fortran: Use pointer arithmetic to index arrays [PR102043]

fortran: Use pointer arithmetic to index arrays [PR102043]

The code generated for array references used to be ARRAY_REF trees as
could be expected.  However, the middle-end may conclude from those
trees that the indexes used are non-negative (more precisely not below
the lower bound), which is a wrong assumption in the case of "reversed-
order" arrays.

The problematic arrays are those with a descriptor and having a negative
stride for at least one dimension.  The descriptor data points to the
first element in array order (which is not the first in memory order in
that case), and the negative stride(s) makes walking the array backwards
(towards lower memory addresses), and we can access elements with
negative index wrt data pointer.

With this change, pointer arithmetic is generated by default for array
references, unless we are in a case where negative indexes can’t happen
(array descriptor’s dim element, substrings, explicit shape,
allocatable, or assumed shape contiguous).  A new flag is added to
choose between array indexing and pointer arithmetic, and it’s set
if the context can tell array indexing is safe (descriptor dim
element, substring, temporary array), or a new method is called
to decide on whether the flag should be set for one given array
expression.

	PR fortran/102043

gcc/fortran/ChangeLog:

	* trans.h (gfc_build_array_ref): Add non_negative_offset
	argument.
	* trans.cc (gfc_build_array_ref): Ditto. Use pointer arithmetic
	if non_negative_offset is false.
	* trans-expr.cc (gfc_conv_substring): Set flag in the call to
	gfc_build_array_ref.
	* trans-array.cc (gfc_get_cfi_dim_item,
	gfc_conv_descriptor_dimension): Same.
	(build_array_ref): Decide on whether to set the flag and update
	the call.
	(gfc_conv_scalarized_array_ref): Same.  New argument tmp_array.
	(gfc_conv_tmp_array_ref): Update call to
	gfc_conv_scalarized_ref.
	(non_negative_strides_array_p): New function.

gcc/testsuite/ChangeLog:

	* gfortran.dg/array_reference_3.f90: New.
	* gfortran.dg/negative_stride_1.f90: New.
	* gfortran.dg/vector_subscript_8.f90: New.
	* gfortran.dg/vector_subscript_9.f90: New.
	* gfortran.dg/c_loc_test_22.f90: Update dump patterns.
	* gfortran.dg/finalize_10.f90: Same.

Co-Authored-By: Richard Biener <rguenther@suse.de>