On Fri, May 20, 2022 at 4:15 AM Andrew MacLeod via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > On 5/19/22 18:23, Segher Boessenkool wrote: > > On Thu, May 19, 2022 at 09:22:32AM -0400, Andrew MacLeod wrote: > >> On 5/18/22 16:40, Segher Boessenkool wrote: > >>> On Wed, May 18, 2022 at 04:24:06PM -0400, Andrew MacLeod wrote: > >>>> On 5/18/22 14:13, Segher Boessenkool wrote: > >>>>> "Side effect" already has a meaning, very commonly used in language > >>>>> theory, and even in the C standard itself: a function has a side effect > >>>>> if it does something more than just return a value: if it changes state. > >>>>> This can be some I/O, or it can just be writing to some non-local data. > >>>>> > >>>>> Side effects are crucial to what a compiler does, and they are used all > >>>>> over the place (the gcc/ dir has some thousand mentions of it for > >>>>> example). > >>>>> > >>>>> Please don't make life hard for everyone by overloading this term. > >>>>> > >>>> I'm open to suggestions for a better term! > >>> Glad to hear that, and this isn't set in stione yet! > >>> > >>>> Is there a commonly used alternate term to describe an observable effect > >>>> on the value of an input operand? > >>> I'd use something with "known" in the name. But: > >>> > >>> As far as I understand what you are doing this is not an effect on the > >>> operand at all! It cannot be one even, the operand is an input only > >>> after all. Instead, it changes what is known about the value of that > >>> input: it cannot be 0 in this case, it is known to not be 0. > >>> > >>> This is similar to other known value things we have in GCC already. Can > >>> you not just use one of those, even? What are the benefit to this new > >>> abstraction? > >> Well, This is a component of ranger tracking value ranges.. it is > >> recording the "side-effect" of the stmt on the known range of an object. > >> The file is called "gimple-range-side-effect.h" > > So the file name is confusingly wrong as well. > > > >> Its a generalization of how ranger tracks non-null pointer values, > >> enabling it to track arbitrary observable ranges values. (The old > >> mechanism also utilized immediate-use chains, which prevented it from > >> being utilized in gimple-folding) > >> > >> WIth this change, we can also track things like a = b / c causing the > >> effect that c is known non-zero after the statement if there were no > >> traps, or https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31178 , which > >> after 15 years, we can now simply indicate that for a = b >> c , its > >> only defined behaviour if c is in the range [0, PRECISION(b)] > >> > >> So its basically just a generalization of how we track known values > >> within the range system. > > Sure. Just the name is harmful :-( > > > Still waiting for a suggestion, since "side effect" is the description > that made sense to me :-)
I think side-effect captures it quite well even if it overlaps with a term used in language standards. Doing c = a << b has the side-effect on imposing a range on 'b' rather than just affecting 'c' (and its range). You could call it 'alternate effect' but that sounds just awkward ;) Richard. > Andrew >
