On Mon, May 14, 2012 at 4:26 AM, Geert Bosch <bo...@adacore.com> wrote: > > On May 13, 2012, at 21:17, amyl...@spamcop.net wrote: >> The expectation is wrap-around. Note that loop strenght reduction can >> cause assumed wrap-around semantics in RTL for strictly conforming C input >> where no such wrap-around is in evidence. > > Really, we should define signed integer arithmetic in RTL to always > be wrap-around. While there may (*) be some benefit in "taking > advantage" of undefined semantics of integer overflow in early tree > optimizers (computing number of loop iterations etc.), at the same > time it also reduces optimization opportunities. > > Transformations cannot introduce overflows where none existed before, > so integer addition and subtraction are not associative. In the > end, the hardware is deterministic and does have wrap-around > semantics. So, it really doesn't make sense to pretend in RTL that > it doesn't.
I agree - but we don't have signed vs. unsigned PLUS on RTL so RTL arith is unsigned anway, no? Richard. > -Geert > > -- > (*) At the end of the day, I think everybody would win by unambiguously > defining signed integer arithmetic as using wrap-around semantics > everywhere. Software is hard enough with defined semantics. >
