Re: IVs optimization issue

[Aurelien Buhrig <aurelien dot buhrig dot gcc at gmail dot com>]

Le 01/03/2012 11:09, Richard Guenther a écrit :
> On Wed, Feb 29, 2012 at 6:02 PM, Aurelien Buhrig
> <aurelien.buhrig.gcc@gmail.com> wrote:
>> Le 29/02/2012 17:08, Richard Guenther a écrit :
>>> On Wed, Feb 29, 2012 at 4:41 PM, Aurelien Buhrig
>>> <aurelien.buhrig.gcc@gmail.com> wrote:
>>>> Le 29/02/2012 16:15, Richard Guenther a écrit :
>>>>> On Wed, Feb 29, 2012 at 4:08 PM, Aurelien Buhrig
>>>>> <aurelien.buhrig.gcc@gmail.com> wrote:
>>>>>>
>>>>>>> The issue is most probably that on GIMPLE we only deal with ptr_mode,
>>>>>>> not Pmode, and IVOPTs thinks that pointer induction variables will
>>>>>>> have ptr_mode.  To fix this the cost computation would need to take
>>>>>>> into account ptr_mode to Pmode conversions _and_ would need to
>>>>>>> consider Pmode IVs in the first place (I'm not sure that will be easy).
>>>>>>
>>>>>>
>>>>>> Thank you Richard for you reply.
>>>>>>
>>>>>> I guess such an issue is not in the top priority tasks of main
>>>>>> developers. So I think I'll have to look at it myself if I feel
>>>>>> confident enough to carry out such a job (I've never worked at tree level).
>>>>>>
>>>>>> My main wonder is about Pmode IVs: since GIMPLE representation only
>>>>>> deals with ptr_mode, what differentiates a Pmode IV from a ptr_mode one?
>>>>>
>>>>> Its TREE_TYPE.  In your case you'd have a POINTER_TYPE with
>>>>> PSImode for Pmode and a POINTER_TYPE with SImode for ptr_mode
>>>>> pointers.  They will differ in TYPE_MODE and TYPE_PRECISION.
>>>>
>>>> Thanks, I will look at it.
>>>>
>>>>>> BTW, this wonder is not limited to IVs. What does control the choice of
>>>>>> Pmode vs ptr_mode when mapping to RTL?
>>>>>
>>>>> ptr_mode is the C language specified mode for all pointers.  Pmode is
>>>>> the mode used for pointers in address operands of CPU instructions.
>>>>> Usually they are the same.  When mapping to RTL all ptr_mode uses
>>>>> for memory accesses are translated to Pmode while operations on
>>>>> the value of ptr_mode quantities are done on ptr_mode (IIRC).
>>>>
>>>> Another point that is not optimal for my backend is when computing the
>>>> address of an array element (M[i]). Now, both the M address and i are
>>>> extended to ptr_mode and the sum is truncated in Pmode; whereas it would
>>>> be much better to extend i to Pmode, and then perform the add in Pmode.
>>>> So if I understand correctly, the later option cannot be generated. Right?
>>>
>>> Not by IVOPTs at least.  There is also the long-standing issue that
>>> POINTER_PLUS_EXPR only accepts sizetype offsets - that may cause
>>> issues if your target does not define sizetype having the same mode as
>>> ptr_mode.  (And of course complicates using Pmode on the gimple level)
>>
>> Sorry, it wasn't related to ivopts, but on the use of Pmode from Gimple,
>> and especially when computing a M[i] address. (My ptr_mode and SIZE_TYPE
>> mode are the same). Can you confirm that it's not possible to compute
>> the address of M[i] in Pmode without truncating from ptr_mode? Because
>> mapping POINTER_PLUS_EXPR directly to Pmode would also be (with ivopts
>> PSI support) a great improvement for Pmode=PSImode targets.
> 
> Not sure what you mean with "not possible", it's not done.

It's what I meant.Thank you for your reply,
Aurélien

> Richard.
> 
>> Thanks for your help,
>> Aurélien
>>
>>> Richard.
>>>
>>>>> Richard.
>>>>>
>>>>>> Thanks,
>>>>>> Aurélien
>>>>>>
>>>>
>>