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.