Re: Question on multiplied address cost computation in ivopt

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Mon, Feb 25, 2013 at 5:39 PM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Fri, Feb 22, 2013 at 9:42 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>> Hi,
>> Function get_address_cost in ivopt computes multiplied address cost
>> with below code:
>>
>> First:
>>       rat = 1;
>>       for (i = 2; i <= MAX_RATIO; i++)
>>     if (multiplier_allowed_in_address_p (i, mem_mode, as))
>>       {
>>         rat = i;
>>         break;
>>       }
>>
>> Then:
>>       if (rat_p)
>>         addr = gen_rtx_fmt_ee (MULT, address_mode, addr,
>>                    gen_int_mode (rat, address_mode));
>>
>> What's the purpose of first iteration? It just finds the first allowed
>> ratio in address, causing the generated ADDR always has the minimal
>> allowed ratio. Is it right?
>
> Looks like so.
>
>> For target doesn't support multiplied address, the generated ADDR is:
>> (MULT reg, 1). The cost generally is equal to address with pure
>> register. What's the meaning of this cost?
>
> Look at its uses.

Thanks, now I understand that it calculates representative cost for
multiplied address, then uses it later. Doesn't matter which ratio is
used generally.

Another question about multiplied address is in function
multiplier_allowed_in_address_p, it constructs rtx like "(MULT reg1,
ratio)" and let backend determine whether multiplied address is
allowed. I suspect the code is implemented/tested on x86, since
machine like ARM only supports multiplied address with non-null base
register, like "base + reg * index". The code causes multiplied
address disabled totally for ARM machine.
Any comments?

Thanks


--
Best Regards.