Re: [PATCH] Add a bit dislike for separate mem alternative when op is REG_P.

[Alexander Monakov <amonakov@ispras.ru>]

On Wed, 25 May 2022, liuhongt via Gcc-patches wrote:

> Rigt now, mem_cost for separate mem alternative is 1 * frequency which
> is pretty small and caused the unnecessary SSE spill in the PR, I've tried
> to rework backend cost model, but RA still not happy with that(regress
> somewhere else). I think the root cause of this is cost for separate 'm'
> alternative cost is too small, especially considering that the mov cost
> of gpr are 2(default for REGISTER_MOVE_COST). So this patch increase mem_cost
> to 2*frequency, also increase 1 for reg_class cost when m alternative.

In the PR, the spill happens in the initial basic block of the function, i.e.
the one with the highest frequency.

Also as noted in the PR, swapping the 'unlikely' branch to 'likely' avoids
the spill, even though it does not affect the frequency of the initial basic
block, and makes the block with the use more rarely executed.

Do you have a root cause analysis that explains the above?

Alexander