Addition of driver-level parallel looping pragma in SCCAnnotate is fragile

[skarppinen]

Hello, filing an issue by request of @reuterbal.

The issue is that in annotate_driver of SCCAnnotate, the following code is used to place the driver-level looping pragma (i.e "acc parallel loop gang vector_length(...)"):

loki/transformations/transformations/single_column_coalesced.py

Lines 657 to 671 in 5c0f821

	if driver_loop.pragma is None or (len(driver_loop.pragma) == 1 and
	driver_loop.pragma[0].keyword.lower() == "loki" and
	driver_loop.pragma[0].content.lower() == "driver-loop"):
	p_content = f'parallel loop gang{private_clause}{vector_length_clause}'
	driver_loop._update(pragma=(ir.Pragma(keyword='acc', content=p_content),))
	driver_loop._update(pragma_post=(ir.Pragma(keyword='acc', content='end parallel loop'),))
	# add acc parallel loop gang if the only existing pragma is acc data
	elif len(driver_loop.pragma) == 1:
	if (driver_loop.pragma[0].keyword == 'acc' and
	driver_loop.pragma[0].content.lower().lstrip().startswith('data ')):
	p_content = f'parallel loop gang{private_clause}{vector_length_clause}'
	driver_loop._update(pragma=(driver_loop.pragma[0], ir.Pragma(keyword='acc', content=p_content)))
	driver_loop._update(pragma_post=(ir.Pragma(keyword='acc', content='end parallel loop'),
	driver_loop.pragma_post[0]))

The condition of the IF statement can easily be missed. For example, I ran into this issue when I used the driver loop as a guide for adding two pragmas ($loki data and $loki update_host, for purposes of DataOffloadTransformation and GlobalVarOffloadTransformation) just above the driver loop. Consequently, these pragmas ended up in driver_loop.pragma, with len(driver_loop.pragma) != 1, and thus no parallel loop pragma appeared as a result of the SCC transformation.