Tuesday, March 23, 2010

impressed with llvm, again

Testing out pattern matching against constants, I wrote the following silly function:
(define thing
  0 1 -> 2
  0 y -> 3
  1 0 -> 4
  1 x -> (+ x 5)
  y z -> (+ y z)
(thing 10 20)
Which turned into a nice set of if statements, doing 4 tests in total. Not bad. But I was curious to see what kind of asm LLVM would generate for it.
        pushq   %rbp
        movq    %rsp, %rbp
        movq    _heap0(%rip), %rax
        movq    $772, (%rax)
        movq    $10, 8(%rax)
        movq    $10, 16(%rax)
        movq    $1, 24(%rax)
        movl    $61, %eax
        popq    %rbp

Huh. So that bit there at the very end, "movl $61, %eax". Yeah. That's the answer, "30". Even with all the tagging/untagging, registers, embedded if statements, etc... LLVM still optimized the entire program away to a constant. Nice.

1 comment:

  1. Haha. LLVM is an amazing compiler toolkit. It builds a use-def graph where every element is dependent on other elements. Once you supply constants to the leafs of the graphs, llvm simply promotes these constants to the root of the graph until you get a single constant.