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Re: Some aliasing questions

On 04/08/2016 11:10 AM, Bill Schmidt wrote:
> The first is an issue with TOC-relative addresses on PowerPC.  These are
> symbolic addresses that are to be loaded from a fixed slot in the table
> of contents, as addressed by the TOC pointer (r2).  In the RTL phases
> prior to register allocation, these are described in an UNSPEC that
> looks like this for an example store:
> (set (mem/c:DI (unspec:DI [
>        (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])
>        (reg:DI 2 2)
>       ] UNSPEC_TOCREL) [1 svul+0 S8 A128])
>      (reg:DI 178))
> The UNSPEC helps keep track of the r2 reference until this is split into
> two or more insns depending on the memory model.

That's why Alpha uses LO_SUM for pre-reload tracking of such things.

Even though that's a bit of a liberty, since there's no HIGH to go along with
the LO_SUM.  But at least it allows the middle-end to continue to find the symbol.

> (1) What is the best way to handle this?  Would it be better to have
> some sort of target hook?

Perhaps, yes.

> Another issue I see involves disjoint alias sets.  If you look closely
> at the rtx's above, they have been marked as disjoint, belonging to
> alias sets 1 and 4, respectively:
>   _11 = (__vector unsigned long *) _10;
>   static unsigned long long svul[2] __attribute__ ((aligned (16)));

Be consistent about unsigned long vs unsigned long long and this will be fixed.


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