[LLVMdev] c const

[Duncan Sands]

Hi Christopher,

> >> In C/C++ a restrict pointer is also assumed not to alias any other
> >> parameters, global values or local value. However the compiler must
> >> not assume that pointers "based on" a restrict pointer do not alias.
> >
> > does "based on" mean something like: copies of the pointer made in the
> > function body?
> 
> To be precise (brackets mine):
> 
> In what follows, a pointer expression E is said to be based on object  
> P [a pointer] if (at some
> sequence point in the execution of B [the enclosing block] prior to  
> the evaluation of E) modifying P to point to
> a copy of the array object into which it formerly pointed would  
> change the value of E.
> Note that ‘‘based’’ is defined only for expressions with pointer types.
> 
> This is from http://www.open-std.org/jtc1/sc22/wg14/www/docs/ 
> n1124.pdf Sec 6.7.3.1 para 3

suppose A and B are array pointers, and are in fact equal.  Consider
the code sequences

(i)
x = A[0]	(a)
B[0] = 1	(b)

and

(ii)
B[0] = 1	(b)
x = A[0]	(a)

If I understand right, (i)(b) is not based on A, either because it
has no value (not sure what "the value of E" means) or because changing
A to point to a copy would make no difference because (i)(a) is only a
read.  Also (i)(a) is not based on B because this is the first instruction.
On the other hand (ii)(a) is based on B because the value of x
is 1, but would be something else if B was changed to point to a copy.

Does this mean that if B is a restricted pointer, then the lines can be
swapped in (i).  That if A is a restricted pointer then the lines can be
swapped in (i) and (ii)?

This sounds very similar to the Ada setup, though somehow "dual" to it.

Ciao,

Duncan.