runtime: use vDSO clock_gettime on linux/mips64x

Speed up nanotime1 and walltime1 on MIPS64 with vDSO, just like the
other vDSO-enabled targets.

Benchmark numbers on Loongson 3A3000 (GOARCH=mips64le, 1.4GHz) against
current master:

benchmark                old ns/op     new ns/op     delta
BenchmarkNow             868           293           -66.24%
BenchmarkNowUnixNano     851           296           -65.22%

Performance hit on fallback case, tested by using a wrong vDSO symbol name:

benchmark                old ns/op     new ns/op     delta
BenchmarkNow             868           889           +2.42%
BenchmarkNowUnixNano     851           893           +4.94%

Change-Id: Ibfb48893cd060536359863ffee7624c00def646b
GitHub-Last-Rev: 03a58ac2e4e036a4f61227cfd013082871e92863
GitHub-Pull-Request: golang/go#35181
Reviewed-on: https://go-review.googlesource.com/c/go/+/203578
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/src/runtime/vdso_linux_mips64x.go b/src/runtime/vdso_linux_mips64x.go
new file mode 100644
index 0000000000..3a0f947612
--- /dev/null
+++ b/src/runtime/vdso_linux_mips64x.go
@@ -0,0 +1,28 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux
+// +build mips64 mips64le
+
+package runtime
+
+const (
+	// vdsoArrayMax is the byte-size of a maximally sized array on this architecture.
+	// See cmd/compile/internal/mips64/galign.go arch.MAXWIDTH initialization.
+	vdsoArrayMax = 1<<50 - 1
+)
+
+// see man 7 vdso : mips
+var vdsoLinuxVersion = vdsoVersionKey{"LINUX_2.6", 0x3ae75f6}
+
+// The symbol name is not __kernel_clock_gettime as suggested by the manpage;
+// according to Linux source code it should be __vdso_clock_gettime instead.
+var vdsoSymbolKeys = []vdsoSymbolKey{
+	{"__vdso_clock_gettime", 0xd35ec75, 0x6e43a318, &vdsoClockgettimeSym},
+}
+
+// initialize to fall back to syscall
+var (
+	vdsoClockgettimeSym uintptr = 0
+)