Efficiency of code generated by Ada compilers
in [ADA]
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From: Robert A Duff on 8 Aug 2010 11:49 Gene <gene.ressler(a)gmail.com> writes: > In some cases, a built-in Ada construct will generate better code than > a "hand-coded" equivalent in C++. An example I ran into recently was > incrementing a modular type. In Ada, you say I := I + 1;, and the > compiler takes care of "wrapping" to zero. In C++, I've frequently > seen people write i = (i + 1) % n; to simulate the same operation in > the absence of modular types. You will see me writing "I := (I + 1) mod N;" (for signed integer I) in Ada. ;-) >...In this case, GCC/C++ generates a div/ > mod instruction, which is very expensive. That's surprising. It wouldn't be hard to optimize the "I := (I + 1) mod N;" case (which would apply to both C and Ada). And it would be desirable, because in most cases the explicit "mod" (or "%") is more readable. - Bob
From: Charles H. Sampson on 8 Aug 2010 13:13 Robert A Duff <bobduff(a)shell01.TheWorld.com> wrote: > Gene <gene.ressler(a)gmail.com> writes: > > > In some cases, a built-in Ada construct will generate better code than > > a "hand-coded" equivalent in C++. An example I ran into recently was > > incrementing a modular type. In Ada, you say I := I + 1;, and the > > compiler takes care of "wrapping" to zero. In C++, I've frequently > > seen people write i = (i + 1) % n; to simulate the same operation in > > the absence of modular types. > > You will see me writing "I := (I + 1) mod N;" (for signed > integer I) in Ada. ;-) > I'm surprised, Bob. Are you saying that you signed integers in preference to a modular type for a variable that cycles? I use modular-typed variables and, if I've got my engineer's hat on, write I := I + 1; -- Modular variable. Wraps. but I have to admit that in the heat of battle the comment might be omitted. Even in that case, the variable name probably indicates that wrapping should be expected. Charlie -- All the world's a stage, and most of us are desperately unrehearsed. Sean O'Casey
From: Dmitry A. Kazakov on 8 Aug 2010 14:11 On Sun, 8 Aug 2010 10:13:51 -0700, Charles H. Sampson wrote: > Robert A Duff <bobduff(a)shell01.TheWorld.com> wrote: > >> Gene <gene.ressler(a)gmail.com> writes: >> >>> In some cases, a built-in Ada construct will generate better code than >>> a "hand-coded" equivalent in C++. An example I ran into recently was >>> incrementing a modular type. In Ada, you say I := I + 1;, and the >>> compiler takes care of "wrapping" to zero. In C++, I've frequently >>> seen people write i = (i + 1) % n; to simulate the same operation in >>> the absence of modular types. >> >> You will see me writing "I := (I + 1) mod N;" (for signed >> integer I) in Ada. ;-) >> > I'm surprised, Bob. Are you saying that you signed integers in > preference to a modular type for a variable that cycles? Sometimes it has to be that way because modular types do not support non-static bounds. E.g.: procedure Foo (N : Positive) is type M is mod 2**N; -- You cannot do that! -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de
From: Robert A Duff on 8 Aug 2010 16:51 csampson(a)inetworld.net (Charles H. Sampson) writes: > Robert A Duff <bobduff(a)shell01.TheWorld.com> wrote: > > I'm surprised, Bob. Are you saying that you signed integers in > preference to a modular type for a variable that cycles? Yes. Unless I'm forced to use modular for some other reason (e.g. I need one extra bit). >...I use > modular-typed variables and, if I've got my engineer's hat on, write > > I := I + 1; -- Modular variable. Wraps. You're not alone. Even Tucker has advocated using modular types for this sort of thing. But I think an explicit "mod N" is clearer than a comment. Variables that cycle are rare, so should be noted explicitly in the code. And, as Dmitry noted, modular types only work when the lower bound is 0. It's not unreasonable to have a circular buffer indexed by a range 1..N. See my point? Still "surprised"? > but I have to admit that in the heat of battle the comment might be > omitted. Even in that case, the variable name probably indicates that > wrapping should be expected. - Bob P.S. Can anybody recall the definition of "not" on modular types, when the modulus is not a power of 2, without looking it up? Hint: It makes no sense. The only feature that's worse than "modular types" is "modular types with a non-power-of-2 modulus". ;-)
From: (see below) on 8 Aug 2010 18:10
On 08/08/2010 21:51, in article wcchbj4j09y.fsf(a)shell01.TheWorld.com, "Robert A Duff" <bobduff(a)shell01.TheWorld.com> wrote: > csampson(a)inetworld.net (Charles H. Sampson) writes: > >> Robert A Duff <bobduff(a)shell01.TheWorld.com> wrote: >> >> I'm surprised, Bob. Are you saying that you signed integers in >> preference to a modular type for a variable that cycles? > > Yes. Unless I'm forced to use modular for some other reason > (e.g. I need one extra bit). > >> ...I use >> modular-typed variables and, if I've got my engineer's hat on, write >> >> I := I + 1; -- Modular variable. Wraps. > > You're not alone. Even Tucker has advocated using modular > types for this sort of thing. > > But I think an explicit "mod N" is clearer than a comment. > > Variables that cycle are rare, so should be noted explicitly > in the code. And, as Dmitry noted, modular types only work > when the lower bound is 0. It's not unreasonable to have > a circular buffer indexed by a range 1..N. > > See my point? Still "surprised"? > >> but I have to admit that in the heat of battle the comment might be >> omitted. Even in that case, the variable name probably indicates that >> wrapping should be expected. > > - Bob > > P.S. Can anybody recall the definition of "not" on modular > types, when the modulus is not a power of 2, without > looking it up? Hint: It makes no sense. The only feature > that's worse than "modular types" is "modular types with a > non-power-of-2 modulus". ;-) I don't use non-power-of-2 mod types, but I write emulators, and I would be stuffed if I could not declare, e.g.: type KDF9_word is mod 2**48; -- Bill Findlay <surname><forename> chez blueyonder.co.uk |