runtime: move arm hardware division support detection to internal/cpu

Assumes mandatory VFP and VFPv3 support to be present by default
but not IDIVA if AT_HWCAP is not available.

Adds GODEBUGCPU options to disable the use of code paths in the runtime
that use hardware support for division.

Change-Id: Ida02311bd9b9701de3fc120697e69445bf6c0853
Reviewed-on: https://go-review.googlesource.com/114826
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/internal/cpu/cpu.go

@@ -66,6 +66,16 @@ type ppc64 struct {
 	_          CacheLinePad
 }
 
+var ARM arm
+
+// The booleans in arm contain the correspondingly named cpu feature bit.
+// The struct is padded to avoid false sharing.
+type arm struct {
+	_        CacheLinePad
+	HasIDIVA bool
+	_        CacheLinePad
+}
+
 var ARM64 arm64
 
 // The booleans in arm64 contain the correspondingly named cpu feature bit.

src/internal/cpu/cpu_arm.go

@@ -5,3 +5,28 @@
 package cpu
 
 const CacheLineSize = 32
+
+// arm doesn't have a 'cpuid' equivalent, so we rely on HWCAP/HWCAP2.
+// These are linknamed in runtime/os_(linux|freebsd)_arm.go and are
+// initialized by archauxv().
+// These should not be changed after they are initialized.
+var HWCap uint
+var HWCap2 uint
+
+// HWCAP/HWCAP2 bits. These are exposed by Linux and FreeBSD.
+const (
+	hwcap_IDIVA = 1 << 17
+)
+
+func doinit() {
+	options = []option{
+		{"idiva", &ARM.HasIDIVA},
+	}
+
+	// HWCAP feature bits
+	ARM.HasIDIVA = isSet(HWCap, hwcap_IDIVA)
+}
+
+func isSet(hwc uint, value uint) bool {
+	return hwc&value != 0
+}

src/internal/cpu/cpu_no_init.go

@@ -5,6 +5,7 @@
 // +build !386
 // +build !amd64
 // +build !amd64p32
+// +build !arm
 // +build !arm64
 // +build !ppc64
 // +build !ppc64le

src/runtime/cpuflags.go

@@ -14,4 +14,6 @@ const (
 	offset_x86_HasAVX2 = unsafe.Offsetof(cpu.X86.HasAVX2)
 	offset_x86_HasERMS = unsafe.Offsetof(cpu.X86.HasERMS)
 	offset_x86_HasSSE2 = unsafe.Offsetof(cpu.X86.HasSSE2)
+
+	offset_arm_HasIDIVA = unsafe.Offsetof(cpu.ARM.HasIDIVA)
 )

src/runtime/os_darwin_arm.go

@@ -4,8 +4,6 @@
 
 package runtime
 
-var hardDiv bool // TODO: set if a hardware divider is available
-
 func checkgoarm() {
 	// TODO(minux): FP checks like in os_linux_arm.go.
 

src/runtime/os_freebsd.go

@@ -389,6 +389,7 @@ const (
 	_AT_PAGESZ   = 6  // Page size in bytes
 	_AT_TIMEKEEP = 22 // Pointer to timehands.
 	_AT_HWCAP    = 25 // CPU feature flags
+	_AT_HWCAP2   = 26 // CPU feature flags 2
 )
 
 func sysauxv(auxv []uintptr) {

src/runtime/os_freebsd_arm.go

@@ -4,22 +4,29 @@
 
 package runtime
 
+import "internal/cpu"
+
 const (
 	_HWCAP_VFP   = 1 << 6
 	_HWCAP_VFPv3 = 1 << 13
-	_HWCAP_IDIVA = 1 << 17
 )
 
-var hwcap = ^uint32(0) // set by archauxv
-var hardDiv bool       // set if a hardware divider is available
+// AT_HWCAP is not available on FreeBSD-11.1-RELEASE or earlier.
+// Default to mandatory VFP hardware support for arm being available.
+// If AT_HWCAP is available goarmHWCap will be updated in archauxv.
+// TODO(moehrmann) remove once all go supported FreeBSD versions support _AT_HWCAP.
+var goarmHWCap uint = (_HWCAP_VFP | _HWCAP_VFPv3)
 
 func checkgoarm() {
-	if goarm > 5 && hwcap&_HWCAP_VFP == 0 {
+	// Update cpu.HWCap to match goarmHWCap in case they were not updated in archauxv.
+	cpu.HWCap = goarmHWCap
+
+	if goarm > 5 && cpu.HWCap&_HWCAP_VFP == 0 {
 		print("runtime: this CPU has no floating point hardware, so it cannot run\n")
 		print("this GOARM=", goarm, " binary. Recompile using GOARM=5.\n")
 		exit(1)
 	}
-	if goarm > 6 && hwcap&_HWCAP_VFPv3 == 0 {
+	if goarm > 6 && cpu.HWCap&_HWCAP_VFPv3 == 0 {
 		print("runtime: this CPU has no VFPv3 floating point hardware, so it cannot run\n")
 		print("this GOARM=", goarm, " binary. Recompile using GOARM=5 or GOARM=6.\n")
 		exit(1)
@@ -35,9 +42,11 @@ func checkgoarm() {
 
