[wide-int] int_traits <tree>

[Kenneth Zadeck]

On 10/17/2013 08:29 AM, Richard Biener wrote:
> On Thu, 17 Oct 2013, Richard Sandiford wrote:
>
>> Richard Biener <rguenther@suse.de> writes:
>>>> The new tree representation can have a length greater than max_len
>>>> for an unsigned tree constant that occupies a whole number of HWIs.
>>>> The tree representation of an unsigned 0x8000 is 0x00 0x80 0x00.
>>>> When extended to max_wide_int the representation is the same.
>>>> But a 2-HWI addr_wide_int would be 0x80 0x00, without the leading zero.
>>>> The MIN trims the length from 3 to 2 in the last case.
>>> Oh, so it was the tree rep that changed?  _Why_ was it changed?
>>> We still cannot use it directly from wide-int and the extra
>>> word is redundant because we have access to TYPE_UNSIGNED (TREE_TYPE ()).
>> It means that we can now use the tree's HWI array directly, without any
>> copying, for addr_wide_int and max_wide_int.  The only part of decompose ()
>> that does a copy is the small_prec case, which is trivially compiled out
>> for addr_wide_int and max_wide_int.
> "     2) addr_wide_int.  This is a fixed size representation that is
>       guaranteed to be large enough to compute any bit or byte sized
>       address calculation on the target.  Currently the value is 64 + 4
>       bits rounded up to the next number even multiple of
>       HOST_BITS_PER_WIDE_INT (but this can be changed when the first
>       port needs more than 64 bits for the size of a pointer).
>
>       This flavor can be used for all address math on the target.  In
>       this representation, the values are sign or zero extended based
>       on their input types to the internal precision.  All math is done
>       in this precision and then the values are truncated to fit in the
>       result type.  Unlike most gimple or rtl intermediate code, it is
>       not useful to perform the address arithmetic at the same
>       precision in which the operands are represented because there has
>       been no effort by the front ends to convert most addressing
>       arithmetic to canonical types.
>
>       In the addr_wide_int, all numbers are represented as signed
>       numbers.  There are enough bits in the internal representation so
>       that no infomation is lost by representing them this way."
>
> so I guess from that that addr_wide_int.get_precision is always
> that "64 + 4 rounded up".  Thus decompose gets that constant precision
> input and the extra zeros make the necessary extension always a no-op.
> Aha.
it is until someone comes up with a port that this will not work for, 
then they will have to add some machinery to sniff the port and make 
this bigger.    I am hoping to be retired by the time this happens.
> For max_wide_int the same rules apply, just its size is larger.
>
> Ok.  So the reps are only canonical wide-int because we only
> ever use them with precision > xprecision (maybe we should assert
> that).
It is now asserted for (as of a few days ago).

>
> Btw, we are not using them directly, but every time we actually
> build a addr_wide_int / max_wide_int we copy them anyway:
>
> /* Initialize the storage from integer X, in precision N.  */
> template <int N>
> template <typename T>
> inline fixed_wide_int_storage <N>::fixed_wide_int_storage (const T &x)
> {
>    /* Check for type compatibility.  We don't want to initialize a
>       fixed-width integer from something like a wide_int.  */
>    WI_BINARY_RESULT (T, FIXED_WIDE_INT (N)) *assertion ATTRIBUTE_UNUSED;
>    wide_int_ref xi (x, N);
>    len = xi.len;
>    for (unsigned int i = 0; i < len; ++i)
>      val[i] = xi.val[i];
> }


> it avoids a 2nd copy though, which shows nicely what was rummaging in
> my head for the last two days - that the int_trais <> abstraction
> was somehow at the wrong level - it should have been traits that
> are specific to the storage model?  or the above should use
> int_traits<>::decompose manually with it always doing the
> copy (that would also optimize away one copy and eventually
> would make the extra zeros not necessary).
this came in with richard's storage manager patch.    In my older code, 
we tried and succeeded many times to just borrow the underlying rep.    
I think that richard needs to work this out.
> I originally thought that extra zeros get rid of all copying from trees
> to all wide-int kinds.
>
> What's the reason again to not use my original proposed encoding
> of the MSB being the sign bit?  RTL constants simply are all signed
> then.  Just you have to also sign-extend in functions like lts_p
> as not all constants are sign-extended.  But we can use both tree
> (with the now appended zero) and RTL constants representation
> unchanged.
I am not following you here.   In trees, the msb is effectively a sign 
bit, even for unsigned numbers because we add that extra block.

but inside of wide int, we do not add extra blocks beyond the 
precision.   That would be messy for a lot of other reasons.