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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>
This commit is contained in:
Keith Randall 2015-04-30 10:41:57 -07:00
parent 71274e4857
commit 135389d0ff
1 changed files with 34 additions and 8 deletions
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42
src/cmd/internal/gc/cgen.go
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@ -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;