Architecture of Lisp Machines (2008) [pdf]

Re:Architecture of Lisp Machines (2008) [pdf] by pfdietz on Friday July 02, 2021 @06:29PM (#27717714)

The single most important trick I remember for Lisp on stock hardware was implementing pointers to cons cells as pointers to the next byte, and doing car/cdr by -1(reg) and 3(reg) (or 7(reg) on a 64 bit machine). This automatically traps on non-conses without any extra cost.

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Re:Architecture of Lisp Machines (2008) [pdf] by rjsw on Saturday July 03, 2021 @08:36AM (#27721206)

The original SPARC CPU is designed to use this tag encoding scheme, section D.4 of the V8 Architecture Manual describes how to use it.

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Re:Architecture of Lisp Machines (2008) [pdf] by pfdietz on Saturday July 03, 2021 @10:46AM (#27721924)

Yes, that gives you flagging/trapping on arithmetic if any args are not fixnums. Aside from that, the idea works on other architectures as well.

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Re:Architecture of Lisp Machines (2008) [pdf] by bitwize on Saturday July 03, 2021 @01:42AM (#27719642)

Oooooh, that is "square root magic constant" levels of dirty.

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Re:Architecture of Lisp Machines (2008) [pdf] by pfdietz on Saturday July 03, 2021 @06:04AM (#27720541)

Also, it lets you implement fixnums with zero low order bits. That is, fixnum x is implemented as x << 2 (on 32 bit machines) or x << 3 (on 64 bit machines). With this encoding, addition and subtraction that is known to produce another fixnum can be done with ordinary add/sub instructions.

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