Re: Quality of VAX compilers (Henry Spencer)
15 Jun 2006 15:03:20 -0400

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From: (Henry Spencer)
Newsgroups: comp.compilers
Date: 15 Jun 2006 15:03:20 -0400
Organization: SP Systems, Toronto, Canada
References: 06-06-009 06-06-016
Keywords: code, history
Posted-Date: 15 Jun 2006 15:03:20 EDT

  our moderator wrote:
>...normal compiler for the late 70s, reasonable common subexpression
>handling and pre-graph-coloring register allocation. They spent a lot
>of effort in instruction selection, to use all of the Vax instructions
>and address modes...

Which in fact was a mistake, because on a good many of the VAX models,
the code ran faster if you used only the simpler instructions and modes,
and built up anything fancy as a multi-instruction sequence using them.
The simple stuff often got fast-tracked during instruction decoding.
Some of the inspiration of the RISC movement came from this.

(A good illustration of the penalty paid for all the VAX's complexity came
from comparing the VAX 730 to the PDP-11/44. They came in the same box,
and were roughly contemporary. The 730 CPU filled almost all of the box,
to the point that some special two-in-one peripheral cards got invented
just so you could build a complete system without needing an expansion
box. The 44 CPU, despite making much less use of custom/semicustom LSI,
occupied less than half of the box. And for problems that would fit in
its 16-bit address space, the 44 was 2-3 times as fast.)
-- is temporarily off the air; | Henry Spencer
mail to henry at instead. |

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