[PATCH] gas: Improve documentation for cfi_remember/restore_state

[Martin Galvan <martin dot galvan at tallertechnologies dot com>]

The previous documentation for these CFI directives wasn't very helpful, so I
reworded it and added an example. Any feedback is welcome.

I have a company-wide copyright assignment for binutils, but I don't have write
access so someone else should commit this for me. Thanks!

gas/ChangeLog:
2016-04-13  Martin Galvan  <martin.galvan@tallertechnologies.com>

	* doc/as.texinfo (.cfi_remember_state, .cfi_restore_state): Improve
	documentation.
---
 gas/doc/as.texinfo | 38 +++++++++++++++++++++++++++++++++-----
 1 file changed, 33 insertions(+), 5 deletions(-)

diff --git a/gas/doc/as.texinfo b/gas/doc/as.texinfo
index 7b36931..01fc17d 100644
--- a/gas/doc/as.texinfo
+++ b/gas/doc/as.texinfo
@@ -4816,11 +4816,39 @@ 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
+@code{.cfi_*} directives that alter the following rows only under certain
+conditions.  For example, we could have something like this:
+
+@smallexample
+        jne  label
+        addq $42, %rsp
+        .cfi_adjust_cfa_offset -42
+        ret
+label:
+        /* Do something else */
+@end smallexample
+
+Here, we want the @code{.cfi} directive to affect only the row corresponding to
+the @code{ret} instruction, but not the code after @code{label}.  To do this we
+can write:
+
+@smallexample
+        .cfi_remember_state
+        jne  label
+        addq $42, %rsp
+        .cfi_adjust_cfa_offset -42
+        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 @{.cfi_adjust_cfa_offset}.
 
 @subsection @code{.cfi_return_column @var{register}}
 Change return column @var{register}, i.e. the return address is either
-- 
2.8.1