Followup on Firefox 85 for POWER: new low-level fix

Shortly after posting my usual update on Firefox on POWER, I started to notice odd occasional tab crashes in Fx85 that weren't happening in Firefox 84. Dan Horák independently E-mailed me to report the same thing. After some digging, it turned out that our fix way back when for Firefox 70 was incomplete: although it renovated the glue that allows scripts to call native functions and fixed a lot of problems, it had an undiagnosed edge case where if we had a whole lot of float arguments we would spill parameters to the wrong place in the stack frame. Guess what type of function was now getting newly called?

This fix is now in the tree as bug 1690152; read that bug for the dirty details. You will need to apply it to Firefox 85 and rebuild, though I plan to ask to land this on beta 86 once it sticks and it will definitely be in Firefox 87. It should also be applied to ESR 78, though that older version doesn't exhibit the crashes to the frequency Fx85 does. This bug also only trips in optimized builds.