cmd/internal/gc: Use shifts for powers-of-two indexing

Fixes #10638

Change-Id: I7bbaad7e5a599aa94d1d158e903596231c7e9897
Reviewed-on: https://go-review.googlesource.com/9535
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

diff --git a/src/cmd/internal/gc/cgen.go b/src/cmd/internal/gc/cgen.go
index 15dca374a31e5e40e946bd7975395432f791de4d..501cdcb1c8782bddb8dd9c70b95ca713624723f9 100644 (file)
--- a/src/cmd/internal/gc/cgen.go
+++ b/src/cmd/internal/gc/cgen.go
@@ -1132,12 +1132,18 @@ func Agenr(n *Node, a *Node, res *Node) {
                        } else if w == 1 {
                                Thearch.Gins(Thearch.Optoas(OADD, Types[Tptr]), &n2, &n3)
                        } else {
-                               Regalloc(&n4, Types[TUINT32], nil)
-                               Nodconst(&n1, Types[TUINT32], int64(w))
-                               Thearch.Gmove(&n1, &n4)
-                               Thearch.Gins(Thearch.Optoas(OMUL, Types[TUINT32]), &n4, &n2)
+                               if w&(w-1) == 0 {
+                                       // Power of 2.  Use shift.
+                                       Thearch.Ginscon(Thearch.Optoas(OLSH, Types[TUINT32]), int64(log2(uint64(w))), &n2)
+                               } else {
+                                       // Not a power of 2.  Use multiply.
+                                       Regalloc(&n4, Types[TUINT32], nil)
+                                       Nodconst(&n1, Types[TUINT32], int64(w))
+                                       Thearch.Gmove(&n1, &n4)
+                                       Thearch.Gins(Thearch.Optoas(OMUL, Types[TUINT32]), &n4, &n2)
+                                       Regfree(&n4)
+                               }
                                Thearch.Gins(Thearch.Optoas(OADD, Types[Tptr]), &n2, &n3)
-                               Regfree(&n4)
                        }
                        *a = n3
                        Regfree(&n2)
@@ -1292,8 +1298,13 @@ func Agenr(n *Node, a *Node, res *Node) {
                        } else if w == 1 {
                                Thearch.Gins(Thearch.Optoas(OADD, Types[Tptr]), &n2, &n3)
                        } else {
-                               Nodconst(&tmp, Types[TUINT32], int64(w))
-                               Thearch.Gins(Thearch.Optoas(OMUL, Types[TUINT32]), &tmp, &n2)
+                               if w&(w-1) == 0 {
+                                       // Power of 2.  Use shift.
+                                       Thearch.Ginscon(Thearch.Optoas(OLSH, Types[TUINT32]), int64(log2(uint64(w))), &n2)
+                               } else {
+                                       // Not a power of 2.  Use multiply.
+                                       Thearch.Ginscon(Thearch.Optoas(OMUL, Types[TUINT32]), int64(w), &n2)
+                               }
                                Thearch.Gins(Thearch.Optoas(OADD, Types[Tptr]), &n2, &n3)
                        }
 
@@ -1485,7 +1496,13 @@ func Agenr(n *Node, a *Node, res *Node) {
                } else if w == 1 {
                        Thearch.Gins(Thearch.Optoas(OADD, Types[Tptr]), &n2, &n3)
                } else {
-                       Thearch.Ginscon(Thearch.Optoas(OMUL, t), int64(w), &n2)
+                       if w&(w-1) == 0 {
+                               // Power of 2.  Use shift.
+                               Thearch.Ginscon(Thearch.Optoas(OLSH, t), int64(log2(w)), &n2)
+                       } else {
+                               // Not a power of 2.  Use multiply.
+                               Thearch.Ginscon(Thearch.Optoas(OMUL, t), int64(w), &n2)
+                       }
                        Thearch.Gins(Thearch.Optoas(OADD, Types[Tptr]), &n2, &n3)
                }
 
@@ -1502,6 +1519,15 @@ func Agenr(n *Node, a *Node, res *Node) {
        }
 }
 
+// log2 returns the logarithm base 2 of n.  n must be a power of 2.
+func log2(n uint64) int {
+       x := 0
+       for n>>uint(x) != 1 {
+               x++
+       }
+       return x
+}
+
 /*
  * generate:
  *     res = &n;