[Bug tree-optimization/27039] [4.1/4.2 Regression] Unable to determine # of iterations for a simple loop

[rguenth at gcc dot gnu dot org]

------- Comment #8 from rguenth at gcc dot gnu dot org  2006-05-04 14:21 -------
Wording of 6.5.6/8 and /9 suggests that array objects larger than the maximum
value that fits in ptrdiff_t (which needs to be signed) invoke undefined
behavior,
not last because of "the expression ((Q)+1)-(P) has the same value as
((Q)-(P))+1 and as -((P)-((Q)+1))" and "The size of the result is
implementation-defined,
and its type (a signed integer type) is ptrdiff_t defined in the <stddef.h>
header.  If the result is not representable in an object of that type, the
behavior is undefined."

So for this reason I believe that folding

  PTR +- CST  CMP  PTR +- CST

(with CST being of pointer type, as represented by the middle-end) as

  +- (ptrdiff_t)CST  CMP  +- (ptrdiff_t)CST

is valid.  Now the intel compiler f.i. does _not_ do this.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27039