1830934 - Add command line flags to Firefox for JS engine options

Status: NEW

(Core :: JavaScript Engine, enhancement, P3)

Description

[Markus Stange [:mstange]]

(I'm filing this in the JS engine component rather than in Toolkit :: Startup and Profile System because I figure this component reaches the more interested audience.)

When running the Spidermonkey shell, you can pass arguments which change its behavior, for example:

% js --enable-ic-frame-pointers --only-inline-selfhosted --spectre-mitigations=off --no-async-stacks index.js

But when running Firefox, passing the same flags is harder. You can get an approximation like this:

% JIT_OPTION_enableICFramePointers=true JIT_OPTION_onlyInlineSelfHosted=true \
JIT_OPTIONS_spectreIndexMasking=false JIT_OPTIONS_spectreObjectMitigations=false \
JIT_OPTIONS_spectreStringMitigations=false JIT_OPTIONS_spectreValueMasking=false \
JIT_OPTIONS_spectreJitToCxxCalls=false firefox

but this is quite unwieldy, and environment variables are harder to use than command line flags. Also, for --no-async-stacks, you'd additionally have to set the pref javascript.options.asyncstack to false on about:config.

It would be great if you were able to do this instead:

% firefox --js-shell="--enable-ic-frame-pointers --only-inline-selfhosted --spectre-mitigations=off --no-async-stacks"

Chrome has a --js-shell argument that does this.

Comment 1

[Jeff Muizelaar [:jrmuizel]]

Having to use environment variables is especially painful on Windows where you need to do:

set JIT_OPTION_enableICFramePointers=true && set JIT_OPTION_onlyInlineSelfHosted=true && \
set JIT_OPTIONS_spectreIndexMasking=false && set JIT_OPTIONS_spectreObjectMitigations=false && \
set JIT_OPTIONS_spectreStringMitigations=false &&  set JIT_OPTIONS_spectreValueMasking=false && \
set JIT_OPTIONS_spectreJitToCxxCalls=false

Comment 2

[Matthew Gaudet (he/him) [:mgaudet]]

This seems possible, mostly, but would be a good chunk of work with a careful scalpel.

Might be better to spend time on cleaning up our option parsing story before tackling this