match.pd: Replace incorrect simplifications into copysign [PR90248]

match.pd: Replace incorrect simplifications into copysign [PR90248]

In the PR Andrew said he has implemented a simplification that has been
added to LLVM, but that actually is not true, what is in there are
X * (X cmp 0.0 ? +-1.0 : -+1.0) simplifications into +-abs(X)
but what has been added into GCC are (X cmp 0.0 ? +-1.0 : -+1.0)
simplifications into copysign(1, +-X) and then
X * copysign (1, +-X) into +-abs (X).
The problem is with the (X cmp 0.0 ? +-1.0 : -+1.0) simplifications,
they don't work correctly when X is zero.
E.g.
(X > 0.0 ? 1.0 : -1.0)
is -1.0 when X is either -0.0 or 0.0, but copysign will make it return
1.0 for 0.0 and -1.0 only for -0.0.
(X >= 0.0 ? 1.0 : -1.0)
is 1.0 when X is either -0.0 or 0.0, but copysign will make it return
still 1.0 for 0.0 and -1.0 for -0.0.
The simplifications were guarded on !HONOR_SIGNED_ZEROS, but as discussed in
the PR, that option doesn't mean that -0.0 will not ever appear as operand
of some operation, it is hard to guarantee that without compiler adding
canonicalizations of -0.0 to 0.0 after most of the operations and thus
making it very slow, but that the user asserts that he doesn't care if the result
of operations will be 0.0 or -0.0.  Not to mention that some of the
transformations are incorrect even for positive 0.0.

So, instead of those simplifications this patch recognizes patterns where
those ?: expressions are multiplied by X, directly into +-abs.
That works fine even for 0.0 and -0.0 (as long as we don't care about
whether the result is exactly 0.0 or -0.0 in those cases), because
whether the result of copysign is -1.0 or 1.0 doesn't matter when it is
multiplied by 0.0 or -0.0.

As a follow-up, maybe we should add the simplification mentioned in the PR,
in particular doing copysign by hand through
VIEW_CONVERT_EXPR <int, float_X> < 0 ? -float_constant : float_constant
into copysign (float_constant, float_X).  But I think that would need to be
done in phiopt.

2021-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/90248
	* match.pd (X cmp 0.0 ? 1.0 : -1.0 -> copysign(1, +-X),
	X cmp 0.0 ? -1.0 : +1.0 -> copysign(1, -+X)): Remove
	simplifications.
	(X * (X cmp 0.0 ? 1.0 : -1.0) -> +-abs(X),
	X * (X cmp 0.0 ? -1.0 : 1.0) -> +-abs(X)): New simplifications.

	* gcc.dg/tree-ssa/copy-sign-1.c: Don't expect any copysign
	builtins.
	* gcc.dg/pr90248.c: New test.