com/emscripten-core/emsdk/blob/6305e91dee5ac2c38af6e8b77011bdf75b50d32f/emscripten-releases-tags.json#L19)

$ git log --pretty=oneline 48ce0b44015d0182fc8c27aa9fbc0a4474b55982..30b9e46ddcea66e91530559379089002d8b692cf
30b9e46ddcea66e91530559379089002d8b692cf 3.1.28 LTO RC
19871a9ea4914d63749b8d4d170e27a8854cb565 Roll emscripten from 3e1b8387691d to f11d6196dd4e (4 revisions)
f74cfa2a3296861a9980b7219f48ed84ea735e69 Roll binaryen from c79548b93a81 to 2cb5cefb6392 (1 revision)
652a3d50f8577cbce8bfa38d86d510f9d08ded65 Roll llvm-project from 19158eb7f06d to ea4be70cea85 (24 revisions)
cf45103f8016a93ac77860d6aea3b0d6187d8549 Roll llvm-project from 5ac633d2de93 to 19158eb7f06d (41 revisions)
f8fa83c0372c2d3f00b758bc7cba5b71844816d9 Roll llvm-project from e5369823bc06 to 5ac633d2de93 (35 revisions)
6e3441d6ad6e4046fc53051b5b77adc70cdfba9d Roll llvm-project from 83b3304dd2a3 to e5369823bc06 (362 revisions)
bb5d857833a1af71fad624b56e312fb93f83ecc4 Roll emscripten from 677a8d097a64 to 3e1b8387691d (9 revisions)
794313e56861552efea30d2e3ae6bc70148f5c2d Roll binaryen from e2add4b09e39 to c79548b93a81 (8 revisions)
c1ade0e9de2092b4bf3a2ee5038d4cab8b06985f Add `ninja` directory to PATH
9ceedfc724ab421aa74b1021056a555328a24a2a Explicitly set CMAKE_MAKE_PROGRAM to point to locally-installed ninja
e46a6259297002060c9f1d5125d596e5eaedf12a Roll binaryen from 145e8f9ece5a to e2add4b09e39 (1 revision)
87bd01aac0daa900f742ad61ffc5efc0204adee3 Remove unused LLVM and Binaryen tools
a7fe7397a07f681cbf4b8dc3ea0a0dfc354e37ac Roll llvm-project from 2402c46b7155 to 83b3304dd2a3 (30 revisions)
8447970ff029f22dfe33c53f389b94c4df15f385 Roll llvm-project from f7a46aa8fbc2 to 2402c46b7155 (20 revisions)
893383d917fce38c437ecc09e3b1093bdbe902df Roll v8 from a044d7325466 to da86ca8ccd8d (48 revisions)
1d4d6bef5d795f07496662ecddca9ce061f1dfe4 Roll llvm-project from 2c7827da4f5b to f7a46aa8fbc2 (32 revisions)
f5473da091e9640871123a571ad60623c16e5863 Roll emscripten from 7c9b97ade051 to 677a8d097a64 (1 revision)
ae37c6a835efa49f4885960706aedc1850353c08 Roll llvm-project from b90ebbc5bd6d to 2c7827da4f5b (38 revisions)
5b5b2a7035c35d41ada91847111b6ddfdecd94bf Specify the ninja path explicitly
839db40665e5de4aaff55a45c47e163817f7ef08 Increase the stack size of LLVM testsuite builds
de39f8e96f761e5d637ed5399c4f21ffea8e2398 Roll binaryen from 69f6235c36eb to 145e8f9ece5a (1 revision)
4e4ab25a9683a6947b3a858a2272c119518488b2 Roll emscripten from c93b3a109583 to 7c9b97ade051 (1 revision)
a8f234fa6b92cccc5f2cf90f632000efeda01042 Roll llvm-project from 798fa4b415ee to b90ebbc5bd6d (65 revisions)
66db713decb3d7b31b1616f6e724c499a877e21b Roll llvm-project from a73db7f1ad7b to 798fa4b415ee (22 revisions)
