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Re: IVs optimization issue
Le 29/02/2012 16:15, Richard Guenther a écrit :
> On Wed, Feb 29, 2012 at 4:08 PM, Aurelien Buhrig
> <email@example.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?