cmd/compile: use masks instead of branches for slicing

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: Cherry Zhang <cherryyz@google.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/test/sliceopt.go b/test/sliceopt.go
index 17959e9326..eb24701f31 100644
--- a/test/sliceopt.go
+++ b/test/sliceopt.go
@@ -43,10 +43,11 @@ func s1(x **[]int, xs **string, i, j int) {
 	z = (**x)[i:0:j] // ERROR "Disproved IsSliceInBounds$"
 	z = (**x)[0:i:j] // ERROR "Proved boolean IsSliceInBounds$"
 	z = (**x)[0:]    // ERROR "slice: omit slice operation$"
-	z = (**x)[2:8]   // ERROR "Disproved Eq(32|64)$"
-	z = (**x)[2:2]   // ERROR "Disproved Eq(32|64)$" "Proved boolean IsSliceInBounds$"
-	z = (**x)[0:i]   // ERROR "Proved boolean IsSliceInBounds$"
-	z = (**x)[2:i:8] // ERROR "Disproved IsSliceInBounds$" "Proved IsSliceInBounds$" "Proved boolean IsSliceInBounds$"
+	z = (**x)[2:8]   // ERROR "Proved slicemask not needed$"
+	println(z)
+	z = (**x)[2:2]
+	z = (**x)[0:i]
+	z = (**x)[2:i:8] // ERROR "Disproved IsSliceInBounds$" "Proved IsSliceInBounds$"
 	z = (**x)[i:2:i] // ERROR "Proved IsSliceInBounds$" "Proved boolean IsSliceInBounds$"
 
 	z = z[0:i] // ERROR "Proved boolean IsSliceInBounds"