Re: Why VAX Was the Ultimate CISC and Not RISC

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.

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