-optc-O2 considered useful

[Don Stewart]

I discovered something today I didn't know.
gcc -O2 can optimise out the computed jumps GHC produces in tight loops.

Consider this program,

    import Data.Array.Vector
    import Data.Bits

    main = print . sumU
                 . mapU (*2)
                 . mapU (`shiftL` 2)
                 $ replicateU (100000000 :: Int) (5::Int)

Yields this core:

    $wfold :: Int# -> Int# -> Int#

    $wfold =
      \ (ww_sMp :: Int#) (ww1_sMt :: Int#) ->
        case ww1_sMt of wild_X10 {
          __DEFAULT -> $wfold (+# ww_sMp 40) (+# wild_X10 1);
          100000000 -> ww_sMp

And -O2 -fasm:

    Main_zdwfold_info:
      movq %rdi,%rax
      cmpq $100000000,%rax
      jne .LcOk
      movq %rsi,%rbx
      jmp *(%rbp)

    .LcOk:
      incq %rax
      addq $40,%rsi
      movq %rax,%rdi
      jmp Main_zdwfold_info

    $ time ./sum
    4000000000
    ./sum  0.19s user 0.00s system 101% cpu 0.188 total


-O2 -fvia-C -optc-O:

    Main_zdwfold_info:
      cmpq        $100000000, %rdi
      jne .L3
      movq        %rsi, %rbx
      movq        (%rbp), %rax
    .L4:
      jmp *%rax
    .L3:
      addq        $40, %rsi
      leaq        1(%rdi), %rdi
      movl        $Main_zdwfold_info, %eax
      jmp .L4

    $ time ./sum
    4000000000
    ./sum  0.34s user 0.00s system 94% cpu 0.361 total

Hmm. That movl, jmp .L4 ; jmp *%rax looks sucky, and performance got worse.
And now with -O2 -fvia-C -optc-O2

    Main_zdwfold_info:
      cmpq        $100000000, %rdi
      je  .L5
    .L3:
      addq        $40, %rsi
      leaq        1(%rdi), %rdi
      jmp Main_zdwfold_info

    $ time ./sum
    4000000000
    ./sum  0.11s user 0.02s system 106% cpu 0.122 total

Woot, back in business.

-- Don