For testing support for CPUs without SSE3, SSE4.1, etc., we currently have JS shell command-line options (--no-sse4, --no-sse3, etc.), but currently there is no mechanism available in the browser. As we move forward with features like bug 1065339, bug 1096684, and bug 1061637, it will be increasingly important to test such features in the browser.
FYI, for ARM we already have something like this, where we either read from the system, or we read from an environment variable: http://dxr.mozilla.org/mozilla-central/source/js/src/jit/arm/Architecture-arm.cpp#185-186
I'm told environment variables are harder to write tests for, that we can write mochitests which set prefs, but it's harder to write tests which set environment variables. Also, I am unfond of environment variables in general, because they can cause effectively invisible differences in behavior.
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