Re: [PATCH 3/3] Add targetm.insn_cost hook

[Sandra Loosemore <sandra@codesourcery.com>]

On 10/09/2017 01:35 PM, Segher Boessenkool wrote:
> This adds a new hook that the insn_cost function uses if a target has
> implemented it (it uses the old pattern_cost nee insn_rtx_cost if not).
>
> I'll commit this now; it was okayed by Jeff at
> https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00204.html .
>
>
> Segher
>
>
> 2017-10-09  Segher Boessenkool  <segher@kernel.crashing.org>
>
> 	* target.def (insn_cost): New hook.
> 	* doc/tm.texi.in (TARGET_INSN_COST): New hook.
> 	* doc/tm.texi: Regenerate.
> 	* rtlanal.c (insn_cost): Use the new hook.

As a target maintainer, I'm kind of confused by this patch, and I don't 
think the tm.texi change gives sufficient guidance about the default 
hook behavior, how it interacts with TARGET_RTX_COSTS and/or 
TARGET_ADDRESS_COST, or the different contexts the three hooks are used 
in.  Do target maintainers need to do something to define this new hook 
to prevent performance regressions?

I could try to write up some advice about cost models and tuning for the 
internals manual, but at present I don't feel like I have any 
understanding of what motivated this change or how it changed the 
recommended practices for back end tuning.  :-(

-Sandra the missing-some-brain-cells