runtime: print signal information during panic

$ 6.out
panic: runtime error: invalid memory address or nil pointer dereference

[signal 11 code=0x1 addr=0x0 pc=0x1c16]

runtime.panic+0xa7 /Users/rsc/g/go/src/pkg/runtime/proc.c:1089
	runtime.panic(0xf6c8, 0x25c010)
runtime.panicstring+0x69 /Users/rsc/g/go/src/pkg/runtime/runtime.c:88
	runtime.panicstring(0x24814, 0x0)
runtime.sigpanic+0x144 /Users/rsc/g/go/src/pkg/runtime/darwin/thread.c:465
	runtime.sigpanic()
main.f+0x16 /Users/rsc/x.go:5
	main.f()
main.main+0x1c /Users/rsc/x.go:9
	main.main()
runtime.mainstart+0xf /Users/rsc/g/go/src/pkg/runtime/amd64/asm.s:77
	runtime.mainstart()
runtime.goexit /Users/rsc/g/go/src/pkg/runtime/proc.c:149
	runtime.goexit()

R=r
CC=golang-dev
https://golang.org/cl/4036042

src/pkg/runtime/darwin/386/signal.c

@@ -66,6 +66,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context)
 		gp->sig = sig;
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = (uintptr)info->si_addr;
+		gp->sigpc = r->eip;
 
 		// Only push runtime·sigpanic if r->eip != 0.
 		// If r->eip == 0, probably panicked because of a

src/pkg/runtime/darwin/amd64/signal.c

@@ -76,6 +76,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context)
 		gp->sig = sig;
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = (uintptr)info->si_addr;
+		gp->sigpc = r->rip;
 		
 		// Only push runtime·sigpanic if r->rip != 0.
 		// If r->rip == 0, probably panicked because of a

src/pkg/runtime/freebsd/386/signal.c

@@ -63,6 +63,7 @@ runtime·sighandler(int32 sig, Siginfo* info, void* context)
 		gp->sig = sig;
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = (uintptr)info->si_addr;
+		gp->sigpc = r->mc_eip;
 
 		// Only push runtime·sigpanic if r->mc_eip != 0.
 		// If r->mc_eip == 0, probably panicked because of a

src/pkg/runtime/freebsd/amd64/signal.c

@@ -71,6 +71,7 @@ runtime·sighandler(int32 sig, Siginfo* info, void* context)
 		gp->sig = sig;
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = (uintptr)info->si_addr;
+		gp->sigpc = r->mc_rip;
 
 		// Only push runtime·sigpanic if r->mc_rip != 0.
 		// If r->mc_rip == 0, probably panicked because of a

src/pkg/runtime/linux/386/signal.c

@@ -60,6 +60,7 @@ runtime·sighandler(int32 sig, Siginfo* info, void* context)
 		gp->sig = sig;
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = ((uintptr*)info)[3];
+		gp->sigpc = r->eip;
 
 		// Only push runtime·sigpanic if r->eip != 0.
 		// If r->eip == 0, probably panicked because of a

src/pkg/runtime/linux/amd64/signal.c

@@ -70,6 +70,7 @@ runtime·sighandler(int32 sig, Siginfo* info, void* context)
 		gp->sig = sig;
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = ((uintptr*)info)[2];
+		gp->sigpc = r->rip;
 
 		// Only push runtime·sigpanic if r->rip != 0.
 		// If r->rip == 0, probably panicked because of a

src/pkg/runtime/linux/arm/signal.c

@@ -67,6 +67,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context)
 		gp->sig = sig;
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = r->fault_address;
+		gp->sigpc = r->arm_pc;
 
 		// If this is a leaf function, we do smash LR,
 		// but we're not going back there anyway.

src/pkg/runtime/proc.c

@@ -1070,6 +1070,8 @@ runtime·panic(Eface e)
 		reflect·call(d->fn, d->args, d->siz);
 		if(p->recovered) {
 			g->panic = p->link;
+			if(g->panic == nil)	// must be done with signal
+				g->sig = 0;
 			runtime·free(p);
 			// put recovering defer back on list
 			// for scheduler to find.

src/pkg/runtime/runtime.c

@@ -30,11 +30,16 @@ runtime·dopanic(int32 unused)
 	}
 	runtime·panicking++;
 
-	runtime·printf("\npanic PC=%X\n", (uint64)(uintptr)&unused);
+	if(g->sig != 0)
+		runtime·printf("\n[signal %d code=%p addr=%p pc=%p]\n",
+			g->sig, g->sigcode0, g->sigcode1, g->sigpc);
+
+	runtime·printf("\n");
 	if(runtime·gotraceback()){
 		runtime·traceback(runtime·getcallerpc(&unused), runtime·getcallersp(&unused), 0, g);
 		runtime·tracebackothers(g);
 	}
+	
 	runtime·breakpoint();  // so we can grab it in a debugger
 	runtime·exit(2);
 }

src/pkg/runtime/runtime.h

@@ -199,6 +199,7 @@ struct	G
 	int32	sig;
 	uintptr	sigcode0;
 	uintptr	sigcode1;
+	uintptr	sigpc;
 };
 struct	M
 {

test/golden.out

@@ -4,22 +4,18 @@
 =========== ./cmp2.go
 panic: runtime error: comparing uncomparable type []int
 
-panic PC=xxx
 
 =========== ./cmp3.go
 panic: runtime error: comparing uncomparable type []int
 
-panic PC=xxx
 
 =========== ./cmp4.go
 panic: runtime error: hash of unhashable type []int
 
-panic PC=xxx
 
 =========== ./cmp5.go
 panic: runtime error: hash of unhashable type []int
 
-panic PC=xxx
 
 =========== ./deferprint.go
 printing: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
@@ -117,12 +113,10 @@ PASS
 =========== interface/fail.go
 panic: interface conversion: *main.S is not main.I: missing method Foo
 
-panic PC=xxx
 
 =========== interface/returntype.go
 panic: interface conversion: *main.S is not main.I2: missing method Name
 
-panic PC=xxx
 
 == nilptr/
 
@@ -164,12 +158,10 @@ M
 =========== fixedbugs/bug113.go
 panic: interface conversion: interface is int, not int32
 
-panic PC=xxx
 
 =========== fixedbugs/bug148.go
 2 3
 panic: interface conversion: interface is main.T, not main.T
 
-panic PC=xxx
 
 == bugs/