Re: IVs optimization issue

[Richard Guenther <richard dot guenther at gmail dot com>]

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.

> 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).

Richard.

> Thanks,
> Aurélien
>