cmd/gc: do not compute dead value maps if they will not be used

Reduces 6g big.go memory usage from 251 MB to 242 MB.
Reduces 6g slow.go memory usage from 529 MB to 453 MB.

Mostly a memory savings; 6g slow.go is only about 5% faster.

The test programs are at
https://rsc.googlecode.com/hg/testdata/big.go (36k lines, 276kB)
https://rsc.googlecode.com/hg/testdata/slow.go (7k lines, 352kB)

R=golang-codereviews, bradfitz, iant
CC=golang-codereviews
https://golang.org/cl/42280045

src/cmd/gc/plive.c

@@ -690,7 +690,7 @@ progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill)
 // liveness computation.  The cfg argument is an array of BasicBlock*s and the
 // vars argument is an array of Node*s.
 static Liveness*
-newliveness(Node *fn, Prog *ptxt, Array *cfg, Array *vars)
+newliveness(Node *fn, Prog *ptxt, Array *cfg, Array *vars, int computedead)
 {
 	Liveness *result;
 	int32 i;
@@ -719,8 +719,13 @@ newliveness(Node *fn, Prog *ptxt, Array *cfg, Array *vars)
 
 	result->livepointers = arraynew(0, sizeof(Bvec*));
 	result->argslivepointers = arraynew(0, sizeof(Bvec*));
+	if(computedead) {
 		result->deadvalues = arraynew(0, sizeof(Bvec*));
 		result->argsdeadvalues = arraynew(0, sizeof(Bvec*));
+	} else {
+		result->deadvalues = nil;
+		result->argsdeadvalues = nil;
+	}
 	return result;
 }
 
@@ -741,6 +746,7 @@ freeliveness(Liveness *lv)
 		free(*(Bvec**)arrayget(lv->argslivepointers, i));
 	arrayfree(lv->argslivepointers);
 
+	if(lv->deadvalues != nil) {
 		for(i = 0; i < arraylength(lv->deadvalues); i++)
 			free(*(Bvec**)arrayget(lv->deadvalues, i));
 		arrayfree(lv->deadvalues);
@@ -748,6 +754,7 @@ freeliveness(Liveness *lv)
 		for(i = 0; i < arraylength(lv->argsdeadvalues); i++)
 			free(*(Bvec**)arrayget(lv->argsdeadvalues, i));
 		arrayfree(lv->argsdeadvalues);
+	}
 
 	for(i = 0; i < arraylength(lv->cfg); i++) {
 		free(lv->uevar[i]);
@@ -1333,11 +1340,13 @@ livenessepilogue(Liveness *lv)
 			arrayadd(lv->livepointers, &locals);
 
 			// Dead stuff second.
+			if(lv->deadvalues != nil) {
 				args = bvalloc(argswords() * BitsPerPointer);
 				arrayadd(lv->argsdeadvalues, &args);
 				locals = bvalloc(localswords() * BitsPerPointer);
 				arrayadd(lv->deadvalues, &locals);
 			}
+		}
 
 		// walk backward, emit pcdata and populate the maps
 		pos = arraylength(lv->livepointers) - 1;
@@ -1391,9 +1400,11 @@ livenessepilogue(Liveness *lv)
 				twobitlivepointermap(lv, liveout, lv->vars, args, locals);
 
 				// Record dead values.
+				if(lv->deadvalues != nil) {
 					args = *(Bvec**)arrayget(lv->argsdeadvalues, pos);
 					locals = *(Bvec**)arrayget(lv->deadvalues, pos);
 					twobitdeadvaluemap(lv, liveout, lv->vars, args, locals);
+				}
 
 				pos--;
 			}
@@ -1487,7 +1498,7 @@ liveness(Node *fn, Prog *firstp, Sym *argssym, Sym *livesym, Sym *deadsym)
 	cfg = newcfg(firstp);
 	if(0) printcfg(cfg);
 	vars = getvariables(fn);
-	lv = newliveness(fn, firstp, cfg, vars);
+	lv = newliveness(fn, firstp, cfg, vars, deadsym != nil);
 
 	// Run the dataflow framework.
 	livenessprologue(lv);