RE: [PATCH] Ignore (possible) signed zeros in operands of FP comparisons.

["Roger Sayle" <roger@nextmovesoftware.com>]

Hi Jeff/Andrew,
> If you're going to do more work in this space, you might want to reach out to
> Aldy and Andrew to see if there's space for collaboration.

One (clever?) suggestion that I do have for ranger would be to add support for
an additional value_range_kind, VR_NONZEROBITS, which would be a variant of
VR_RANGE (for unsigned types?) and require very few changes to the existing
code.  Just like VR_RANGE all values would lie in [MIN, MAX], so by default
treating this value_range_kind identically to VR_RANGE there should be no
visible changes, but the change in semantics is that MIN has the minimum bits
set, and MAX, the maximum bits set [equivalent to the RVAL and RMASK pairs
from CCP's bit_value_{bin,un}op].  Hence, the VR_NONZEROBITS range [2,7]
would represent the possible values {2, 3, 6, 7} rather than {2, 3, 4, 5, 6, 7}. 
For a small number of bits, int_range can already handle this with multiple
irange spans, but adding this representation would allow the unification of the
bit-based propagation performed in tree-ssa-ccp with the range-value based
propagation performed in EVRP/ranger, allowing the clever forwards/backwards
functionality.

As Andrew's recent (partial) review points out, tracking the effect of operations
like BIT_XOR_EXPR on VR_RANGE is much more complicated than on the
proposed VR_NONZEROBITS.

Alas, I'm not the sort of contributor to make large infrastructure changes
myself, but if the above functionality were in place, I/the compiler would
be able to make use of it.

Cheers,
Roger
--

> -----Original Message-----
> From: Jeff Law <jeffreyalaw@gmail.com>
> Sent: 17 March 2022 23:28
> To: Roger Sayle <roger@nextmovesoftware.com>; 'Richard Biener'
> <richard.guenther@gmail.com>
> Cc: 'GCC Patches' <gcc-patches@gcc.gnu.org>
> Subject: Re: [PATCH] Ignore (possible) signed zeros in operands of FP
> comparisons.
> 
> 
> On 3/15/2022 2:03 AM, Roger Sayle wrote:
> >> -----Original Message-----
> >> From: Richard Biener <richard.guenther@gmail.com>
> >> Sent: 15 March 2022 07:29
> >> To: Roger Sayle <roger@nextmovesoftware.com>
> >> Cc: GCC Patches <gcc-patches@gcc.gnu.org>
> >> Subject: Re: [PATCH] Ignore (possible) signed zeros in operands of FP
> >> comparisons.
> >>
> >> On Mon, Mar 14, 2022 at 8:26 PM Roger Sayle
> >> <roger@nextmovesoftware.com> wrote:
> >>>
> >>> I've been wondering about the possible
> >>> performance/missed-optimization impact of my patch for PR
> >>> middle-end/98420 and similar IEEE correctness fixes that disable
> >>> constant folding optimizations when worrying
> >> about -0.0.
> >>> In the common situation where the floating point result is used by a
> >>> FP comparison, there's no distinction between +0.0 and -0.0, so some
> >>> HONOR_SIGNED_ZEROS optimizations that we'd usually disable, are safe.
> >>>
> >>> Consider the following interesting example:
> >>>
> >>> int foo(int x, double y) {
> >>>      return (x * 0.0) < y;
> >>> }
> >>>
> >>> Although we know that x (when converted to double) can't be NaN or
> >>> Inf, we still worry that for negative values of x that (x * 0.0) may
> >>> be -0.0 and so perform the multiplication at run-time.  But in this
> >>> case, the result of the comparison (-0.0 < y) will be exactly the
> >>> same as (+0.0 < y) for any y, hence the above may be safely constant
> >>> folded to "0.0 <
> >> y"
> >>> avoiding the multiplication at run-time.
> >>>
> >>> This patch has been tested on x86_64-pc-linux-gnu with make
> >>> bootstrap and make -k check with no new failures, and allows GCC to
> >>> continue to optimize cases that we optimized in GCC 11 (without regard to
> correctness).
> >>> Ok for mainline?
> >> Isn't that something that gimple-ssa-backprop.c is designed to
> >> handle?  I wonder if you can see whether the signed zero speciality can be
> retrofitted there?
> >> It currently tracks "sign does not matter", so possibly another
> >> state, "sign of zero does not matter" could be introduced there.
> > Two questions. Would adding tracking of "sign of zero does not matter"
> > to gimple-ssa-backprop.c be suitable for stage4?  Secondly, even if
> > gimple-ssa-backprop.c performed this kind of optimization, would that
> > be a reason not to support these transformations in match.pd?  Perhaps
> > someone could open a missed optimization PR for backprop in Bugzilla,
> > but the above patch still needs to be reviewed on its own merits.
> 
> Can't see how it's appropriate for stage4, but definitely interesting for gcc-13.
> 
> It'd fit well into some of the Ranger plans too -- Aldy and Andrew have been
> talking about tracking the special FP values in Ranger.   This is related, though
> not exactly the same since rather than tracking the special value, you're tracking
> if those special values actually matter. If you're going to do more work in this
> space, you might want to reach out to Aldy and Andrew to see if there's space
> for collaboration.
> 
> 
> >
> > Speaking of tree-ssa passes that could be improved, I was wondering
> > whether you could review my EVRP patch to fix regression PR/102950.  Pretty
> please?
> > https://gcc.gnu.org/pipermail/gcc-patches/2022-February/589569.html
> 
> I forwarded this to Aldy & Andrew.  I suspect they missed it.
> 
> 
> >
> > Thanks (as always),
> 
> No, thank you.  I'm so happy to see you contributing to GCC regularly again!
> 
> 
> Jeff
> 
> >