On 09/27/2013 10:28 AM, Vincent Lefevre wrote: > On 2013-09-27 09:23:35 +0100, Andrew Haley wrote: >> On 09/27/2013 08:57 AM, Vincent Lefevre wrote: >>> On 2013-09-26 18:30:10 +0100, Andrew Haley wrote: >>>> On 09/26/2013 06:02 PM, Vincent Lefevre wrote: >>>>> On 2013-09-26 15:49:05 +0100, Andrew Haley wrote: >>>>>> On 09/26/2013 09:29 AM, Vincent Lefevre wrote: >>>>>>> On 2013-09-25 22:29:58 -0400, James K. Lowden wrote: >>>>>>>> You mean that a naïve rendering of the source code implies an overflow >>>>>>>> where none might exist in the actual emitted object code. And, >>>>>>>> presumably, the converse: that even if the source is written such that >>>>>>>> there logically can't be an overflow, the compiler might render object >>>>>>>> code that does. >>>>>>> >>>>>>> The converse is forbidden. >>>>>> >>>>>> You'll find it hard to justify that by any language in the standard. >>>>> >>>>> What do you mean? >>>> >>>> There is no reason why a compiler should not generate an overflow >>>> where none is written in the program, as long as it doesn't generate >>>> a different result. >>> >>> OK, I wouldn't call that an overflow, then. >> >> As far as the processor is concerned, what sets the overflow flag is >> an overflow. That's the context of this discussion. > > No, it isn't. If you regard the CPU overflow flag as a part of the > result, then the compiler is not allowed to generate overflows not > expressed in the source. Never. For instance, it would be really > wrong to get spurious crashes with -ftrapv just because gcc modified > the order of operations or just because the overflow flag has been > set with an unsigned operation (at the C level). Sure, but if -ftrapv is turned off, gcc can generate instructions that will overflow. I suspect that it would be very hard to get GCC to do this correctly in all cases when -ftrapv is turned on. > If you disregard the CPU overflow flag, then what the CPU does is > not regarded as an overflow. It is by the CPU. And it is by me. If you choose not to regard that as an overflow, I have no quarrel with you, but I will not agree. By the definition I am using, what sets the overflow flag is an overflow. > Note that Dave Allured asked whether there is a way to check the > CPU overflow flag on an example where there may be an overflow > *in the source*. Well, there probably isn't. As I have explained. Andrew.
