When is the stack pointer unreliable?

[John Wehle]

I noticed that i386 backend reloads the stack pointer in
situations where it shouldn't be necessary.  In i386.c
(ix86_epilogue) is the comment:

  /* sp is often  unreliable so we must go off the frame pointer.

Since reloading the stack pointer causes a stall on some processors
it should be avoided when possible.  Under what situations is sp
unreliable?

-- John
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