Re: [PATCH][C] Fixup pointer-int-sum

[Richard Guenther <rguenther@suse.de>]

On Thu, 7 Jul 2011, Joseph S. Myers wrote:

> On Thu, 7 Jul 2011, Richard Guenther wrote:
> 
> > not overflow (what is actually the C semantics - is the
> > multiplication allowed to overflow for unsigned intop?  If not
> 
> Overflow is not allowed.  Formally the multiplication is as-if to infinite 
> precision, and then there is undefined behavior if the result of the 
> addition (to infinite precision) is outside the array pointed to - 
> wrapping around by some multiple of the whole address space is not 
> allowed.
> 
> In practice, as previously discussed objects half or more of the address 
> space do not work reliably because of the problems doing pointer 
> subtraction, so always using a signed type shouldn't break anything that 
> actually worked reliably (though how unreliable things were with large 
> malloced objects - which unfortunately glibc's malloc can provide - if the 
> source code didn't use pointer subtraction, I don't know).
> 
> In GCC's terms half or more of the address space generally means half the 
> range of size_t.  (m32c has ptrdiff_t wider than size_t in some cases.  On 
> such unusual architectures it ought to be possible to have objects whose 
> size is up to SIZE_MAX bytes and have pointer addition and subtraction 
> work reliably, which would suggest using ptrdiff_t for arithmetic in such 
> cases, but the code checking sizes for arrays of constant size uses the 
> signed type corresponding to size_t, so you could only get a larger object 
> through malloc or VLAs.)
> 
> The patch is OK.  Unconditionally signed is also OK, though I don't see 
> any advantage over this version.

Ok, I'll defer the decision to the time I have settled on a final
solution to get rid of the (unsigned) sizetype offset operand
for POINTER_PLUS_EXPR.  The least invasive idea is to introduce a
new signed ptrofftype to replace all sizetype conversions at places
we build POINTER_PLUS_EXPRs.  That would favor unconditionally
signed.  The moderate invasive idea is to allow both a signed
and an unsigned ptrofftype (but still force a common precision),
with all the fun that arises from combining (ptr p+ off1) p+ off2
with different signs for the offset operand ...

Thanks,
Richard.