build: disable precise collection of stack frames

The code for call site-specific pointer bitmaps was not ready in time,
but the zeroing required without it is too expensive to use by default.
We will have to wait for precise collection of stack frames until Go 1.3.

The precise collection can be re-enabled by

        GOEXPERIMENT=precisestack ./all.bash

but that will not be the default for a Go 1.2 build.

Fixes #6087.

R=golang-dev, jeremyjackins, dan.kortschak, r
CC=golang-dev
https://golang.org/cl/13677045

src/cmd/dist/a.h

@@ -101,6 +101,7 @@ void	mkzgoarch(char*, char*);
 void	mkzgoos(char*, char*);
 void	mkzruntimedefs(char*, char*);
 void	mkzversion(char*, char*);
+void	mkzexperiment(char*, char*);
 
 // buildgo.c
 void	mkzdefaultcc(char*, char*);

src/cmd/dist/build.c

@@ -557,6 +557,7 @@ static struct {
 		"$GOROOT/pkg/obj/$GOOS_$GOARCH/lib9.a",
 	}},
 	{"pkg/runtime", {
+		"zaexperiment.h", // must sort above zasm
 		"zasm_$GOOS_$GOARCH.h",
 		"zsys_$GOOS_$GOARCH.s",
 		"zgoarch_$GOARCH.go",
@@ -589,6 +590,7 @@ static struct {
 	{"zgoos_", mkzgoos},
 	{"zruntime_defs_", mkzruntimedefs},
 	{"zversion.go", mkzversion},
+	{"zaexperiment.h", mkzexperiment},
 };
 
 // install installs the library, package, or binary associated with dir,

src/cmd/dist/buildruntime.c

@@ -38,6 +38,34 @@ mkzversion(char *dir, char *file)
 	bfree(&out);
 }
 
+// mkzexperiment writes zaexperiment.h (sic):
+//
+//	#define GOEXPERIMENT "experiment string"
+//
+void
+mkzexperiment(char *dir, char *file)
+{
+	Buf b, out, exp;
+	
+	USED(dir);
+
+	binit(&b);
+	binit(&out);
+	binit(&exp);
+	
+	xgetenv(&exp, "GOEXPERIMENT");
+	bwritestr(&out, bprintf(&b,
+		"// auto generated by go tool dist\n"
+		"\n"
+		"#define GOEXPERIMENT \"%s\"\n", bstr(&exp)));
+
+	writefile(&out, file, 0);
+	
+	bfree(&b);
+	bfree(&out);
+	bfree(&exp);
+}
+
 // mkzgoarch writes zgoarch_$GOARCH.go:
 //
 //	package runtime
@@ -193,12 +221,13 @@ mkzasm(char *dir, char *file)
 {
 	int i, n;
 	char *aggr, *p;
-	Buf in, b, out;
+	Buf in, b, out, exp;
 	Vec argv, lines, fields;
 
 	binit(&in);
 	binit(&b);
 	binit(&out);
+	binit(&exp);
 	vinit(&argv);
 	vinit(&lines);
 	vinit(&fields);
@@ -284,6 +313,9 @@ ok:
 		bwritestr(&out, bprintf(&b, "#define cb_max %d\n", MAXWINCB));
 	}
 	
+	xgetenv(&exp, "GOEXPERIMENT");
+	bwritestr(&out, bprintf(&b, "#define GOEXPERIMENT \"%s\"\n", bstr(&exp)));
+	
 	// Write both to file and to workdir/zasm_GOOS_GOARCH.h.
 	writefile(&out, file, 0);
 	writefile(&out, bprintf(&b, "%s/zasm_GOOS_GOARCH.h", workdir), 0);
@@ -291,6 +323,7 @@ ok:
 	bfree(&in);
 	bfree(&b);
 	bfree(&out);
+	bfree(&exp);
 	vfree(&argv);
 	vfree(&lines);
 	vfree(&fields);

src/cmd/gc/go.h

@@ -982,6 +982,7 @@ EXTERN	int	noescape;
 
 EXTERN	int	nointerface;
 EXTERN	int	fieldtrack_enabled;
+EXTERN	int	precisestack_enabled;
 
 /*
  *	y.tab.c

src/cmd/gc/lex.c

@@ -41,6 +41,7 @@ static struct {
 } exper[] = {
 //	{"rune32", &rune32},
 	{"fieldtrack", &fieldtrack_enabled},
+	{"precisestack", &precisestack_enabled},
 	{nil, nil},
 };
 

src/cmd/gc/pgen.c

@@ -316,7 +316,7 @@ walktype(Type *type, Bvec *bv)
 	walktype1(type, &xoffset, bv);
 }
 
