https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68136
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2015-10-28
CC| |rguenth at gcc dot gnu.org
Ever confirmed|0 |1
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
int
foo (s64 a, s64 b, s64 c)
{
s64 d = a - b;
if (d == 0)
return a + c;
else
return b + d + c;
}
ok, so the issue here is that the additions are narrowed to unsigned int
because of the appearant cast to 'int' for the return while the
d = a - b assignment is not narrowed in this way.
(u32)b + (u32)(a - b) + (u32)c
can not be easily simplified to the desired (u32)a + (u32)c.
As 'd' is multi-use this way we don't change the compare to a == b either
which would expose the then single-use 'd' to reassoc (which could get
some tricks to perform narrowing).
I don't see a very good answer here. Pattern-matching the whole thing
is of course an option but I don't see that as a scalable solution.
Maybe hacking up reassoc to do that narrowing trick and also consider
non-single-use chains (marked specially so they only participate in
simplification but never actual association or code-gen).
