cmd/gc: correct liveness for func ending in panic

The registerization code needs the function to end in a RET,
even if that RET is actually unreachable.

The liveness code needs to avoid such unreachable RETs.
It had a special case for final RET after JMP, but no case
for final RET after UNDEF. Instead of expanding the special
cases, let fixjmp - which already knows what is and is not
reachable definitively - mark the unreachable RET so that
the liveness code can identify it.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/63680043

src/cmd/gc/plive.c

@@ -519,12 +519,10 @@ newcfg(Prog *firstp)
 				break;
 			bb->last = p;
 
-			// Pattern match an unconditional branch followed by a
-			// dead return instruction.  This avoids a creating
+			// Stop before an unreachable RET, to avoid creating
 			// unreachable control flow nodes.
-			if(p->link != nil && p->link->link == nil)
-				if (p->as == AJMP && p->link->as == ARET && p->link->opt == nil)
-					break;
+			if(p->link != nil && p->link->as == ARET && p->link->mode == -1)
+				break;
 
 			// Collect basic blocks with selectgo calls.
 			if(isselectgocall(p))

src/cmd/gc/popt.c

@@ -146,7 +146,13 @@ fixjmp(Prog *firstp)
 		if(p->opt == dead) {
 			if(p->link == P && p->as == ARET && last && last->as != ARET) {
 				// This is the final ARET, and the code so far doesn't have one.
-				// Let it stay.
+				// Let it stay. The register allocator assumes that all live code in
+				// the function can be traversed by starting at all the RET instructions
+				// and following predecessor links. If we remove the final RET,
+				// this assumption will not hold in the case of an infinite loop
+				// at the end of a function.
+				// Keep the RET but mark it dead for the liveness analysis.
+				p->mode = -1;
 			} else {
 				if(debug['R'] && debug['v'])
 					print("del %P\n", p);

test/live.go

@@ -113,3 +113,11 @@ func f9() bool {
 	x := i9
 	return x != 99
 }
+
+// liveness formerly confused by UNDEF followed by RET,
+// leading to "live at entry to f10: ~r1" (unnamed result).
+
+func f10() string {
+	panic(1)
+}
+