Passing an in parameter and aliasing

Bill Baxter dnewsgroup at billbaxter.com
Fri May 25 22:30:07 PDT 2007


Dave wrote:
> Bill Baxter wrote:
>> renoX wrote:
>>> Bill Baxter a écrit :
>>
>>>> The thing with this is it may catch the error but it doesn't help 
>>>> the compiler generate more optimized code.  The C99 'restrict' 
>>>> keyword does, though.  I suppose the compiler could be smart enough 
>>>> to deduce 'restrict' semantics are in effect from the assert message 
>>>> above.  That would be pretty neat.
>>>
>>> Well, if I understood correctly Jarrett Billingsley's answer, the 
>>> semantic on 'in' is the same as 'restrict': the fastest as the 
>>> compiler will be free to optimize but the most fragile from 
>>> programmers point of view because if you violate the restrict 
>>> restriction the result is unpredictable (no bug, a bug depending on 
>>> optimization flags, fun!)
>>>
>>> Sure, it'd be nice if the compiler was able to do this kind of 
>>> optimization without the in|restrict hint but this is hard.
>>
>> The 'in' parameter type does nothing currently.  Leaving it off is the 
>> same as putting it in.  So I don't believe it acts like 'restrict'.  I 
> 
> That's right (verifiable through the asm).
> 
>> certainly don't remember reading anywhere in the spec that D assumes 
>> all 'in' array parameters are unaliased.  That would be a big break 
>> from C/C++ if it were so, so I think the spec would go out of its way 
>> to point out the difference.
>>
> 
> I believe that's part of the overall justification for the new for 2.0 
> 'in', which will mean 'scope const final' for any type of reference 
> parameter (correct me if I'm wrong Walter). Then the optimizer will be 
> free to do its thing with those references, w/o any complicated pointer 
> analysis or runtime checking.
> 
> IIUC, the compiler front-end will do it's best to enforce it (unlike C's 
> 'restrict') and I believe it won't be as easy to cast away like C++'s 
> 'const&'.
> 
> If so that'd be pretty cool, because it would make D competitive with 
> Fortran in this regard.

I don't think Walter has said that 'in' as 'scope const final' is going 
to imply no aliasing.  I think it's kind of an orthogonal concern.

Say you have a vector difference function:

     void sub(mutable float[] ret, in float[] a, in float[] b) {
         for(size_t i=1; i<ret.length; i++) {
            ret[i] = a[i] - b[i];
         }
     }

Should you expect this to fail?
     float[10] c;
     for(i=0; i<=10; i++) c[i]=i;
     sub(c[0..9],c[1..$],c[0..9]);

Looking at the loop it should be fine.  It should calc
         c[0] = c[1] - c[0];
         c[1] = c[2] - c[1];
         c[2] = c[3] - c[2];
         ...
and leave you with c[0..9] containing all 1's.
But a smarty pants compiler might decide for some reason that it's 
faster to rewrite the loop as:
        memcpy(ret,a,float.sizeof*a.length);
        for (i=0;i<N;i++) { ret[i] -= b[i]; }

That's a fine transformation if b and ret are distinct.  But it isn't 
fine when b and ret are aliased.

That's the reason why C doesn't have restrict as default. The thinking 
is that whatever the compiler does under the hood, the result should be 
output that looks exactly like if you really did run the C code on a C 
virtual machine in the exact order written.  Even if users get tricky 
with inputs and do things that are guaranteed to give incorrect results, 
it should still give the "exact" incorrect result.  It really is a 
separate issue from whether the parameter can be modified or not.

That's C's philosophy anyway.  And I haven't heard anything from Walter 
that sounded to me like D was taking a different approach.  I think a 
'restrict' type keyword probably is the best way to handle it in a 
C-like language.  Most code doesn't need the performance boost as much 
as it needs predictability and reliability.  Aliasing bugs can also be 
very tricky to debug because things might work great on your computer 
but only fail on some other machine when compiled with SSE3 instructions 
and -03 optimizations.  So it makes sense for performance critical code 
to declare in a visible way that it's placing some restrictions on the 
kind of inputs that will work.

So I think D could still use a separate restrict-like keyword.  But I 
think it could be made a little more user friendly than C's by allowing 
it to apply to a whole function rather than having to say 'restrict' 
'restrict' 'restrict' for every single parameter.

--bb



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