https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38401
--- Comment #28 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue with the patch is that it does speculation for expressions that might
invoke undefined behavior. Like for gcc.c-torture/execute/20000801-2.c where
it speculates a load from the next-next iteration:
<bb 2>:
a.next = &b;
b.next = 0B;
<bb 3>:
# node_9 = PHI <prephitmp_4(4), &a(2)>
# prephitmp_4 = PHI <prephitmp_10(4), &b(2)>
# prephitmp_10 = PHI <pretmp_7(4), 0B(2)>
if (prephitmp_4 != 0B)
goto <bb 4>;
else
goto <bb 5>;
<bb 4>:
pretmp_7 = prephitmp_10->next;
goto <bb 3>;
similar issues can appear for non-backedges, too, and for example undefined
integer overflow. We'd need to be much more careful with what we'd insert
here.
