See commit 8120bee4dc for the original
problem description and solution. That commit and a couple of related
ones had to be reverted when we found they had introduced GC-safety
regressions leading to crashes.
This commit restores the reverted code and fixes the regressions.
We're seeing race conditions which occasionally lead to assertion
failures and thus crashes, so I'm reverting these changes for now:
29309fb414e92674cb738120bee4dc
This is important on the 32-bit OS X PowerPC ABI, since the location
of the low 32-bits of a return value change depending on whether the
entire value is 64-bits or not.
