https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94836
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
OK, so it's not that easy to do. Consider
int f(int x)
{
static int s;
if (!s)
s = x;
return s;
}
which we cannot optimize in this way. VN would need to work in a way
assigning the value 0 to 's' optimistically and if it eventually
arrives at s = x it would need to invalidate that optimistic assumption
and iterate. That is it would need to see the function as
int f(int x)
{
static int s;
# s = PHI <0, s'>
if (!s)
s = x;
return s;
s' = s;
goto start;
}
with the goto of course not explicit. I suppose rather than integrating
the feature into their value-numbering scheme compilers implement special
purpose optimization (IIRC this pattern appears in SPEC).
