New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

cmd/compile: combine contiguous multi-byte comparisons to a constant #17106

Open
josharian opened this Issue Sep 14, 2016 · 0 comments

Comments

Projects
None yet
1 participant
@josharian
Contributor

josharian commented Sep 14, 2016

On architectures that allow unaligned loads, we should rewrite b[0] == c1 && b[1] == c2 to *(*uint16)(&b[0]) == (c1 << 8) + c2. And do all architectures for cases in which we know b[0] is appropriately aligned. Also uint32 and uint64 (as appropriate).

See CL 26758 for more discussion; that CL will cause many of these to be generated.

We might also independently want to update the front end (near OCMPSTR) to generate the larger comparisons directly.

package p

import "testing"

var (
    sink   bool
    b1, b2 byte
)

func Benchmark8(b *testing.B) {
    for i := 0; i < b.N; i++ {
        sink = b1 == 5 && b2 == 9
    }
}

func Benchmark16(b *testing.B) {
    u16 := uint16(b1<<8) + uint16(b2)
    const c16 = (5 << 8) + 9
    for i := 0; i < b.N; i++ {
        sink = u16 == c16
    }
}
Benchmark8-8                2000000000           0.99 ns/op
Benchmark16-8               2000000000           0.68 ns/op

@josharian josharian added this to the Unplanned milestone Sep 14, 2016

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment