RE: [PATCH] PR middle-end/111701: signbit(x*x) vs -fsignaling-nans

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

> From: Richard Biener <richard.guenther@gmail.com>
> On Fri, Apr 26, 2024 at 10:19 AM Roger Sayle <roger@nextmovesoftware.com>
> wrote:
> >
> > This patch addresses PR middle-end/111701 where optimization of
> > signbit(x*x) using tree_nonnegative_p incorrectly eliminates a
> > floating point multiplication when the operands may potentially be signaling
> NaNs.
> >
> > The above bug fix also provides a solution or work-around to the
> > tricky issue in PR middle-end/111701, that the results of IEEE
> > operations on NaNs are specified to return a NaN result, but fail to
> > (precisely) specify the exact NaN representation of this result.
> > Hence for the operation "-NaN*-NaN" different hardware implementations
> > (targets) return different results.  Ultimately knowing what the
> > resulting NaN "payload" of an operation is can only be known by
> > executing that operation at run-time, and I'd suggest that GCC's
> > -fsignaling-nans provides a mechanism for handling code that uses NaN
> > representations for communication/signaling (which is a different but related
> concept to IEEE's sNaN).
> >
> > One nice thing about this patch, which may or may not be a P2
> > regression fix, is that it only affects (improves) code compiled with
> > -fsignaling-nans so should be extremely safe even for this point in stage 3.
> >
> > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> > and make -k check, both with and without --target_board=unix{-m32}
> > with no new failures.  Ok for mainline?
> 
> Hmm, but the bugreports are not about sNaN but about the fact that the sign of
> the NaN produced by 0/0 or by -NaN*-NaN is not well-defined.
> So I don't think this is the correct approach to fix this.  We'd instead have to use
> tree_expr_maybe_nan_p () - and if NaN*NaN cannot be -NaN (is that at least
> specified?) then the RECURSE path should still work as well.

If we ignore the bugzilla PR for now, can we agree that if x is a signaling NaN,
that we shouldn't be eliminating x*x?  i.e. that this patch fixes a real bug, but
perhaps not (precisely) the one described in PR middle-end/111701.

Once the signaling NaN case is correctly handled, the use of -fsignaling-nans
can be used as a workaround for PR 111701, allowing it to perhaps be reduced
from a P2 to a P3 regression (or even not a bug if the qNaN case is undefined behavior).
When I wrote this patch I was trying to help with GCC 14's stage 3.
 
> > 2024-04-26  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         PR middle-end/111701
> >         * fold-const.cc (tree_binary_nonnegative_warnv_p) <case MULT_EXPR>:
> >         Split handling of floating point and integer types.  For equal
> >         floating point operands, avoid optimization if the operand may be
> >         a signaling NaN.
> >
> > gcc/testsuite/ChangeLog
> >         PR middle-end/111701
> >         * gcc.dg/pr111701-1.c: New test case.
> >         * gcc.dg/pr111701-2.c: Likewise.
> >