Re: [PLUG] The mysterious case of the Linux Page Table Isolation patches

[gary@duzan.org]

Rich wrote:
=> On Wed, Jan 3, 2018 at 12:23 PM, George Zipperlen
=> <george.zipperlen@mail.com> wrote:
=>> How is it that CISC beat out RISC for high performance computing?
=>> I haven't payed attention to hardware for 20 years,  but I still
=>> remember
=>> the Pentium FDIV bug...
=>
=> [...]
=>
=> I can't really think of any technical reasons why CISC would beat RISC
=> other than legacy code that can't be recompiled using a decent
=> compiler.  To the degree that CISC can give hints to the processor to
=> better perform speculative execution I'd argue that a RISC instruction
=> set could be created that does the same (have the compiler always
=> output code that makes a particular branch the most common one).

   I can think of one: bandwidth for getting instructions from main
memory. CISC instructions tend to be shorter and require fewer
instructions to get the job done than RISC, so you can get more
instructions started with the same memory bandwidth with CISC.

                                 Gary Duzan



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