8f087c46c3b1a237af050244362f2d0c4cc3708b Roll llvm-project from 5fa43db46ef9 to a73db7f1ad7b (27 revisions)
408ffd5d6301d79b7743953618c546505c0cc3e0 Roll llvm-project from 4f33339fb4d9 to 5fa43db46ef9 (21 revisions)
530a165d9b3e3f241529bd2d01c7761fdd79b2f6 Roll v8 from 8f886969838a to a044d7325466 (2 revisions)
752043f9f8cc7d05e2d2e058088291bab566f955 Roll llvm-project from 7a194cfb327a to 4f33339fb4d9 (13 revisions)
0b248f30d163a90c61a3cb585bee30d37b4541f2 Roll llvm-project from 8be0d8fb83aa to 7a194cfb327a (9 revisions)
210a63b96ee4fad339a44a43acdc00f32c879538 Roll llvm-project from 63b63c3dcd54 to 8be0d8fb83aa (4 revisions)
c9bc396b50a8bce1382a910f7d2a2108b992358c Roll llvm-project from 1d650d4f1dd3 to 63b63c3dcd54 (4 revisions)
db3eefae32bca18d2e35ce4d823c655991cff87c Roll llvm-project from 16a72a0f8748 to 1d650d4f1dd3 (42 revisions)
09329ed8ba2a7c25694c7c804de1974952a65ced Roll v8 from be25ee73979a to 8f886969838a (4 revisions)
5d48fee3da0ffb9197a50c57c5c3193ed5eb75ee Roll llvm-project from 5ceaeed65962 to 16a72a0f8748 (6 revisions)
7b0b034cdab1177871d64f6ea91ef56602b931fe Roll llvm-project from b6a01caa64aa to 5ceaeed65962 (11 revisions)
80118106fc47873358b5731108f8a4d95a54567c Roll llvm-project from fccab9f90b03 to b6a01caa64aa (17 revisions)
7667d62651090a4dd721fc2a1e427c28ef5df3b8 Roll emscripten from ae2f8d8ab7bc to c93b3a109583 (4 revisions)
5230f7cef51d76cbd7fe26f67ea34991a73663ef Roll binaryen from 4c79085f1bc6 to 69f6235c36eb (2 revisions)
e2e035f8896f23266b83e665549d4bee0f179480 Roll llvm-project from 0b2473936d36 to fccab9f90b03 (56 revisions)
3311d61089de55c1fb8682f629cc361c424b4167 Roll emscripten from 7c3be8e4361b to ae2f8d8ab7bc (1 revision)
db6948742ff457f46f501f8a4fe5aced60fb713b Update Binaryen CMake variable name
7fdbda10257edad93d2466f96ef35dc19e282655 Roll emscripten from dd96490f2112 to 7c3be8e4361b (2 revisions)
24c6e06328dd1c1f305cca7ed54868b18d925c33 Roll llvm-project from 683b9fc7bd23 to 0b2473936d36 (24 revisions)
6b108b4ad8f0cd57d03e458df69fe8eb50625bb6 Roll v8 from 90fe7dc9ce48 to be25ee73979a (22 revisions)
65ed76b603418db8a0eb38ee301f5b1a15d039db Roll binaryen from 197b6da7afe6 to e8b41f4a92b8 (2 revisions)
bdcaf3d5d5efa0b5ca6357b30599171996316e8a Roll emscripten from 6b1e844a76c4 to dd96490f2112 (1 revision)
25be4826744e15b5558c42d6ce3123eaa1b62aeb Roll llvm-project from 1522a3b7b34b to 683b9fc7bd23 (26 revisions)
07d8c899070e5209c12e393cdc7aade167d9d722 Roll llvm-project from d85e849ff4d5 to 1522a3b7b34b (36 revisions)
6cc40ae524952ab2ab62922d3762e38cb240e599 Roll emscripten from 2d75aa804d89 to 6b1e844a76c4 (2 revisions)
3d001d441dcb8f6d31a9a5fa65594aaf21fc7907 Roll llvm-project from 6d12599fd413 to d85e849ff4d5 (65 revisions)
