runtime: run on GOARM=5 and GOARM=6 uniprocessor freebsd/arm systems

Also, crash early on non-Linux SMP ARM systems when GOARM < 7;
without the proper synchronization, SMP cannot work.

Linux is okay because we call kernel-provided routines for
synchronization and barriers, and the kernel takes care of
providing the right routines for the current system.
On non-Linux systems we are left to fend for ourselves.

It is possible to use different synchronization on GOARM=6,
but it's too late to do that in the Go 1.5 cycle.
We don't believe there are any non-Linux SMP GOARM=6 systems anyway.

Fixes #12067.

Change-Id: I771a556e47893ed540ec2cd33d23c06720157ea3
Reviewed-on: https://go-review.googlesource.com/13363
Reviewed-by: Austin Clements <austin@google.com>

9 files changed, 53 insertions, 8 deletions

diff --git a/src/runtime/asm_arm.s b/src/runtime/asm_arm.s
index 91dccdc381..9c32e42afd 100644
--- a/src/runtime/asm_arm.s
+++ b/src/runtime/asm_arm.s
@@ -752,6 +752,8 @@ TEXT runtime·atomicstoreuintptr(SB),NOSPLIT,$0-8
 	B	runtime·atomicstore(SB)
 
 // armPublicationBarrier is a native store/store barrier for ARMv7+.
+// On earlier ARM revisions, armPublicationBarrier is a no-op.
+// This will not work on SMP ARMv6 machines, if any are in use.
 // To implement publiationBarrier in sys_$GOOS_arm.s using the native
 // instructions, use:
 //
@@ -759,6 +761,9 @@ TEXT runtime·atomicstoreuintptr(SB),NOSPLIT,$0-8
 //		B	runtime·armPublicationBarrier(SB)
 //
 TEXT runtime·armPublicationBarrier(SB),NOSPLIT,$-4-0
+	MOVB	runtime·goarm(SB), R11
+	CMP	$7, R11
+	BLT	2(PC)
 	WORD $0xf57ff05e	// DMB ST
 	RET
 
diff --git a/src/runtime/os1_darwin.go b/src/runtime/os1_darwin.go
index 08ec611d43..e07022997c 100644
--- a/src/runtime/os1_darwin.go
+++ b/src/runtime/os1_darwin.go
@@ -34,14 +34,19 @@ func osinit() {
 	// bsdthread_register delayed until end of goenvs so that we
 	// can look at the environment first.
 
+	ncpu = getncpu()
+}
+
+func getncpu() int32 {
 	// Use sysctl to fetch hw.ncpu.
 	mib := [2]uint32{6, 3}
 	out := uint32(0)
 	nout := unsafe.Sizeof(out)
 	ret := sysctl(&mib[0], 2, (*byte)(unsafe.Pointer(&out)), &nout, nil, 0)
-	if ret >= 0 {
-		ncpu = int32(out)
+	if ret >= 0 && int32(out) > 0 {
+		return int32(out)
 	}
+	return 1
 }
 
 var urandom_dev = []byte("/dev/urandom\x00")
diff --git a/src/runtime/os_darwin_arm.go b/src/runtime/os_darwin_arm.go
index d3336c012a..1ccc9592da 100644
--- a/src/runtime/os_darwin_arm.go
+++ b/src/runtime/os_darwin_arm.go
@@ -5,7 +5,14 @@
 package runtime
 
 func checkgoarm() {
-	return // TODO(minux)
+	// TODO(minux): FP checks like in os_linux_arm.go.
+
+	// osinit not called yet, so ncpu not set: must use getncpu directly.
+	if getncpu() > 1 && goarm < 7 {
+		print("runtime: this system has multiple CPUs and must use\n")
+		print("atomic synchronization instructions. Recompile using GOARM=7.\n")
+		exit(1)
+	}
 }
 
