This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Division by INT64_MIN


On Mon, 5 Oct 2015, Matthew Fernandez wrote:

> tl;dr: Is dividing by INT64_MIN in C undefined behaviour?

No, only dividing by 0 or dividing the minimum value of the type by -1 
(and the latter should be defined for -fwrapv, see bug 30484).

> on x86 is emitted as a call to __divdi3. If the numerator or denominator are
> negative, __divdi3 negates them. If either of these values is INT64_MIN, I
> believe this negation is undefined. Is this correct? If this is the case, then
> it seems code like "INT64_MIN / INT64_MIN" which should be perfectly legal
> accidentally causes undefined behaviour via libgcc. In practice, everything
> seems to work as expected, but it seems to me that the C code of __divdi3
> should not be relying on these negations working consistently.

It sounds like some casts to UDWtype should be inserted so the negation 
takes place on an unsigned type.

-- 
Joseph S. Myers
joseph@codesourcery.com


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]