2c1bc71f2ca992236054f2b4581148de75080278 Roll emscripten from 4681c920d0de to 2d75aa804d89 (3 revisions)
54323a2b83ee4f06d3f7a3f952fbad272a20d623 Roll binaryen from f70bc4d6634c to 197b6da7afe6 (1 revision)
4599a25f205089e99ca769b0bee45a750cd2d24a Roll binaryen from 73b048770937 to f70bc4d6634c (1 revision)
cdb02cb91268764fb4e4832e593785297a463a81 Roll emscripten from a197a6bd0dd3 to 4681c920d0de (1 revision)
ce1ae37ef6a46f06af66dfb443fae83fae167d76 Roll llvm-project from 2ab7c7e50a85 to 6d12599fd413 (29 revisions)
38daf11b8e88457dc50f55ce25ce2cb59bf2f2ce Roll llvm-project from f82f5b0507a2 to 2ab7c7e50a85 (33 revisions)
e86af751fe4133c0494256758fd49b3bd1e3c060 Roll v8 from 1b2cef85c79f to 90fe7dc9ce48 (29 revisions)
dff6def80eda488c4ba40ff70d932ecf3db1b0e8 Roll llvm-project from 942f3ee40866 to f82f5b0507a2 (16 revisions)
4b9dbe73a1a123507eb46274528f136680320ed7 Roll llvm-project from 0689edd68717 to 942f3ee40866 (26 revisions)
20827fbe911a1aaf5df4033cb24b2cba6aa46d90 Roll emscripten from 71ee39fd6310 to a197a6bd0dd3 (1 revision)
2262e37ff93e97a763b9afb7411aacf07ffe61d6 Roll binaryen from 4f6cb8e54aa0 to 73b048770937 (1 revision)
bf424779a73e39c11bbeedb76970a4ab66058793 Roll llvm-project from 3d9e226081cf to 0689edd68717 (37 revisions)
fe79edc0a0ec540168ffc41cfbc2dcc69e12443b Roll emscripten from 6a93f7db16cd to 71ee39fd6310 (1 revision)
1aea1ecb031d5cf4c4784c2c48cc0aaf39aff1e5 Roll binaryen from fe3ad0e6319c to 4f6cb8e54aa0 (3 revisions)
e131b3b9d023a909857cbfc885e12b2567fa519e Roll emscripten from 15bf099c5fed to 6a93f7db16cd (1 revision)
4ae44fb73a4f14c1428449329cb49a835316b01e Roll llvm-project from 0e6f0b7cc383 to 3d9e226081cf (44 revisions)
d560989afefb61cc976a54c4c9e2c03579c9f43f Roll llvm-project from afa60d08a2d4 to 0e6f0b7cc383 (28 revisions)
647408b33bf0061975e51ff78431d72b940c0cb7 Roll v8 from b49cea5506ad to 1b2cef85c79f (43 revisions)
fe67b123c858aa9942fb779ec74ca3abd605dc67 Roll llvm-project from 51679dc1c9d5 to afa60d08a2d4 (11 revisions)
67826f44f0ce93c6388931a511a1d96833729ca7 Roll emscripten from 0c10ba8ea3fa to 15bf099c5fed (4 revisions)
3cb2850d4fb361bf8facc522881467da3bfa1987 Roll llvm-project from ca856fff1c0f to 51679dc1c9d5 (33 revisions)
949e0243de74c216ac7e89e2a3604ddd586e5a77 Roll binaryen from 0c97bb81325a to fe3ad0e6319c (1 revision)
8ad7b2833969ba4d69b2b3f5760d3651abb7a427 Roll emscripten from afa75f342eef to 0c10ba8ea3fa (4 revisions)
f65ed23f9ab7e79d19e083d34d2909f61a90083c Roll llvm-project from dee009d3b5c7 to ca856fff1c0f (53 revisions)
34abebf36d7a0a091e2fe17cb58c6d6317e523b6 Roll binaryen from 83cceaca5c9c to 0c97bb81325a (3 revisions)
ca82b6d1b1514c3bd31f85d978f9c2d84232766e Roll binaryen from eeb838155dbf to 83cceaca5c9c (3 revisions)