Re: [RFC] Removal of loop notes

[Jim Wilson <wilson at specifix dot com>]

Zdenek Dvorak wrote:
> *sched* -- no idea what happens there; it seems to make REG_SAVE_NOTE
>   notes from loop notes, and then makes some magic, but I do not
>   understand what and why.

The loop notes are converted into REG_NOTES, and attached to an adjacent 
instruction.  The REG_SAVE_NOTES are treated as scheduling barriers. 
Nothing is allowed to move forwards and backwards across them.  After 
scheduling, the REG_SAVE_NOTES are converted back to loop notes.  This 
ensures that loop structure is preserved by the instruction scheduler. 
All insns inside the loop before sched will still be inside the loop 
after sched.  All insns outside the loop before sched will still be 
outside the loop after sched.

The reason for doing this is because some of the register lifetime info 
computed by flow is loop dependent.  For instance, reg_n_refs is 
multiplied by a constant factor if a use occurs inside a loop.  This 
ensures the registers used inside a loop are more likely to get a hard 
register than pseudo registers used outside a loop.  However, a side 
effect of this means that the scheduler either has to preserve loop 
structure to prevent the numbers from getting out of whack, or else we 
need to recompute flow info after sched.  The decision was made long ago 
to preserve loop structure.

This is my recollection of how and why things work they do.  My 
recollection could be wrong and/or out-of-date.

I see that flow no longer uses loop_depth when computing REG_N_REFS, so 
the original reason for the sched support seems to be gone.
--
Jim Wilson, GNU Tools Support, http://www.specifix.com