[PATCH] Tree-level fix for PR 69526

Richard Biener richard.guenther@gmail.com
Tue Jan 17 09:48:00 GMT 2017

On Tue, Jan 10, 2017 at 2:32 PM, Robin Dapp <rdapp@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

 (uint64_t)(uint32_t + uint32_t-CST) + uint64_t-CST


 (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.



> Regards
>  Robin

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