------- Comment #6 from rguenth at gcc dot gnu dot org 2009-01-09 22:57 -------
Ok, I'm convinced. That jump-threading is really weird. But only because
__builtin_constant_p isn't a regular function that is executed on a path
of the CFG, it really is supposed to be evaluating all paths through the CFG
going through it. Which of course conflicts with what jump threading does.
OTOH
void foo (bool b)
{
if (__builtin_constant_p (b))
boo ();
else
yeah ();
}
is then not equivalent to
void foo (bool b)
{
if (b == true)
{
if (__builtin_constant_p (b))
...
}
else if (b == false)
{
if (__builtin_constant_p (b))
....
}
}
That makes exact definition of the behavior of __builtin_constant_p not
easier. That is, the manual states
"You can use the built-in function @code{__builtin_constant_p} to
determine if a value is known to be constant at compile-time and hence
that GCC can perform constant-folding on expressions involving that
value."
which is true for the second variant but not for the first. Still if
__builtin_constant_p were a regular function both programs would be
equivalent.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever Confirmed|0 |1
Last reconfirmed|0000-00-00 00:00:00 |2009-01-09 22:57:03
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38789
