[Bug target/68110] __builtin_sub_overflow unsigned performance issue

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68110

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
value-numbering would need to special-case them via the insertion trick it does
for conversions.  somehow.  not sure if feasible or worthwhile.