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- From: kirchner@uklira.informatik.uni-kl.de (Reinhard Kirchner)
- Subject: How many vector registers are useful?
- Reply-To: kirchner@uklira.informatik.uni-kl.de (Reinhard Kirchner)
- Organization: University of Kaiserslautern, Germany
- Date: Mon, 25 Jan 1993 13:11:46 GMT
- Approved: compilers@iecc.cambridge.ma.us
- Message-ID: <93-01-174@comp.compilers>
- Keywords: architecture, question
- Sender: compilers-sender@iecc.cambridge.ma.us
- Lines: 44
-
- Hello,
-
- On discussing various merrits of different vector machines we came about
- the issue of the register architectures.
-
- There are on one side the cray and convex with 8 vector registers a 64 or
- 128 words, and on the other side,
-
- The Fujitsu machines with their reconfigurable register file of 32 or
- 64kb, which is 4k or 8k words, being grouped from 256 register a 16/32
- words to 8 registers a 512/1024 words.
-
- Now there is the question: is such a large register file useful at all ?
-
- Some thoughts:
-
- 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.
-
- But how is this on vector machines ? The register creates a speedup only
- when it can hold an entire vector, which can be used again later. This
- requires a register long enough to do so. That means vectors of e.g. a
- length of 5000 can not be held anyway, every machine must load, process,
- and store it in pieces, and only a lot of memory bandwidth helps.
-
- When configured as a few long vectors the Fujitsu vector registers may
- help, but then comes the second question: Are there any statistics on the
- reusing of vectors? I know about such things for scalar registers, where
- people found that 32 is plenty enough, and only 8 help a lot. But in these
- cases registers are used for loop indexes, addresses etc., which can not
- be compared to the use of vector registers.
-
- So: what can be gained with such a big vector register file ? Or is it
- only of limited help ? Can the register file be traded against bandwith to
- load and store from memory ?
-
- Reinhard Kirchner
- Univ. of Kaiserslautern, Germany
- kirchner@informatik.uni-kl.de
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