liblink: add leaf bit to object file format

Without the leaf bit, the linker cannot record
the correct frame size in the symbol table, and
then stack traces get mangled. (Only for ARM.)

Fixes #7338.
Fixes #7347.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/88550043

src/cmd/ld/pcln.c

@@ -112,7 +112,7 @@ pclntab(void)
 {
 	int32 i, nfunc, start, funcstart;
 	LSym *ftab, *s;
-	int32 off, end;
+	int32 off, end, frameptrsize;
 	int64 funcdata_bytes;
 	Pcln *pcln;
 	Pciter it;
@@ -173,7 +173,10 @@ pclntab(void)
 		// when a called function doesn't have argument information.
 		// We need to make sure everything has argument information
 		// and then remove this.
-		off = setuint32(ctxt, ftab, off, ctxt->cursym->locals + PtrSize);
+		frameptrsize = PtrSize;
+		if(ctxt->cursym->leaf)
+			frameptrsize = 0;
+		off = setuint32(ctxt, ftab, off, ctxt->cursym->locals + frameptrsize);
 		
 		if(pcln != &zpcln) {
 			renumberfiles(pcln->file, pcln->nfile, &pcln->pcfile);

src/liblink/objfile.c

@@ -49,6 +49,7 @@
 //
 //	- args [int]
 //	- locals [int]
+//	- leaf [int]
 //	- nlocal [int]
 //	- local [nlocal automatics]
 //	- pcln [pcln table]
@@ -291,8 +292,11 @@ writesym(Link *ctxt, Biobuf *b, LSym *s)
 		if(s->dupok)
 			Bprint(ctxt->bso, "dupok ");
 		Bprint(ctxt->bso, "size=%lld value=%lld", (vlong)s->size, (vlong)s->value);
-		if(s->type == STEXT)
+		if(s->type == STEXT) {
 			Bprint(ctxt->bso, " args=%#llux locals=%#llux", (uvlong)s->args, (uvlong)s->locals);
+			if(s->leaf)
+				Bprint(ctxt->bso, " leaf");
+		}
 		Bprint(ctxt->bso, "\n");
 		for(p=s->text; p != nil; p = p->link)
 			Bprint(ctxt->bso, "\t%#06ux %P\n", (int)p->pc, p);
@@ -346,6 +350,7 @@ writesym(Link *ctxt, Biobuf *b, LSym *s)
 	if(s->type == STEXT) {
 		wrint(b, s->args);
 		wrint(b, s->locals);
+		wrint(b, s->leaf);
 		n = 0;
 		for(a = s->autom; a != nil; a = a->link)
 			n++;
@@ -566,6 +571,7 @@ readsym(Link *ctxt, Biobuf *f, char *pkg, char *pn)
 	if(s->type == STEXT) {
 		s->args = rdint(f);
 		s->locals = rdint(f);
+		s->leaf = rdint(f);
 		n = rdint(f);
 		for(i=0; i<n; i++) {
 			a = emallocz(sizeof *a);

src/pkg/debug/goobj/read.go

@@ -190,6 +190,7 @@ type Var struct {
 type Func struct {
 	Args     int        // size in bytes of of argument frame: inputs and outputs
 	Frame    int        // size in bytes of local variable frame
+	Leaf     bool       // function omits save of link register (ARM)
 	Var      []Var      // detail about local variables
 	PCSP     Data       // PC → SP offset map
 	PCFile   Data       // PC → file number map (index into File)
@@ -621,6 +622,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 			s.Func = f
 			f.Args = r.readInt()
 			f.Frame = r.readInt()
+			f.Leaf = r.readInt() != 0
 			f.Var = make([]Var, r.readInt())
 			for i := range f.Var {
 				v := &f.Var[i]