runtime: use vDSO on linux/386 to improve time.Now performance

This change adds support for accelerating time.Now by using
the __vdso_clock_gettime fast-path via the vDSO on linux/386
if it is available.

When the vDSO path to the clocks is available, it is typically
5x-10x faster than the syscall path (see benchmark extract
below).  Two such calls are made for each time.Now() call
on most platforms as of go 1.9.

- Add vdso_linux_386.go, containing the ELF32 definitions
  for use by vdso_linux.go, the maximum array size, and
  the symbols to be located in the vDSO.

- Modify runtime.walltime and runtime.nanotime to check for
  and use the vDSO fast-path if available, or fall back to
  the existing syscall path.

- Reduce the stack reservations for runtime.walltime and
  runtime.monotime from 32 to 16 bytes. It appears the syscall
  path actually only needed 8 bytes, but 16 is now needed to
  cover the syscall and vDSO paths.

- Remove clearing DX from the syscall paths as clock_gettime
  only takes 2 args (BX, CX in syscall calling convention),
  so there should be no need to clear DX.

The included BenchmarkTimeNow was run with -cpu=1 -count=20
on an "Intel(R) Celeron(R) CPU J1900 @ 1.99GHz", comparing
released go 1.9.1 vs this change. This shows a gain in
performance on linux/386 (6.89x), and that no regression
occurred on linux/amd64 due to this change.

Kernel: linux/i686, GOOS=linux GOARCH=386
   name      old time/op  new time/op  delta
   TimeNow   978ns ± 0%   142ns ± 0%  -85.48%  (p=0.000 n=16+20)

Kernel: linux/x86_64, GOOS=linux GOARCH=amd64
   name      old time/op  new time/op  delta
   TimeNow   125ns ± 0%   125ns ± 0%   ~       (all equal)

Gains are more dramatic in virtualized environments,
presumably due to the overhead of virtualizing the syscall.

Fixes #22190

Change-Id: I2f83ce60cb1b8b310c9ced0706bb463c1b3aedf8
Reviewed-on: https://go-review.googlesource.com/69390
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os_linux_noauxv.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64,!arm,!arm64,!mips,!mipsle,!mips64,!mips64le,!s390x,!ppc64,!ppc64le
+// +build !386,!amd64,!arm,!arm64,!mips,!mipsle,!mips64,!mips64le,!s390x,!ppc64,!ppc64le
 
 package runtime
 

src/runtime/sys_linux_386.s

@@ -203,12 +203,34 @@ TEXT runtime·mincore(SB),NOSPLIT,$0-16
 	RET
 
 // func walltime() (sec int64, nsec int32)
-TEXT runtime·walltime(SB), NOSPLIT, $32
+TEXT runtime·walltime(SB), NOSPLIT, $16
+	// Stack layout, depending on call path:
+	//  x(SP)   vDSO            INVOKE_SYSCALL
+	//    12    ts.tv_nsec      ts.tv_nsec
+	//     8    ts.tv_sec       ts.tv_sec
+	//     4    &ts             -
+	//     0    CLOCK_<id>      -
+	//
+	// If we take the vDSO path, we're calling a function with gcc calling convention.
+	// We're guaranteed 128 bytes on entry. We've taken 16, and the call uses another 4,
+	// leaving 108 for __vdso_clock_gettime to use.
+	MOVL	runtime·__vdso_clock_gettime_sym(SB), AX
+	CMPL	AX, $0
+	JEQ	fallback
+
+	LEAL	8(SP), BX	// &ts (struct timespec)
+	MOVL	BX, 4(SP)
+	MOVL	$0, 0(SP)	// CLOCK_REALTIME
+	CALL	AX
+	JMP finish
+
+fallback:
 	MOVL	$SYS_clock_gettime, AX
 	MOVL	$0, BX		// CLOCK_REALTIME
 	LEAL	8(SP), CX
-	MOVL	$0, DX
 	INVOKE_SYSCALL
+
+finish:
 	MOVL	8(SP), AX	// sec
 	MOVL	12(SP), BX	// nsec
 
@@ -220,12 +242,25 @@ TEXT runtime·walltime(SB), NOSPLIT, $32
 
 // int64 nanotime(void) so really
 // void nanotime(int64 *nsec)
-TEXT runtime·nanotime(SB), NOSPLIT, $32
+TEXT runtime·nanotime(SB), NOSPLIT, $16
+	// See comments above in walltime() about stack space usage and layout.
+	MOVL	runtime·__vdso_clock_gettime_sym(SB), AX
+	CMPL	AX, $0
+	JEQ	fallback
+
+	LEAL	8(SP), BX	// &ts (struct timespec)
+	MOVL	BX, 4(SP)
+	MOVL	$1, 0(SP)	// CLOCK_MONOTONIC
+	CALL	AX
+	JMP finish
+
+fallback:
 	MOVL	$SYS_clock_gettime, AX
 	MOVL	$1, BX		// CLOCK_MONOTONIC
 	LEAL	8(SP), CX
-	MOVL	$0, DX
 	INVOKE_SYSCALL
+
+finish:
 	MOVL	8(SP), AX	// sec
 	MOVL	12(SP), BX	// nsec
 

src/runtime/vdso_linux.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build linux
-// +build amd64
+// +build 386 amd64
 
