[Xapian-tickets] [Xapian] #151: Use function attributes to mark functions as "const" or "pure" (was: Use function attributes to mark functions as "const", "pure", and "nothrow")

[Xapian]

#151: Use function attributes to mark functions as "const" or "pure"
-------------------------+--------------------------------------------------
 Reporter:  olly         |        Owner:  olly     
     Type:  enhancement  |       Status:  assigned 
 Priority:  normal       |    Milestone:  1.2.x    
Component:  Other        |      Version:  SVN trunk
 Severity:  minor        |   Resolution:           
 Keywords:               |    Blockedby:           
 Platform:  All          |     Blocking:           
-------------------------+--------------------------------------------------

Old description:

> GCC allows functions to be annotate with __attribute__((const)) if they
> "do not
> examine any values except their arguments, and have no effects except the
> return
> value", which allows the compiler to use CSE to eliminate calls to them
> with
> identical arguments.  This would probably be very useful for
> Xapian::Unicode::get_category() for example.
>
> URL: http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Function-Attributes.html
> #Function-Attributes

New description:

 GCC allows functions to be annotated with __attribute__((const)) if they
 "do not
 examine any values except their arguments, and have no effects except the
 return
 value", which allows the compiler to use CSE to eliminate calls to them
 with
 identical arguments.  This would probably be very useful for
 Xapian::Unicode::get_category() for example.

 attribute((const)) is available for (at least) GCC 2.95 onwards, which is
 all the versions we support.  MSVC doesn't seem to have an equivalent.
 I've not looked at other compilers.

 ----

 There's also attribute((pure)) which is like const, but the function may
 also examine global memory, perhaps via pointer (or presumably reference)
 parameters. This is available from GCC 2.96 onwards, which covers all the
 versions we support for 1.1.x and higher.

 MSVC doesn't seem to have an equivalent.  I've not looked at other
 compilers.

 ----

 Both GCC and MSVC support nothrow - this is now ticket #454.

 ----

 GCC also allows functions to be marked listing pointer parameters which
 can't be NULL, which may be worth considering, or perhaps
 such parameters should be changed to references.

 URL: http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Function-Attributes.html
 #Function-Attributes

--

Comment(by olly):

 Hmm, I created #454 for nothrow without realising we already had a ticket.

 Since we're unlikely to address all of these at once, it seems useful to
 have a separate ticket, so removing nothrow from this one.

-- 
Ticket URL: <http://trac.xapian.org/ticket/151#comment:9>
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