fix bug in stack overflow check.

if stack is in low memory and frame size is huge, test could wrap around!
fix by omitting test altogether if we know frame is big.

R=gri
OCL=14782
CL=14782

src/cmd/6l/pass.c

@@ -669,34 +669,36 @@ dostkoff(void)
 			q = P;
 			if(pmorestack != P)
 			if(!(p->from.scale & NOSPLIT)) {
-				if(autoffset <= 75) {
-					// small stack
-					p = appendp(p);
-					p->as = ACMPQ;
-					p->from.type = D_SP;
-					p->to.type = D_INDIR+D_R15;
-					
-				} else {
-					// large stack
-					p = appendp(p);
-					p->as = ALEAQ;
-					p->from.type = D_INDIR+D_SP;
-					p->from.offset = -(autoffset-75);
-					p->to.type = D_AX;
-
+				if(autoffset < 4096) {  // do we need to call morestack
+					if(autoffset <= 75) {
+						// small stack
+						p = appendp(p);
+						p->as = ACMPQ;
+						p->from.type = D_SP;
+						p->to.type = D_INDIR+D_R15;
+						
+					} else {
+						// large stack
+						p = appendp(p);
+						p->as = ALEAQ;
+						p->from.type = D_INDIR+D_SP;
+						p->from.offset = -(autoffset-75);
+						p->to.type = D_AX;
+	
+						p = appendp(p);
+						p->as = ACMPQ;
+						p->from.type = D_AX;
+						p->to.type = D_INDIR+D_R15;
+					}
+	
+					// common
 					p = appendp(p);
-					p->as = ACMPQ;
-					p->from.type = D_AX;
-					p->to.type = D_INDIR+D_R15;
+					p->as = AJHI;
+					p->to.type = D_BRANCH;
+					p->to.offset = 4;
+					q = p;
 				}
 
-				// common
-				p = appendp(p);
-				p->as = AJHI;
-				p->to.type = D_BRANCH;
-				p->to.offset = 4;
-				q = p;
-
 				p = appendp(p);
 				p->as = AMOVQ;
 				p->from.type = D_CONST;

test/golden.out

@@ -145,10 +145,6 @@ BUG: fails incorrectly
 =========== bugs/bug090.go
 BUG: compilation succeeds incorrectly
 
-=========== bugs/bug092.go
-BUG should not crash
-$RUNFILE: line 1: PID Illegal instruction     ./$A.out
-
 =========== fixedbugs/bug015.go
 fixedbugs/bug015.go:7: overflow converting constant to <int64>INT64