 package runtime
 

src/runtime/vdso_linux_386.go

+// Copyright 2012 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.
+
+package runtime
+
+// ELF32 structure definitions for use by the Linux vDSO loader
+
+type elfSym struct {
+	st_name  uint32
+	st_value uint32
+	st_size  uint32
+	st_info  byte
+	st_other byte
+	st_shndx uint16
+}
+
+type elfVerdef struct {
+	vd_version uint16 /* Version revision */
+	vd_flags   uint16 /* Version information */
+	vd_ndx     uint16 /* Version Index */
+	vd_cnt     uint16 /* Number of associated aux entries */
+	vd_hash    uint32 /* Version name hash value */
+	vd_aux     uint32 /* Offset in bytes to verdaux array */
+	vd_next    uint32 /* Offset in bytes to next verdef entry */
+}
+
+type elfEhdr struct {
+	e_ident     [_EI_NIDENT]byte /* Magic number and other info */
+	e_type      uint16           /* Object file type */
+	e_machine   uint16           /* Architecture */
+	e_version   uint32           /* Object file version */
+	e_entry     uint32           /* Entry point virtual address */
+	e_phoff     uint32           /* Program header table file offset */
+	e_shoff     uint32           /* Section header table file offset */
+	e_flags     uint32           /* Processor-specific flags */
+	e_ehsize    uint16           /* ELF header size in bytes */
+	e_phentsize uint16           /* Program header table entry size */
+	e_phnum     uint16           /* Program header table entry count */
+	e_shentsize uint16           /* Section header table entry size */
+	e_shnum     uint16           /* Section header table entry count */
+	e_shstrndx  uint16           /* Section header string table index */
+}
+
+type elfPhdr struct {
+	p_type   uint32 /* Segment type */
+	p_offset uint32 /* Segment file offset */
+	p_vaddr  uint32 /* Segment virtual address */
+	p_paddr  uint32 /* Segment physical address */
+	p_filesz uint32 /* Segment size in file */
+	p_memsz  uint32 /* Segment size in memory */
+	p_flags  uint32 /* Segment flags */
+	p_align  uint32 /* Segment alignment */
+}
+
+type elfShdr struct {
+	sh_name      uint32 /* Section name (string tbl index) */
+	sh_type      uint32 /* Section type */
+	sh_flags     uint32 /* Section flags */
+	sh_addr      uint32 /* Section virtual addr at execution */
+	sh_offset    uint32 /* Section file offset */
+	sh_size      uint32 /* Section size in bytes */
+	sh_link      uint32 /* Link to another section */
+	sh_info      uint32 /* Additional section information */
+	sh_addralign uint32 /* Section alignment */
+	sh_entsize   uint32 /* Entry size if section holds table */
+}
+
+type elfDyn struct {
+	d_tag int32  /* Dynamic entry type */
+	d_val uint32 /* Integer value */
+}
+
+type elfVerdaux struct {
+	vda_name uint32 /* Version or dependency names */
+	vda_next uint32 /* Offset in bytes to next verdaux entry */
+}
+
+const (
+	// vdsoArrayMax is the byte-size of a maximally sized array on this architecture.
+	// See cmd/compile/internal/x86/galign.go arch.MAXWIDTH initialization, but must also
+	// be constrained to max +ve int.
+	vdsoArrayMax = 1<<31 - 1
+)
+
+var sym_keys = []symbol_key{
+	{"__vdso_clock_gettime", 0xd35ec75, 0x6e43a318, &__vdso_clock_gettime_sym},
+}
+
+// initialize to fall back to syscall
+var (
+	__vdso_clock_gettime_sym uintptr = 0
+)

src/runtime/vdso_linux_test.go

+// Copyright 2017 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 386 amd64
+
+package runtime_test
+
+import (
+	"testing"
+	"time"
+	_ "unsafe"
+)
+
+// These tests are a little risky because they overwrite the __vdso_clock_gettime_sym value.
+// It's normally initialized at startup and remains unchanged after that.
+
+//go:linkname __vdso_clock_gettime_sym runtime.__vdso_clock_gettime_sym
+var __vdso_clock_gettime_sym uintptr
+
+func TestClockVDSOAndFallbackPaths(t *testing.T) {
+	// Check that we can call walltime() and nanotime() with and without their (1st) fast-paths.
+	// This just checks that fast and fallback paths can be called, rather than testing their
+	// results.
+	//
+	// Call them indirectly via time.Now(), so we don't need auxiliary .s files to allow us to
+	// use go:linkname to refer to the functions directly.
+
+	save := __vdso_clock_gettime_sym
+	if save == 0 {
+		t.Log("__vdso_clock_gettime symbol not found; fallback path will be used by default")
+	}
+
+	// Call with fast-path enabled (if vDSO symbol found at startup)
+	time.Now()
+
+	// Call with fast-path disabled
+	__vdso_clock_gettime_sym = 0
+	time.Now()
+	__vdso_clock_gettime_sym = save
+}
+
+func BenchmarkClockVDSOAndFallbackPaths(b *testing.B) {
+	run := func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			// Call via time.Now() - see comment in test above.
+			time.Now()
+		}
+	}
+
+	save := __vdso_clock_gettime_sym
+	b.Run("vDSO", run)
+	__vdso_clock_gettime_sym = 0
+	b.Run("Fallback", run)
+	__vdso_clock_gettime_sym = save
+}
+
+func BenchmarkTimeNow(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		time.Now()
+	}
+}