------- Comment #6 from matz at gcc dot gnu dot org 2008-02-02 16:08 -------
(written before richis comment, essentially the same info)
The compare routine doesn't need to be a template to show the bug. But it
needs to take reference parameters. The difference is in the call. E.g.
with this testcase:
% cat x.cc
enum EBorderStyle { bla = 1 };
inline bool compare_ref(const unsigned int &t, const EBorderStyle &u)
{ return t == u; }
inline bool compare_val(const unsigned int t, const EBorderStyle u)
{ return t == u; }
struct S {
unsigned m_style : 4;
};
void call_ref (S *s, EBorderStyle v)
{ if (!compare_ref(s->m_style, v)) s->m_style = v; }
void call_val (S *s, EBorderStyle v)
{ if (!compare_val(s->m_style, v)) s->m_style = v; }
The bodies of the two compare routines is sensible (after all nothing
fancy happens as not bitfield types are involved there anymore):
bool compare_ref(const unsigned int&, const EBorderStyle&) (t, u)
{
bool D.1655;
unsigned int D.1656;
EBorderStyle D.1657;
D.1656 = *t;
D.1657 = *u;
D.1655 = D.1656 == D.1657;
return D.1655;
}
bool compare_val(unsigned int, EBorderStyle) (t, u)
{
bool D.1662;
D.1662 = t == u;
return D.1662;
}
But the difference in the call setup is revealing:
void call_ref(S*, EBorderStyle) (s, v)
<unnamed-unsigned:4> D.1672;
unsigned int D.1670;
...
D.1672 = s->m_style;
D.1670 = D.1672;
D.1673 = compare_ref (&D.1670, &v);
}
void call_val(S*, EBorderStyle) (s, v)
{
...
<unnamed-unsigned:4> D.1682;
unsigned int D.1683;
...
D.1682 = s->m_style;
D.1683 = (unsigned int) D.1682;
D.1684 = compare_val (D.1683, v);
}
Note how the call with the reference parameter copies the value from the
bitfield into an unsigned temp without a cast, and the call by value copies
it into a unsigned temp with a cast. That latter cast will stay there until
130.final_cleanup, and hence the compare instruction will be expanded
correctly.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35056
