Try to emulate D language exception state save/restore on W64 context switch. #279
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Reserve extra space on the fiber stack for saving/restoring the three 64-bit pointers at GS:[0]; initialize the new slots in each new fiber stack; but do not yet attempt to save or restore the special GS:[0] block.
Unfortunately this also produces immediate segfaults.
This reverts commit 7647c2a.
This commit doesn't yet restore it.
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This hint about the D language fiber implementation points to this tactic for 32-bit Windows, and apparently to a simpler tactic for 64-bit Windows.
Specifically, it appears that for 64-bit Windows, the D fiber implementation saves and restores a three-pointer block at
GS:[0]on every context switch. The root block is initialized to contain0xFFFF'FFFF'FFFF'FFFF(next block in chain, sentinel value)respectively.
This PR attempts to adopt the same tactic for the Windows 64 fcontext implementation of Boost.Context.
Sadly, with the Boost.Context built from this PR, these three programs that switch context during exception handling behave no differently than with unmodified Boost.Context.