On Tue, Jan 10, 2017 at 2:32 PM, Robin Dapp <rd...@linux.vnet.ibm.com> wrote: > Perhaps I'm still missing how some cases are handled or not handled, > sorry for the noise. > >> I'm not sure there is anything to "interpret" -- the operation is unsigned >> and overflow is when the operation may wrap around zero. There might >> be clever ways of re-writing the expression to >> (uint64_t)((uint32_t)((int32_t)uint32 + -1) + 1) >> avoiding the overflow and thus allowing the transform but I'm not sure that's >> good. > > The extra work I introduced was to discern between > > (uint64_t)(a + UINT_MAX) + 1 -> (uint64_t)(a), > (uint64_t)(a + UINT_MAX) + 1 -> (uint64_t)(a) + (uint64_t)(UINT_MAX + 1), > > For a's range of [1,1] there is an overflow in both cases. > We still want to simplify the first case by combining UINT_MAX + 1 -> 0.
So there's the case where a == 0 where (uint64_t)(0 + UINT_MAX) + 1 is not zero. I think we're still searching for the proper condition on when it is allowed to re-write (uint64_t)(uint32_t + uint32_t-CST) + uint64_t-CST to (uint64_t)(uint32_t) + (uint64_t)uint32_t-CST + uint64_t-CST or to (uint64_t)(uint32_t) + (uint64_t)(uint32_t-CST + (uint32_t)uint64_t-CST) > If "interpreting" UINT_MAX as -1 is not the correct thing to do, perhaps > (uint64_t)((uint32_t)(UINT_MAX + 1)) is? This fails, however, if the > outer constant is larger than UINT_MAX. What else can we do here? > Do we see cases like the second one at all? If it's not needed, the > extra work is likely not needed. We do have the need to associate and simplfy across conversions but of course we have to do it conservatively correct. >> A related thing would be canonicalizing unsigned X plus CST to >> unsigned X minus CST' >> if CST' has a smaller absolute value than CST. I think currently we >> simply canonicalize >> to 'plus CST' but also only in fold-const.c, not in match.pd (ah we >> do, but only in a simplified manner). > > I can imagine this could simplify the treatment of some cases, yet I'm > already a bit lost with the current cases :) Yes, but I am lost a bit as well. I don't know the correct conditions to test off-head -- I know we may not introduce new undefined overflow ops and of course we need to not compute wrong numbers either. >> That said, can we leave that "trick" out of the patch? I think your >> more complicated >> "overflows" result from extract_range_from_binary_expr_1 doesn't apply to all >> ops (like MULT_EXPR where more complicated cases can arise). > > There is certainly additional work to be done for MULT_EXPR, I > disregarded it so far. For this patch, I'd rather conservatively assume > overflow. Ok... Richard. > Regards > Robin >
