Re: [PATCH V3] RISC-V: Support Dynamic LMUL Cost model

[Robin Dapp <rdapp.gcc@gmail.com>]

Hi Juzhe,

glad that we can use the dominator info directly.  Could we move the
calculation of the info to the beginning (if it's not available)?  That
makes it clearer that it's a prerequisite.  Function comments look
good now.

Some general remarks kind of similar to v1:

 - I would prefer a hash_map or similar to hold the end point for a range
   instead of looking through potentially all ranges in contrived cases.

 - As long as we're just looking for the maximum number of live registers,
   we can use a sliding-window approach:  create a structure with all
   start and end points, sort it, and increase the current pressure
   if we start a new range or decrease.  That's O(n log n).

> +      const ssa_use_operand_t *const head = &(SSA_NAME_IMM_USE_NODE (t));
> +      const ssa_use_operand_t *ptr;
> +
> +      for (ptr = head->next; ptr != head; ptr = ptr->next)
> +	{

Why does FOR_EACH_IMM_USE not work here?

> +		      unsigned int max_point
> +			= (*program_points_per_bb.get (e->src)).length () - 1;
> +		      for (k = 0; k < (*live_ranges).length (); k++)
> +			{
> +			  if ((*live_ranges)[i].var == def)

Would also be nice not having to search through all ranges but just index/hash
it via var (or similar).

What about one test with global live ranges?  Not a necessity IMHO we can still
add it later.

Regards
 Robin