Hi Richard,
> No. It's never correct to completely wipe out the existing cost - you
> don't know the context where this is being used.
>
> The most you can do is not add any additional cost.
Remember that aarch64_rtx_costs starts like this:
/* By default, assume that everything has equivalent cost to the
cheapest instruction. Any additional costs are applied as a delta
above this default. */
*cost = COSTS_N_INSNS (1);
This is literally the last statement executed before the big switch...
Given the cost is always initialized, there is no existing cost besides this
default value, and thus changing it to something else is not an issue.
We could of course do something like:
*cost -= COSTS_N_INSNS (1);
But that is less clear and problematic if the default value ever changes.
Cheers,
Wilco
