• Keith Randall's avatar
    cmd/compile: use masks instead of branches for slicing · deb4177c
    Keith Randall authored
    When we do
    
      var x []byte = ...
      y := x[i:]
    
    We can't just use y.ptr = x.ptr + i, as the new pointer may point to the
    next object in memory after the backing array.
    We used to fix this by doing:
    
      y.cap = x.cap - i
      delta := i
      if y.cap == 0 {
        delta = 0
      }
      y.ptr = x.ptr + delta
    
    That generates a branch in what is otherwise straight-line code.
    
    Better to do:
    
      y.cap = x.cap - i
      mask := (y.cap - 1) >> 63 // -1 if y.cap==0, 0 otherwise
      y.ptr = x.ptr + i &^ mask
    
    It's about the same number of instructions (~4, depending on what
    parts are constant, and the target architecture), but it is all
    inline. It plays nicely with CSE, and the mask can be computed
    in parallel with the index (in cases where a multiply is required).
    
    It is a minor win in both speed and space.
    
    Change-Id: Ied60465a0b8abb683c02208402e5bb7ac0e8370f
    Reviewed-on: https://go-review.googlesource.com/32022
    Run-TryBot: Keith Randall <khr@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarCherry Zhang <cherryyz@google.com>
    deb4177c
rewriteARM.go 363 KB