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Compiling

Compiling. “premature optimization is the root of all evil.” -Donald Knuth. Optimization. In LISP, you can write slow code fast or fast code slow Experience will help you overcome common pitfalls (over use of append in recursive algorithms for example)

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Compiling

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  1. Compiling “premature optimization is the root of all evil.” -Donald Knuth

  2. Optimization • In LISP, you can write slow code fast or fast code slow • Experience will help you overcome common pitfalls (over use of append in recursive algorithms for example) • In general, worry about the top level algorithms first. You can always speed it up later

  3. Compiling • LISP can be compiled into machine code just like other languages • Typically results in a drastic speed up for LISP programs • Compilation can reduce both running time and memory consumption

  4. Time a Function • You can see how long it takes to evaluate a function by wrapping the function call in a call to time (defun waste-time (n) (dotimes (i n) (* i i))) >(time (waste-time 50000)) real time : 0.020 secs run-gbc time : 0.020 secs child run time : 0.000 secs gbc time : 0.000 secs NIL

  5. Compile >(time (waste-time 50000000)) real time : 21.690 secs run-gbc time : 20.770 secs child run time : 0.000 secs gbc time : 0.870 secs NIL >(compile 'waste-time) […Compiler output…] >(time (waste-time 50000000)) real time : 11.220 secs run-gbc time : 10.200 secs child run time : 0.000 secs gbc time : 0.950 secs NIL

  6. Disassemble • We can look at the assembly code produced by compile using disassemble • System and implementation dependent • Here’s an example in CLISP >(defun add1 (n) (1+ n)) ADD1 >(disassemble 'add1) Disassembly of function ADD1 1 required argument 0 optional arguments No rest parameter No keyword parameters 3 byte-code instructions: 0 (LOAD&PUSH 1) 1 (CALLS2 151) ; 1+ 3 (SKIP&RET 2) NIL • GCL output can be a mess since it actually transforms LISP into C code

  7. Compile-File • You can also compile all functions in a given file (compile-file "file.lisp") • To run functions from the compiled file, load it like any other file (load "file.fas") • The specific file extension depends on implementation

  8. Inlining • You can make a function be an inline function by using declare (defun f (x) (* x x)) (defun g (x) (declare (inline f)) (+ (f x) (f x))) • Now, upon compiling g, the actual body of f will be copied into g in the pertinent locations, though only within g. • To make f be an inline function everywhere, use declaim instead of declare, within the global scope. • Cannot be used with recursive functions

  9. More Type Safety • declare and declaim can specify other compiler directives • You can use to streamline compiled functions to only expect certain types using type >(defun f (x y) (declare (type integer x y)) (+ x y)) F >(f 5.6 3.4) 9.0 >(compile 'f) …Compiler output… >(f 3 4) 7 >(f 5.6 3.4) Error

  10. Optimizing • The optimize declaration allows several optimization priorities to be set • Priorities are on a scale from 0 to 3, with 3 the most important • compilation-speed: speed of the compilation process • debug: ease of debugging • safety: run-time error checking • space: both code size and run-time space • speed: speed of the object code • Example specification (defun f (x y) (declare (optimize (safety 2) (speed 3))) (+ x y))

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