liblink, cmd/ld: reenable nosplit checking and test

The new code is adapted from the Go 1.2 nosplit code,
but it does not have the bug reported in issue 7623:

g% go run nosplit.go
g% go1.2 run nosplit.go
BUG
rejected incorrectly:
        main 0 call f; f 120

        linker output:
        # _/tmp/go-test-nosplit021064539
        main.main: nosplit stack overflow
                120	guaranteed after split check in main.main
                112	on entry to main.f
                -8	after main.f uses 120

g%

Fixes issue 6931.
Fixes issue 7623.

LGTM=iant
R=golang-codereviews, iant, ality
CC=golang-codereviews, r
https://codereview.appspot.com/88190043

1 files changed, 13 insertions, 4 deletions

diff --git a/src/liblink/asm6.c b/src/liblink/asm6.c
index b8029e10c..2c719a775 100644
--- a/src/liblink/asm6.c
+++ b/src/liblink/asm6.c
@@ -124,6 +124,7 @@ enum
 	Z_rp,
 	Zbr,
 	Zcall,
+	Zcallindreg,
 	Zib_,
 	Zib_rp,
 	Zibo_m,
@@ -503,8 +504,8 @@ static uchar	yloop[] =
 };
 static uchar	ycall[] =
 {
-	Ynone,	Yml,	Zo_m64,	0,
-	Yrx,	Yrx,	Zo_m64,	2,
+	Ynone,	Yml,	Zcallindreg,	0,
+	Yrx,	Yrx,	Zcallindreg,	2,
 	Ynone,	Ybr,	Zcall,	1,
 	0
 };
@@ -2903,6 +2904,7 @@ found:
 		break;
 
 	case Zo_m64:
+	case_Zo_m64:
 		*ctxt->andptr++ = op;
 		asmandsz(ctxt, &p->to, o->op[z+1], 0, 1);
 		break;
@@ -3074,11 +3076,18 @@ found:
 		r->off = p->pc + ctxt->andptr - ctxt->and;
 		r->sym = p->to.sym;
 		r->add = p->to.offset;
-		r->type = R_PCREL;
+		r->type = R_CALL;
 		r->siz = 4;
 		put4(ctxt, 0);
 		break;
 
+	case Zcallindreg:
+		r = addrel(ctxt->cursym);
+		r->off = p->pc;
+		r->type = R_CALLIND;
+		r->siz = 0;
+		goto case_Zo_m64;
+
 	case Zbr:
 	case Zjmp:
 	case Zloop:
@@ -3548,7 +3557,7 @@ asmins(Link *ctxt, Prog *p)
 			break;
 		if(ctxt->rexflag)
 			r->off++;
-		if(r->type == R_PCREL)
+		if(r->type == R_PCREL || r->type == R_CALL)
 			r->add -= p->pc + n - (r->off + r->siz);
 	}