[Bug c/6906] warn about asserts with side effects

[Bug c/6906] warn about asserts with side effects

[felix.von.s at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=6906

felix <felix.von.s at posteo dot de> changed:

           What    |Removed                     |Added
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                 CC|                            |felix.von.s at posteo dot de

--- Comment #9 from felix <felix.von.s at posteo dot de> ---
I have just realised that __builtin_pure_p is already expressible:

#define __builtin_pure_p(expr) \
    (__builtin_object_size(((void)(expr), ""), 2))

If (expr) is free of side effects, this is the same as
__builtin_object_size("", 2), which is 1. Otherwise, the expression returns 0.
This is documented behaviour, by the way. The documentation for
__builtin_object_size at
<https://gcc.gnu.org/onlinedocs/gcc/Object-Size-Checking.html> states:

> __builtin_object_size never evaluates its arguments for side effects.
> If there are any side effects in them, it returns (size_t) -1 for
> type 0 or 1 and (size_t) 0 for type 2 or 3.

Experimentation reveals __attribute__((pure)) is enough for
__builtin_object_size to consider a function free of side effects.

[Bug c/6906] warn about asserts with side effects

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2006-10-15 19:45 -------
*** Bug 29480 has been marked as a duplicate of this bug. ***


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pinskia at gcc dot gnu dot org changed:

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                 CC|                            |robert at linuxfromscratch
                   |                            |dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=6906

[Bug c/6906] warn about asserts with side effects

[chris dot pickett at mail dot mcgill dot ca]

------- Comment #5 from chris dot pickett at mail dot mcgill dot ca  2008-03-03 02:55 -------
Maybe you could fix the preprocessor to warn about operators with known side
effects.  Can cpp see the entire expr in assert (expr)?  If so, just search the
tokens for =, ++, --, +=, -=, etc.


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chris dot pickett at mail dot mcgill dot ca changed:

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                 CC|                            |chris dot pickett at mail
                   |                            |dot mcgill dot ca


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=6906

[Bug c/6906] warn about asserts with side effects

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
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           Keywords|                            |diagnostic
   Last reconfirmed|2003-10-01 04:02:16         |2003-12-31 16:03:17
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=6906

[Bug c/6906] warn about asserts with side effects

[mbp at sourcefrog dot net]

------- Additional Comments From mbp at sourcefrog dot net  2004-08-10 23:56 -------
It might be nice if there were a compile-time function similar to
__builtin_constant_p which checked whether an expression was free of
side-effects.    One could then write

# define assert(expr) \
      (__builtin_pure_p((expr)) ? _real_assert(expr) :
warning_assert_expr_not_pure())

So this would be true for things like
  
  __builtin_pure_p(arry[0] == 0)
  __builtin_pure_p(strlen(s) == 5)

assuming the functions had the right attributes.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=6906