On 10/17/2013 10:05 AM, Richard Sandiford wrote:
Richard Biener <rguent...@suse.de> writes:
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.
The answer's the same as always. RTL constants don't have a sign.
Any time we extend an RTL constant, we need to say whether the extension
should be sign extension or zero extension. So the natural model for
RTL is that an SImode addition is done to SImode width, not some
arbitrary wider width.
RTL constants are sign-extended (whether you call them then "signed"
is up to you). They have a precision. This is how they are
valid reps for wide-ints, and that doesn't change.
I was saying that if we make not _all_ wide-ints sign-extended
then we can use the tree rep as-is. We'd then have the wide-int
rep being either zero or sign extended but not arbitrary random
bits outside of the precision (same as the tree rep).
OK, but does that have any practical value over leaving them arbitrary,
as in Kenny's original implementation? Saying that upper bits can be
either signs or zeros means that readers wouldn't be able to rely on
one particular extension (so would have to do the work that they did
in Kenny's implementation). At the same time it means that writers
can't leave the upper bits in an arbitrary state, so would have to
make sure that they are signs or zeros (presumably having a free
choice of which).
Thanks,
Richard
not so easy here. We would still need to clean things up when we
convert back to tree-cst or rtl at least for the short precisions.
Otherwise, the parts of the compiler that look inside of trees, in
particular the hash functions have to clean the value them selves.
As much as my arm still hurts from richi forcing me to change this, it
is true that it seems to have been the right thing to do.
the change for the wider unsigned trees perhaps should be backed out.
kenny
