cmd/dist: add -check-armv6k command to check for ARMv6K

so that our release note can reference a simple command to check if
the processor implements ARMv6K or not.

Updates #17082.

Change-Id: I9ca52051e5517394a7cd6b778fb822c3ee435f84
Reviewed-on: https://go-review.googlesource.com/33686
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/util.go

@@ -393,6 +393,14 @@ func errprintf(format string, args ...interface{}) {
 func main() {
 	os.Setenv("TERM", "dumb") // disable escape codes in clang errors
 
+	// provide -check-armv6k first, before checking for $GOROOT so that
+	// it is possible to run this check without having $GOROOT available.
+	if len(os.Args) > 1 && os.Args[1] == "-check-armv6k" {
+		useARMv6K() // might fail with SIGILL
+		println("ARMv6K supported.")
+		os.Exit(0)
+	}
+
 	slash = string(filepath.Separator)
 
 	gohostos = runtime.GOOS

src/cmd/dist/util_gc.go

@@ -25,3 +25,8 @@ func useVFPv1()
 // useVFPv3 tries to execute one VFPv3 instruction on ARM.
 // It will crash the current process if VFPv3 is missing.
 func useVFPv3()
+
+// useARMv6K tries to run ARMv6K instructions on ARM.
+// It will crash the current process if it doesn't implement
+// ARMv6K or above.
+func useARMv6K()

src/cmd/dist/util_gccgo.go

@@ -22,3 +22,5 @@ func cansse2() bool { return C.supports_sse2() != 0 }
 func useVFPv1() {}
 
 func useVFPv3() {}
+
+func useARMv6K() {}

src/cmd/dist/vfp_arm.s

@@ -15,3 +15,12 @@ TEXT ·useVFPv1(SB),NOSPLIT,$0
 TEXT ·useVFPv3(SB),NOSPLIT,$0
 	WORD $0xeeb70b00	// vmov.f64 d0, #112
 	RET
+
+// try to run ARMv6K (or above) "ldrexd" instruction
+TEXT ·useARMv6K(SB),NOSPLIT,$32
+	MOVW R13, R2
+	BIC  $15, R13
+	WORD $0xe1bd0f9f	// ldrexd r0, r1, [sp]
+	WORD $0xf57ff01f	// clrex
+	MOVW R2, R13
+	RET

src/cmd/dist/vfp_default.s

@@ -11,3 +11,6 @@ TEXT ·useVFPv1(SB),NOSPLIT,$0
 
 TEXT ·useVFPv3(SB),NOSPLIT,$0
 	RET
+
+TEXT ·useARMv6K(SB),NOSPLIT,$0
+	RET