OpenMP/Fortran: Non-rectangular loops with constant steps other than 1 or -1 [PR107424]

Before this commit, gfortran produced with OpenMP for 'do i = 1,10,2'
the code
  for (count.0 = 0; count.0 < 5; count.0 = count.0 + 1)
    i = count.0 * 2 + 1;

While such an inner loop can be collapsed, a non-rectangular could not.
With this commit and for all constant loop steps, a simple loop such
as 'for (i = 1; i <= 10; i = i + 2)' is created. (Before only for the
constant steps of 1 and -1.)

The constant step permits to know the direction (increasing/decreasing)
that is required for the loop condition.

The new code is only valid if one assumes no overflow of the loop variable.
However, the Fortran standard can be read that this must be ensured by
the user. Namely, the Fortran standard requires (F2023, 10.1.5.2.4):
"The execution of any numeric operation whose result is not defined by
the arithmetic used by the processor is prohibited."

And, for DO loops, F2023's "11.1.7.4.3 The execution cycle" has the
following: The number of loop iterations handled by an iteration count,
which would permit code like 'do i = huge(i)-5, huge(i),4'. However,
in step (3), this count is not only decremented by one but also:
  "... The DO variable, if any, is incremented by the value of the
  incrementation parameter m3."
And for the example above, 'i' would be 'huge(i)+3' in the last
execution cycle, which exceeds the largest model number and should
render the example as invalid.

	PR fortran/107424

gcc/fortran/ChangeLog:

	* trans-openmp.cc (gfc_nonrect_loop_expr): Accept all
	constant loop steps.
	(gfc_trans_omp_do): Likewise; use sign to determine
	loop direction.

libgomp/ChangeLog:

	* libgomp.texi (Impl. Status 5.0): Add link to new PR110735.
	* testsuite/libgomp.fortran/non-rectangular-loop-1.f90: Enable
	commented tests.
	* testsuite/libgomp.fortran/non-rectangular-loop-1a.f90: Remove
	test file; tests are in non-rectangular-loop-1.f90.
	* testsuite/libgomp.fortran/non-rectangular-loop-5.f90: Change
	testcase to use a non-constant step to retain the 'sorry' test.
	* testsuite/libgomp.fortran/non-rectangular-loop-6.f90: New test.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/linear-2.f90: Update dump to remove
	the additional count variable.

1 files changed, 3 insertions, 1 deletions

diff --git a/libgomp/libgomp.texi b/libgomp/libgomp.texi
index 639dd05..a8582b5 100644
--- a/libgomp/libgomp.texi
+++ b/libgomp/libgomp.texi
@@ -194,7 +194,9 @@ The OpenMP 4.5 specification is fully supported.
 @item @code{requires} directive @tab P
       @tab complete but no non-host device provides @code{unified_shared_memory}
 @item @code{teams} construct outside an enclosing target region @tab Y @tab
-@item Non-rectangular loop nests @tab P @tab Full support for C/C++, partial for Fortran
+@item Non-rectangular loop nests @tab P
+      @tab Full support for C/C++, partial for Fortran
+           (@uref{https://gcc.gnu.org/PR110735,PR110735})
 @item @code{!=} as relational-op in canonical loop form for C/C++ @tab Y @tab
 @item @code{nonmonotonic} as default loop schedule modifier for worksharing-loop
       constructs @tab Y @tab