runtime: arm: abort if VFPv3 support missing

Fixes #3456.

This proposal is a reformulation of CL 5987063. This CL resets
the default GOARM value to 6 and allows the use of the VFPv3
optimisation if GOARM=7. Binaries built with this CL in place
will abort if GOARM=7 was used and the target host does not
support VFPv3.

R=minux.ma, rsc, ajstarks
CC=golang-dev
https://golang.org/cl/6501099

src/cmd/5l/asm.c

@@ -2213,7 +2213,8 @@ omvl(Prog *p, Adr *a, int dr)
 int
 chipzero(Ieee *e)
 {
-	if(e->l != 0 || e->h != 0)
+	// We use GOARM=7 to gate the use of VFPv3 vmov (imm) instructions.
+	if(goarm < 7 || e->l != 0 || e->h != 0)
 		return -1;
 	return 0;
 }
@@ -2224,6 +2225,10 @@ chipfloat(Ieee *e)
 	int n;
 	ulong h;
 
+	// We use GOARM=7 to gate the use of VFPv3 vmov (imm) instructions.
+	if(goarm < 7)
+		goto no;
+
 	if(e->l != 0 || (e->h&0xffff) != 0)
 		goto no;
 	h = e->h & 0x7fc00000;

src/cmd/5l/l.h

@@ -316,6 +316,8 @@ void	addpool(Prog*, Adr*);
 EXTERN	Prog*	blitrl;
 EXTERN	Prog*	elitrl;
 
+EXTERN	int	goarm;
+
 void	initdiv(void);
 EXTERN	Prog*	prog_div;
 EXTERN	Prog*	prog_divu;

src/cmd/5l/obj.c

@@ -93,7 +93,7 @@ main(int argc, char *argv[])
 	if(p != nil)
 		goarm = atoi(p);
 	else
-		goarm = 7;
+		goarm = 6;
 	if(goarm == 5)
 		debug['F'] = 1;
 

src/pkg/runtime/signal_linux_arm.c

@@ -147,7 +147,8 @@ runtime·setsig(int32 i, void (*fn)(int32, Siginfo*, void*, G*), bool restart)
 #define AT_PLATFORM	15 // introduced in at least 2.6.11
 #define AT_HWCAP	16 // introduced in at least 2.6.11
 #define AT_RANDOM	25 // introduced in 2.6.29
-#define HWCAP_VFP	(1 << 6)
+#define HWCAP_VFP	(1 << 6) // introduced in at least 2.6.11
+#define HWCAP_VFPv3	(1 << 13) // introduced in 2.6.30
 static uint32 runtime·randomNumber;
 uint8  runtime·armArch = 6;	// we default to ARMv6
 uint32 runtime·hwcap;	// set by setup_auxv
@@ -161,6 +162,11 @@ runtime·checkgoarm(void)
 		runtime·printf("this GOARM=%d binary. Recompile using GOARM=5.\n", runtime·goarm);
 		runtime·exit(1);
 	}
+	if(runtime·goarm > 6 && !(runtime·hwcap & HWCAP_VFPv3)) {
+		runtime·printf("runtime: this CPU has no VFPv3 floating point hardware, so it cannot run\n");
+		runtime·printf("this GOARM=%d binary. Recompile using GOARM=6.\n", runtime·goarm);
+		runtime·exit(1);
+	}
 }
 
 #pragma textflag 7