-// Compute a bit vector to describes the pointer containing locations
+// Compute a bit vector to describe the pointer-containing locations
 // in the in and out argument list and dump the bitvector length and
 // data to the provided symbol.
 static void
@@ -344,9 +344,9 @@ dumpgcargs(Node *fn, Sym *sym)
 	ggloblsym(sym, off, 0, 1);
 }
 
-// Compute a bit vector to describes the pointer containing locations
-// in local variables and dumps the bitvector length and data out to
-// the provided symbol. Returns the vector for use and freeing by caller.
+// Compute a bit vector to describe the pointer-containing locations
+// in local variables and dump the bitvector length and data out to
+// the provided symbol. Return the vector for use and freeing by caller.
 static Bvec*
 dumpgclocals(Node* fn, Sym *sym)
 {
@@ -438,11 +438,13 @@ allocauto(Prog* ptxt)
 			ll->n->used = 0;
 
 	markautoused(ptxt);
-	
-	// TODO: Remove when liveness analysis sets needzero instead.
-	for(ll=curfn->dcl; ll != nil; ll=ll->next)
-		if (ll->n->class == PAUTO)
-			ll->n->needzero = 1; // ll->n->addrtaken;
+
+	if(precisestack_enabled) {
+		// TODO: Remove when liveness analysis sets needzero instead.
+		for(ll=curfn->dcl; ll != nil; ll=ll->next)
+			if(ll->n->class == PAUTO)
+				ll->n->needzero = 1; // ll->n->addrtaken;
+	}
 
 	listsort(&curfn->dcl, cmpstackvar);
 

src/cmd/gc/walk.c

@@ -2331,9 +2331,9 @@ paramstoheap(Type **argin, int out)
 		v = t->nname;
 		if(v && v->sym && v->sym->name[0] == '~')
 			v = N;
-		// The garbage collector assumes results are always live,
-		// so zero them always (1 ||).
-		if(out && (1 || (v == N && hasdefer))) {
+		// In precisestack mode, the garbage collector assumes results
+		// are always live, so zero them always.
+		if(out && (precisestack_enabled || (v == N && hasdefer))) {
 			// Defer might stop a panic and show the
 			// return values as they exist at the time of panic.
 			// Make sure to zero them on entry to the function.

src/pkg/runtime/mgc0.c

@@ -1345,7 +1345,7 @@ scaninterfacedata(uintptr bits, byte *scanp, bool afterprologue)
 	Itab *tab;
 	Type *type;
 
-	if(afterprologue) {
+	if(runtime·precisestack && afterprologue) {
 		if(bits == BitsIface) {
 			tab = *(Itab**)scanp;
 			if(tab->type->size <= sizeof(void*) && (tab->type->kind & KindNoPointers))

src/pkg/runtime/proc.c

@@ -4,6 +4,7 @@
 
 #include "runtime.h"
 #include "arch_GOARCH.h"
+#include "zaexperiment.h"
 #include "malloc.h"
 #include "stack.h"
 #include "race.h"
@@ -112,6 +113,7 @@ static void injectglist(G*);
 static bool preemptall(void);
 static bool preemptone(P*);
 static bool exitsyscallfast(void);
+static bool haveexperiment(int8*);
 
 // The bootstrap sequence is:
 //
@@ -128,6 +130,7 @@ runtime·schedinit(void)
 	byte *p;
 
 	runtime·sched.maxmcount = 10000;
+	runtime·precisestack = haveexperiment("precisestack");
 
 	m->nomemprof++;
 	runtime·mprofinit();
@@ -2929,3 +2932,24 @@ runtime∕debug·setMaxThreads(intgo in, intgo out)
 	runtime·unlock(&runtime·sched);
 	FLUSH(&out);
 }
+
+static int8 experiment[] = GOEXPERIMENT; // defined in zaexperiment.h
+
+static bool
+haveexperiment(int8 *name)
+{
+	int32 i, j;
+	
+	for(i=0; i<sizeof(experiment); i++) {
+		if((i == 0 || experiment[i-1] == ',') && experiment[i] == name[0]) {
+			for(j=0; name[j]; j++)
+				if(experiment[i+j] != name[j])
+					goto nomatch;
+			if(experiment[i+j] != '\0' && experiment[i+j] != ',')
+				goto nomatch;
+			return 1;
+		}
+	nomatch:;
+	}
+	return 0;
+}

src/pkg/runtime/runtime.h

@@ -539,6 +539,8 @@ struct DebugVars
 	int32	scheddetail;
 };
 
+extern bool runtime·precisestack;
+
 /*
  * defined macros
  *    you need super-gopher-guru privilege