 //go:nosplit
diff --git a/src/runtime/os_freebsd_arm.go b/src/runtime/os_freebsd_arm.go
index e049cbf9a1..1f2add279f 100644
--- a/src/runtime/os_freebsd_arm.go
+++ b/src/runtime/os_freebsd_arm.go
@@ -5,7 +5,14 @@
 package runtime
 
 func checkgoarm() {
-	// TODO(minux)
+	// TODO(minux): FP checks like in os_linux_arm.go.
+
+	// osinit not called yet, so ncpu not set: must use getncpu directly.
+	if getncpu() > 1 && goarm < 7 {
+		print("runtime: this system has multiple CPUs and must use\n")
+		print("atomic synchronization instructions. Recompile using GOARM=7.\n")
+		exit(1)
+	}
 }
 
 //go:nosplit
diff --git a/src/runtime/os_linux_arm.go b/src/runtime/os_linux_arm.go
index 6c74c81859..3749640ee5 100644
--- a/src/runtime/os_linux_arm.go
+++ b/src/runtime/os_linux_arm.go
@@ -19,7 +19,6 @@ const (
 var randomNumber uint32
 var armArch uint8 = 6 // we default to ARMv6
 var hwcap uint32      // set by setup_auxv
-var goarm uint8       // set by 5l
 
 func checkgoarm() {
 	if goarm > 5 && hwcap&_HWCAP_VFP == 0 {
diff --git a/src/runtime/os_nacl_arm.go b/src/runtime/os_nacl_arm.go
index a43e7c47b7..f94c183e87 100644
--- a/src/runtime/os_nacl_arm.go
+++ b/src/runtime/os_nacl_arm.go
@@ -5,7 +5,13 @@
 package runtime
 
 func checkgoarm() {
-	return // NaCl/ARM only supports ARMv7
+	// TODO(minux): FP checks like in os_linux_arm.go.
+
+	// NaCl/ARM only supports ARMv7
+	if goarm != 7 {
+		print("runtime: NaCl requires ARMv7. Recompile using GOARM=7.\n")
+		exit(1)
+	}
 }
 
 //go:nosplit
diff --git a/src/runtime/os_netbsd_arm.go b/src/runtime/os_netbsd_arm.go
index 83c4c06cf9..03032e8bea 100644
--- a/src/runtime/os_netbsd_arm.go
+++ b/src/runtime/os_netbsd_arm.go
@@ -16,7 +16,14 @@ func lwp_mcontext_init(mc *mcontextt, stk unsafe.Pointer, mp *m, gp *g, fn uintp
 }
 
 func checkgoarm() {
-	// TODO(minux)
+	// TODO(minux): FP checks like in os_linux_arm.go.
+
+	// osinit not called yet, so ncpu not set: must use getncpu directly.
+	if getncpu() > 1 && goarm < 7 {
+		print("runtime: this system has multiple CPUs and must use\n")
+		print("atomic synchronization instructions. Recompile using GOARM=7.\n")
+		exit(1)
+	}
 }
 
 //go:nosplit
diff --git a/src/runtime/os_openbsd_arm.go b/src/runtime/os_openbsd_arm.go
index be3f330dfb..b46fef0090 100644
--- a/src/runtime/os_openbsd_arm.go
+++ b/src/runtime/os_openbsd_arm.go
@@ -5,7 +5,14 @@
 package runtime
 
 func checkgoarm() {
-	// TODO(minux)
+	// TODO(minux): FP checks like in os_linux_arm.go.
+
+	// osinit not called yet, so ncpu not set: must use getncpu directly.
+	if getncpu() > 1 && goarm < 7 {
+		print("runtime: this system has multiple CPUs and must use\n")
+		print("atomic synchronization instructions. Recompile using GOARM=7.\n")
+		exit(1)
+	}
 }
 
 //go:nosplit
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index a157f016d1..57cd869d88 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -645,6 +645,8 @@ var (
 	cpuid_ecx         uint32
 	cpuid_edx         uint32
 	lfenceBeforeRdtsc bool
+
+	goarm uint8 // set by cmd/link on arm systems
 )
 
 // Set by the linker so the runtime can determine the buildmode.