On 05/03/2013 08:12 AM, Richard Biener wrote:
On Fri, May 3, 2013 at 1:49 PM, Kenneth Zadeck <zad...@naturalbridge.com> wrote:
On 05/03/2013 07:34 AM, Richard Biener wrote:
On Thu, Apr 25, 2013 at 1:18 AM, Kenneth Zadeck
<zad...@naturalbridge.com> wrote:
On 04/24/2013 11:13 AM, Richard Biener wrote:
On Wed, Apr 24, 2013 at 5:00 PM, Richard Sandiford
<rdsandif...@googlemail.com> wrote:
Richard Biener<richard.guent...@gmail.com> writes:
On Wed, Apr 24, 2013 at 4:29 PM, Richard Sandiford
<rdsandif...@googlemail.com> wrote:
In other words, one of the reasons wide_int can't be exactly 1:1
in practice is because it is clearing out these mistakes (GEN_INT
rather than gen_int_mode) and missing features (non-power-of-2
widths).
Note that the argument should be about CONST_WIDE_INT here,
not wide-int. Indeed CONST_WIDE_INT has the desired feature
and can be properly truncated/extended according to mode at the time
we
build it
via immed_wide_int_cst (w, mode). I don't see the requirement that
wide-int itself is automagically providing that truncation/extension
(though it is a possibility, one that does not match existing behavior
of
HWI for CONST_INT or double-int for CONST_DOUBLE).
I agree it doesn't match the existing behaviour of HWI for CONST_INT or
double-int for CONST_DOUBLE, but I think that's very much a good thing.
The model for HWIs at the moment is that you have to truncate results
to the canonical form after every operation where it matters. As you
proved in your earlier message about the plus_constant bug, that's
easily
forgotten. I don't think the rtl code is doing all CONST_INT
arithmetic
on full HWIs because it wants to: it's doing it because that's the way
C/C++ arithmetic on primitive types works. In other words, the current
CONST_INT code is trying to emulate N-bit arithmetic (for gcc runtime
N)
using a single primitive integer type. wide_int gives us N-bit
arithmetic
directly; no emulation is needed.
Ok, so what wide-int provides is integer values encoded in 'len' HWI
words that fit in 'precision' or more bits (and often in less).
wide-int
also provides N-bit arithmetic operations. IMHO both are tied
too closely together. A give constant doesn't really have a precision.
Associating one with it to give a precision to an arithmetic operation
looks wrong to me and are a source of mismatches.
What RTL currently has looks better to me - operations have
explicitely specified precisions.
I have tried very hard to make wide-int work very efficiently with both
tree
and rtl without biasing the rep towards either representation. Both rtl
and
trees constants have a precision. In tree, constants are done better
than
in rtl because the tree really does have a field that is filled in that
points to a type. However, that does not mean that rtl constants do not
have
a precision: currently you have to look around at the context to find the
mode of a constant that is in your hand, but it is in fact always there.
At the rtl level, you can see the entire patch - we always find an
appropriate mode.
Appearantly you cannot. See Richard S. examples.
As of "better", the tree has the issue that we have so many unshared
constants because they only differ in type but not in their
representation.
That's the nice part of RTL constants all having VOIDmode ...
Richard.
I said we could always find a mode, i did not say that in order to find the
mode we did not have to stand on our head, juggle chainsaws and say "mother
may i". The decision to leave the mode as void in rtl integer constants
was made to save space, but comes with an otherwise very high cost and in
today's world of cheap memory seems fairly dated. It is a decision that i
and others would love to change and the truth is wide int is one step in
that direction (in that it gets rid of the pun of using double-int for both
integers and floats where the discriminator is voidmode for ints.) But for
now we have to live with that poor decision.
As far as I have read your wide-int patches the CONST_WIDE_INT RTX
object does not include a mode. So I don't see it as a step forward in
any way (other than that it makes it explicit that you _do_ need a mode
to do any operation on a constant).
Richard.
There are several problems with just dropping a mode into the already
existing mode field of an rtx constant.
1) There may be places where the a back end is testing equality to see
if constants of different modes are in fact the same value.
2) Most of the places what build int constants use GEN_INT which does
not take a mode, even though about 95% of those places have a mode right
there and the rest just take a little work. There are constructor
that do take a mode, but in the end they just throw the mode on the floor.
3) The canonical test to see if a CONST_DOUBLE contains an int or float
is to test if the mode is VOIDmode.
Any port that is converted to have TARGET_SUPPORTS_WIDE_INT has no more
of problem (3). I admit that rooting out (1) is likely to be the worst
of the problems. But we were careful to at least make this work move
us in the correct direction.
