5l/6l/8l: undo spadj cleanup at ARET for following instructions in the same stackframe.

5l was already correct, clarified comment and added diags for unmaintained code.

R=rsc
CC=golang-dev
https://golang.org/cl/4277070

src/cmd/5l/noop.c

@@ -450,6 +450,8 @@ noops(void)
 					}
 				}
 				if(thumb){
+					diag("thumb not maintained");
+					errorexit();
 					if(cursym->text->mark & LEAF){
 						if(autosize){
 							p->as = AADD;
@@ -492,6 +494,8 @@ noops(void)
 					break;
 				}
 				if(foreign) {
+					diag("foreign not maintained");
+					errorexit();
 // if(foreign) print("ABXRET 3 %s\n", cursym->name);
 #define	R	1
 					p->as = AMOVW;
@@ -530,7 +534,9 @@ noops(void)
 					p->from.reg = REGSP;
 					p->to.type = D_REG;
 					p->to.reg = REGPC;
-					// no spadj because it doesn't fall through
+					// If there are instructions following
+					// this ARET, they come from a branch
+					// with the same stackframe, so no spadj.
 				}
 				break;
 	

src/cmd/6l/pass.c

@@ -674,6 +674,11 @@ dostkoff(void)
 				p->spadj = -autoffset;
 				p = appendp(p);
 				p->as = ARET;
+				// If there are instructions following
+				// this ARET, they come from a branch
+				// with the same stackframe, so undo
+				// the cleanup.
+				p->spadj = +autoffset;
 			}
 		}
 	}

src/cmd/8l/pass.c

@@ -614,14 +614,17 @@ dostkoff(void)
 				diag("unbalanced PUSH/POP");
 	
 			if(autoffset) {
-				q = p;
+				p->as = AADJSP;
+				p->from.type = D_CONST;
+				p->from.offset = -autoffset;
+				p->spadj = -autoffset;
 				p = appendp(p);
 				p->as = ARET;
-	
-				q->as = AADJSP;
-				q->from.type = D_CONST;
-				q->from.offset = -autoffset;
-				p->spadj = -autoffset;
+				// If there are instructions following
+				// this ARET, they come from a branch
+				// with the same stackframe, so undo
+				// the cleanup.
+				p->spadj = +autoffset;
 			}
 		}
 	}