The x86-64 path searched for free memory near the code, and if it found some,
committed to it - if that VirtualAlloc failed, ptr was left null and we returned
null, never reaching the else branch that exists precisely to say "can still run,
thanks to RipAccessible".
Finding a free region isn't the same as being able to reserve it. VirtualAlloc
rounds a non-null lpAddress down to the 64K allocation granularity while
SearchForFreeMem only guarantees page alignment, so the rounded-down base can land
back inside a committed region; a concurrent allocation between the VirtualQuery and
the VirtualAlloc does it too. Callers don't check the result - AllocCodeSpace stores
it unchecked and the emitters write from there - so this turned into a wild write
rather than a clean JIT-unavailable fallback.
Henrik Rydgård
f094752f21