On 7/15/2017 2:57 PM, Michael Niedermayer wrote: > Requested-by: Kieran Kunhya <kier...@obe.tv> > > Signed-off-by: Michael Niedermayer <mich...@niedermayer.cc> > --- > doc/undefined.txt | 47 +++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 47 insertions(+) > create mode 100644 doc/undefined.txt > > diff --git a/doc/undefined.txt b/doc/undefined.txt > new file mode 100644 > index 0000000000..957db3e2a9 > --- /dev/null > +++ b/doc/undefined.txt > @@ -0,0 +1,47 @@ > +Undefined Behavior > +------------------ > +In the C language, some operations are undefined, like signed integer > overflow > +dereferencing freed pointers, accessing outside allocated space, ... > + > +Undefined Behavior must not occur in a C program, it is not safe even if the > +output of undefined operations is unused. The unsaftey may seem nit picking> > +but Optimizing compilers have in fact optimized code on the assumtation that
Assumption. > +no undefined Behavior occurs. > +Optimizing code based on wrong assumtations can and has in some cases lead to Assumptions. > +effects beyond the output of computations. > + > + > +The signed integer overflow problem in speed critical code > +---------------------------------------------------------- > +Code which is highly optimized and works with signed integers sometimes has > the > +problem that some (invalid) inputs can trigger overflows (undefined > behavior). > +In these cases, often the output of the computation does not matter (as it is > +from invalid input). > +In some cases the input can be checked easily in others checking the input is > +computationally too intensive. > +In these remaining cases a unsigned type can be used instead of a signed > type. > +unsigned overflows are defined in C. > + > +SUINT > +----- > +As we have above established there is a need to use "unsigned" sometimes in > +computations which work with signed integers (which overflow). > +Using "unsigned" for signed integers has the very significant potential to > +cause confusion > +as in > +unsigned a,b,c; > +... > +a+b*c; > +The reader does not expect b to be semantically -5 here and if the code is > +changed by maybe adding a cast, a division or other the signeness will almost > +certainly be mistaken. > +To avoid this confusion a new type was introduced, "SUINT" is the C > "unsigned" > +type but it holds a signed "int". > +to use the same example > +SUINT a,b,c; > +... > +a+b*c; > +here the reader knows that a,b,c are meant to be signed integers but for C > +standard compliance / to avoid undefined behavior they are stored in unsigned > +ints. > + > _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
