runtime: fix wall time computation in macOS High Sierra

Latest macOS High Sierra changed how the wall time information
is exported into the commpage. Backward compatibility was partly
preserved, that is previous Go versions are basically forced to
go through a syscall which is much slower and is not able to
get nanosecond precision.

Implement the new commpage layout and wall time computation,
using a version check to fallback to the previous code on
older operating systems.

Fixes #22037

Change-Id: I8c2176eaca83a5d7be23443946a6b4c653ec7f68
Reviewed-on: https://go-review.googlesource.com/67332
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/sys_darwin_amd64.s

@@ -107,15 +107,24 @@ TEXT runtime·madvise(SB), NOSPLIT, $0
 	RET
 
 // OS X comm page time offsets
-// http://www.opensource.apple.com/source/xnu/xnu-1699.26.8/osfmk/i386/cpu_capabilities.h
+// https://opensource.apple.com/source/xnu/xnu-4570.1.46/osfmk/i386/cpu_capabilities.h
+
+#define	commpage_version	0x1e
+
 #define	v12_nt_tsc_base	0x50
 #define	v12_nt_scale	0x58
 #define	v12_nt_shift	0x5c
 #define	v12_nt_ns_base	0x60
 #define	v12_nt_generation	0x68
-#define	v12_gtod_generation	0x6c
-#define	v12_gtod_ns_base	0x70
-#define	v12_gtod_sec_base	0x78
+#define	v12_gtod_generation	0x6c  // obsolete since High Sierra (v13)
+#define	v12_gtod_ns_base	0x70  // obsolete since High Sierra (v13)
+#define	v12_gtod_sec_base	0x78  // obsolete since High Sierra (v13)
+
+#define	v13_gtod_ns_base	0xd0
+#define	v13_gtod_sec_ofs	0xd8
+#define	v13_gtod_frac_ofs	0xe0
+#define	v13_gtod_scale		0xe8
+#define	v13_gtod_tkspersec	0xf0
 
 TEXT runtime·nanotime(SB),NOSPLIT,$0-8
 	MOVQ	$0x7fffffe00000, BP	/* comm page base */
@@ -151,6 +160,75 @@ TEXT time·now(SB), NOSPLIT, $32-24
 	// are used in the systime fallback, as the timeval address
 	// filled in by the system call.
 	MOVQ	$0x7fffffe00000, BP	/* comm page base */
+	CMPW	commpage_version(BP), $13
+	JB		v12 /* sierra and older */
+
+	// This is the new code, for macOS High Sierra (v13) and newer.
+v13:
+	// Loop trying to take a consistent snapshot
+	// of the time parameters.
+timeloop13:
+	MOVQ 	v13_gtod_ns_base(BP), R12
+
+	MOVL	v12_nt_generation(BP), CX
+	TESTL	CX, CX
+	JZ		timeloop13
+	RDTSC
+	MOVQ	v12_nt_tsc_base(BP), SI
+	MOVL	v12_nt_scale(BP), DI
+	MOVQ	v12_nt_ns_base(BP), BX
+	CMPL	v12_nt_generation(BP), CX
+	JNE		timeloop13
+
+	MOVQ 	v13_gtod_sec_ofs(BP), R8
+	MOVQ 	v13_gtod_frac_ofs(BP), R9
+	MOVQ 	v13_gtod_scale(BP), R10
+	MOVQ 	v13_gtod_tkspersec(BP), R11
+	CMPQ 	v13_gtod_ns_base(BP), R12
+	JNE 	timeloop13
+
+	// Compute monotonic time
+	//	mono = ((tsc - nt_tsc_base) * nt_scale) >> 32 + nt_ns_base
+	// The multiply and shift extracts the top 64 bits of the 96-bit product.
+	SHLQ	$32, DX
+	ADDQ	DX, AX
+	SUBQ	SI, AX
+	MULQ	DI
+	SHRQ	$32, AX:DX
+	ADDQ	BX, AX
+
+	// Subtract startNano base to return the monotonic runtime timer
+	// which is an offset from process boot.
+	MOVQ	AX, BX
+	MOVQ	runtime·startNano(SB), CX
+	SUBQ	CX, BX
+	MOVQ	BX, monotonic+16(FP)
+
+	// Now compute the 128-bit wall time:
+	//  wall = ((mono - gtod_ns_base) * gtod_scale) + gtod_offs
+	// The parameters are updated every second, so if we found them
+	// outdated (that is, more than one second is passed from the ns base),
+	// fallback to the syscall.
+	TESTQ	R12, R12
+	JZ		systime
+	SUBQ	R12, AX
+	CMPQ	R11, AX
+	JB		systime
+	MULQ 	R10
+	ADDQ	R9, AX
+	ADCQ	R8, DX
+
+	// Convert the 128-bit wall time into (sec,nsec).
+	// High part (seconds) is already good to go, while low part
+	// (fraction of seconds) must be converted to nanoseconds.
+	MOVQ	DX, sec+0(FP)
+	MOVQ 	$1000000000, CX
+	MULQ	CX
+	MOVQ	DX, nsec+8(FP)
+	RET
+
+	// This is the legacy code needed for macOS Sierra (v12) and older.
+v12:
 	// Loop trying to take a consistent snapshot
 	// of the time parameters.
 timeloop: