[LLVMdev] Avoiding MCRegAliasIterator with register units

[Renato Golin]

On 24 May 2013 17:39, Chad Rosier <mcrosier at apple.com> wrote:

> One side effect of dynamically computing the aliases is that the iterator
> does not guarantee that the entries are ordered or that duplicates have
> been removed.
>

Hi Chad,

Sounds like you're growing the list (thus the lookup time), rather than
shrinking, as I take it was Jacob's original intention?


The documentation seems to imply this is a safe assumption and I haven't
> found a client that requires these attributes (i.e., strict ordering and
> uniqueness).
>

I don't think it should matter for the purposes of aliasing.


I also setup a LNT tester and found no execution-time failures on x86 for
> -O0 and -O2 using the llvm test-suite and externals test-suite (i.e.,
> SPEC95/2K/2K6, etc.).
>

I'm wondering why not O3, too? That'll expose vector selection as well, not
just VFP, which is a big source of super-registers, at least on ARM.

Somehow it sounds as though you'll find more performance regressions than
improvements, but that's my poor assumptions based on nothing concrete. ;)

cheers,
--renato
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