 func archauxv(tag, val uintptr) {
 	switch tag {
-	case _AT_HWCAP: // CPU capability bit flags
-		hwcap = uint32(val)
-		hardDiv = (hwcap & _HWCAP_IDIVA) != 0
+	case _AT_HWCAP:
+		cpu.HWCap = uint(val)
+		goarmHWCap = cpu.HWCap
+	case _AT_HWCAP2:
+		cpu.HWCap2 = uint(val)
 	}
 }
 

src/runtime/os_linux_arm.go

@@ -4,20 +4,20 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"internal/cpu"
+	"unsafe"
+)
 
 const (
 	_AT_PLATFORM = 15 //  introduced in at least 2.6.11
 
 	_HWCAP_VFP   = 1 << 6  // introduced in at least 2.6.11
 	_HWCAP_VFPv3 = 1 << 13 // introduced in 2.6.30
-	_HWCAP_IDIVA = 1 << 17
 )
 
 var randomNumber uint32
 var armArch uint8 = 6 // we default to ARMv6
-var hwcap uint32      // set by archauxv
-var hardDiv bool      // set if a hardware divider is available
 
 func checkgoarm() {
 	// On Android, /proc/self/auxv might be unreadable and hwcap won't
@@ -26,12 +26,12 @@ func checkgoarm() {
 	if GOOS == "android" {
 		return
 	}
-	if goarm > 5 && hwcap&_HWCAP_VFP == 0 {
+	if goarm > 5 && cpu.HWCap&_HWCAP_VFP == 0 {
 		print("runtime: this CPU has no floating point hardware, so it cannot run\n")
 		print("this GOARM=", goarm, " binary. Recompile using GOARM=5.\n")
 		exit(1)
 	}
-	if goarm > 6 && hwcap&_HWCAP_VFPv3 == 0 {
+	if goarm > 6 && cpu.HWCap&_HWCAP_VFPv3 == 0 {
 		print("runtime: this CPU has no VFPv3 floating point hardware, so it cannot run\n")
 		print("this GOARM=", goarm, " binary. Recompile using GOARM=5 or GOARM=6.\n")
 		exit(1)
@@ -53,9 +53,10 @@ func archauxv(tag, val uintptr) {
 			armArch = t - '0'
 		}
 
-	case _AT_HWCAP: // CPU capability bit flags
-		hwcap = uint32(val)
-		hardDiv = (hwcap & _HWCAP_IDIVA) != 0
+	case _AT_HWCAP:
+		cpu.HWCap = uint(val)
+	case _AT_HWCAP2:
+		cpu.HWCap2 = uint(val)
 	}
 }
 

src/runtime/os_nacl_arm.go

@@ -4,8 +4,6 @@
 
 package runtime
 
-var hardDiv bool // TODO: set if a hardware divider is available
-
 func checkgoarm() {
 	// TODO(minux): FP checks like in os_linux_arm.go.
 

src/runtime/os_netbsd_arm.go

@@ -6,8 +6,6 @@ package runtime
 
 import "unsafe"
 
-var hardDiv bool // TODO: set if a hardware divider is available
-
 func lwp_mcontext_init(mc *mcontextt, stk unsafe.Pointer, mp *m, gp *g, fn uintptr) {
 	// Machine dependent mcontext initialisation for LWP.
 	mc.__gregs[_REG_R15] = uint32(funcPC(lwp_tramp))

src/runtime/os_openbsd_arm.go

@@ -4,8 +4,6 @@
 
 package runtime
 
-var hardDiv bool // TODO: set if a hardware divider is available
-
 func checkgoarm() {
 	// TODO(minux): FP checks like in os_linux_arm.go.
 

src/runtime/os_plan9_arm.go

@@ -4,8 +4,6 @@
 
 package runtime
 
-var hardDiv bool // TODO: set if a hardware divider is available
-
 func checkgoarm() {
 	return // TODO(minux)
 }

src/runtime/vlop_arm.s

@@ -44,7 +44,7 @@
 // the RET instruction will clobber R12 on nacl, and the compiler's register
 // allocator needs to know.
 TEXT runtime·udiv(SB),NOSPLIT|NOFRAME,$0
-	MOVBU	runtime·hardDiv(SB), Ra
+	MOVBU	internal∕cpu·ARM+const_offset_arm_HasIDIVA(SB), Ra
 	CMP	$0, Ra
 	BNE	udiv_hardware