[Bug c/51971] New: unclear/unverified restrictions on attribute((const|pure))

["akim.demaille at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51971

             Bug #: 51971
           Summary: unclear/unverified restrictions on
                    attribute((const|pure))
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: akim.demaille@gmail.com


Created attachment 26434
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26434
Declare pure functions which obviously do not return

Hi,

The documentation for const and pure is not clear about the fact that functions
should "return normally":

  Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as feof in a multithreading environment).

In particular, at that point nothing is said about abort().

This is something which does appear in the documentation of
-Wsuggest-attribute, yet at that point it is still unclear (to me?):

  The compiler only warns for functions
  visible in other compilation units or (in the case of pure and
  const) if it cannot prove that the function returns normally.

It does not say "do not flag as pure if the function does not return normally".
 It does not tell either how to silence the suggestion if the function is not
pure.

The warning itself is clearer though:

  akim@boss ~/src/gcc $ g++-mp-4.6 -O2 -Wsuggest-attribute=pure -c
gcc/testsuite/gcc.dg/pure-2.c
  gcc/testsuite/gcc.dg/pure-2.c: In function 'int foo3(int)':
  gcc/testsuite/gcc.dg/pure-2.c:38:1: warning: function might be candidate for
attribute 'pure' if it is known to return normally [-Wsuggest-attribute=pure]

I think that the documentation should also unveil why there is this
restriction.  The documentation for "pure" mentions:

  Such a function can be subject to common subexpression elimination and loop
optimization just as an arithmetic operator would be.

It also rules out every use of assert, which is a serious limitation, even for
pure functions.

Does that mean that the function might be called _because_ of the optimization?
 In which case, yes, indeed, looping or aborting becomes a problem :)  Can CSE
really introduce extraneous calls?  The doc does not say so (it says "fewer",
not "more"):

  For example,
         int square (int) __attribute__ ((pure));
  says that the hypothetical function square is safe to call fewer times than
the program says.

Finally, if really "non-normally returning functions" are ruled out, then GCC
should diagnose misuses, such as the attached one.

akim@padam /tmp $ gcc-mp-4.6 -O2 -Wall -Wextra -Wsuggest-attribute=pure
-Wsuggest-attribute=const -c /tmp/pure.c
akim@padam /tmp $ echo $?
0

akim@padam /tmp $ gcc-mp-4.6 --version
gcc-mp-4.6 (GCC) 4.6.2
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.