[PATCH v4 0/5] openmp: Add support for iterators in OpenMP mapping clauses

[Kwok Cheung Yeung]

This is a revised version of the patch series posted at: 
https://gcc.gnu.org/pipermail/gcc-patches/2024-October/664541.html

The previous version performed the expansion of the iterators into loops 
at the omp-lowering stage, but that meant that the Gimplification of the 
clauses with iterators was also delayed to that point, which is undesirable.

To fix this, I have split the handling of iterators into two phases. The 
first stage happens during the gimplification pass - the iterator loops 
are created, then immediately Gimplified. The resulting Gimple sequence 
is later stored separately as an operand in the Gimple target statement 
rather than incorporated directly into the main program code. When the 
clause decl and sizes are gimplified in gimplify_scan_omp_clauses and 
gimplify_adjust_omp_clauses, we enter the bind context of the iterator 
loop body corresponding to the clause then gimplify the decl/size 
directly into the loop body (which works because the iterator variables 
are in scope at that point).

In the omp lowering pass, lower_omp_map_iterator_expr and 
lower_omp_map_iterator_size as still called just before sending the 
hostaddrs/sizes to the arrays read by libgomp, but they now enter the 
bind context of the relevant loop body and generate code there. After 
lowering is done, the gimple sequence containing the loops is inserted 
just before the Gimple code for the target statement.

The behaviour with clauses not using all variables specified in an 
iterator is also changed. It is now made into a warning when detected 
instead of an error, and the iterator is trimmed to exclude the unused 
variables. If no variables are used then the iterator is removed 
entirely. If an iterator is shared between multiple clauses with 
different sets of variables used, then the iterator is unshared and 
trimmed accordingly.