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.cfi_remember_state/.cfi_restore_state documentation

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* doc/as.texinfo (.cfi_remember_state, .cfi_restore_state): Improve
	documentation.
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Martin Galvan 2016-04-19 09:16:09 +09:30 committed by Alan Modra
parent eabc9d9f96
commit 48eac74cb7
2 changed files with 53 additions and 5 deletions
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@ -1,3 +1,8 @@
2016-04-19 Martin Galvan <martin.galvan@tallertechnologies.com>
* doc/as.texinfo (.cfi_remember_state, .cfi_restore_state): Improve
documentation.
2016-04-17 Andrew Burgess <andrew.burgess@embecosm.com>
Revert prevous change.

53
gas/doc/as.texinfo
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@ -4816,11 +4816,54 @@ From now on the previous value of @var{register} can't be restored anymore.
Current value of @var{register} is the same like in the previous frame,
i.e. no restoration needed.
@subsection @code{.cfi_remember_state},
First save all current rules for all registers by @code{.cfi_remember_state},
then totally screw them up by subsequent @code{.cfi_*} directives and when
everything is hopelessly bad, use @code{.cfi_restore_state} to restore
the previous saved state.
@subsection @code{.cfi_remember_state} and @code{.cfi_restore_state}
@code{.cfi_remember_state} pushes the set of rules for every register onto an
implicit stack, while @code{.cfi_restore_state} pops them off the stack and
places them in the current row. This is useful for situations where you have
multiple @code{.cfi_*} directives that need to be undone due to the control
flow of the program. For example, we could have something like this (assuming
the CFA is the value of @code{rbp}):
@smallexample
je label
popq %rbx
.cfi_restore %rbx
popq %r12
.cfi_restore %r12
popq %rbp
.cfi_restore %rbp
.cfi_def_cfa %rsp, 8
ret
label:
/* Do something else */
@end smallexample
Here, we want the @code{.cfi} directives to affect only the rows corresponding
to the instructions before @code{label}. This means we'd have to add multiple
@code{.cfi} directives after @code{label} to recreate the original save
locations of the registers, as well as setting the CFA back to the value of
@code{rbp}. This would be clumsy, and result in a larger binary size. Instead,
we can write:
@smallexample
je label
popq %rbx
.cfi_remember_state
.cfi_restore %rbx
popq %r12
.cfi_restore %r12
popq %rbp
.cfi_restore %rbp
.cfi_def_cfa %rsp, 8
ret
label:
.cfi_restore_state
/* Do something else */
@end smallexample
That way, the rules for the instructions after @code{label} will be the same
as before the first @code{.cfi_restore} without having to use multiple
@code{.cfi} directives.
@subsection @code{.cfi_return_column @var{register}}
Change return column @var{register}, i.e. the return address is either