Re: How many vector registers are useful?

jlg@cochiti.lanl.gov (J. Giles)
Tue, 26 Jan 1993 18:33:43 GMT

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Newsgroups: comp.sys.super,comp.arch,comp.compilers
From: jlg@cochiti.lanl.gov (J. Giles)
Organization: Los Alamos National Laboratory
Date: Tue, 26 Jan 1993 18:33:43 GMT
References: 93-01-174
Keywords: architecture, vector

kirchner@uklira.informatik.uni-kl.de (Reinhard Kirchner) writes:
> A register has an optimizing effect only when the value in it can be used
                                                                            ^^^^
> several times, at least twice, to avoid loading the value again from
> memory. We all know that this is to a great extent possible on scalar
> machines.


Vectors and scalars are not really all that different. Unless you have
overlap between the vectors they really behave just like large scalars.
My objection to the above is the statement that multiple use is the *only*
optimizing effect. The other (also true of scalars) is when the value is
an intermediate. A value which was just produced as the result of an
operation, which is needed as an operand almost immediately, and that's
the last you'll ever use it will not benefit from being written to memory
and read back in again. Of course, if you can chain your functional units
together without using intermediaries at all, you won't need registers for
this purpose.
--
J. Giles
--

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