phiopt: Optimize (x <=> y) cmp z [PR94589]

genericize_spaceship genericizes i <=> j to approximately
({ int c; if (i == j) c = 0; else if (i < j) c = -1; else c = 1; c; })
for strong ordering and
({ int c; if (i == j) c = 0; else if (i < j) c = -1; else if (i > j) c = 1; else c = 2; c; })
for partial ordering.
The C++ standard supports then == or != comparisons of that against
strong/partial ordering enums, or </<=/==/!=/>/>= comparisons of <=> result
against literal 0.

In some cases we already optimize that but in many cases we keep performing
all the 2 or 3 comparisons, compute the spaceship value and then compare
that.

The following patch recognizes those patterns if the <=> operands are
integral types or floating point (the latter only for -ffast-math) and
optimizes it to the single comparison that is needed (plus adds debug stmts
if needed for the spaceship result).

There is one thing I'd like to address in a follow-up: the pr94589-2.C
testcase should be matching just 12 times each, but runs
into operator>=(partial_ordering, unspecified) being defined as
(_M_value&1)==_M_value
rather than _M_value>=0.  When not honoring NaNs, the 2 case should be
unreachable and so (_M_value&1)==_M_value is then equivalent to _M_value>=0,
but is not a single use but two uses.  I'll need to pattern match that case
specially.

2021-05-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94589
	* tree-ssa-phiopt.c (tree_ssa_phiopt_worker): Call
	spaceship_replacement.
	(cond_only_block_p, spaceship_replacement): New functions.

	* gcc.dg/pr94589-1.c: New test.
	* gcc.dg/pr94589-2.c: New test.
	* gcc.dg/pr94589-3.c: New test.
	* gcc.dg/pr94589-4.c: New test.
	* g++.dg/opt/pr94589-1.C: New test.
	* g++.dg/opt/pr94589-2.C: New test.
	* g++.dg/opt/pr94589-3.C: New test.
	* g++.dg/opt/pr94589-4.C: New test.

0 files changed, 0 insertions, 0 deletions