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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Thu, 6 Mar 2025 02:30:55 -0000 (UTC)
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According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:
>By contrast, making good use of the complex instructions of VAX in a
>compiler consumed significant resources (e.g., Figure 2 of
>https://dl.acm.org/doi/pdf/10.1145/502874.502876 reports about a
>factor 1.5 more code in the code generator for VAX than for RISC-II).
>Compilers at the time did not use the CISCy features much, which is
>one reason why the IBM 801 project and later the Berkeley RISC and
>Stanford MIPS proposed replacing them with a load/store architecture.

I'm not so sure. The IBM Fortran H compiler used a lot of the 360's instruction
set and it is my recollection that even the dmr C compiler would generate memory
to memory instructions when appropriate. The PL.8 compiler generated code for 5
architectures including S/360 and 68K, and I think I read somewhere that its
S/360 code was considrably better than the native PL/I compilers.

I get the impression that they found that once you have a reasonable number of
registers, like 16 or more, the benefit of complex instructions drops because
you can make good use of the values in the registers.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly