Andrew Haley <[EMAIL PROTECTED]> writes: > In practice, %ebp either points to a call frame -- not necessarily the > most recent one -- or is null. I don't think that having an optional > frame pointer mees you can use %ebp for anything random at all, but we > need to make a clarification request of the ABI.
I don't see that as feasible. If %ebp/%rbp may be used as a general callee-saved register, then it can hold any value. And permitting %ebp/%rbp to hold any value is a very useful optimization in a function which does not require a frame pointer, since it gives the compiler an extra register to use. If you want to require %ebp/%rbp to hold a non-zero value, then you are effectively saying that this optimization is forbidden. There is no meaningful way to tell gcc "this is a general register, but you may not store zero in it." It would be a poor tradeoff to forbid that optimization in order to provide better support for exception handling: exception handling is supposed to be unusual. Ian
