Fix END pseudo in util/ego/sr; closes #203

The strength reduction (SR) phase mishandled the END pseudo when SR
found at least 2 different expressions in the same loop, and SR made a
new header for the loop.

Since 1987 (commit 159b84e), SR appended the new header to the end of
the procedure.  Later, SR fixed the header by adding a BRA branch to
the loop's entry, and moving the END pseudo from the previous basic
block to the header.  If SR found multiple expressions, it called
fix_header() multiple times, and tried to move the END again.  The
extra move failed assert(INSTR(e) == ps_end), or moved another line
after the END, so opt2 (the peephole optimizer) would crash.

Fix by removing fix_header() and moving the code to make_header(), so
SR adds the BRA and moves the END when it makes the header, and does
so only once for each header.

Adjust init_code(), which inserts code into a header.  Stop checking
for an empty header, because make_header() now adds BRA.  After
inserting code before BRA, don't move LP_INSTR, because later
insertions should go before BRA, not after END.

util/ego/sr/sr_reduce.c

@@ -171,9 +171,7 @@ STATIC line_p add_code(pl, l)
 
 
 
-STATIC void init_code(code,tmp)
+STATIC void init_code(code_p code, offset tmp)
-	code_p code;
-	offset tmp;
 {
 	/* Generate code to set up the temporary local.
 	 * For multiplication, its initial value is const*iv_expr,
@@ -223,20 +221,19 @@ STATIC void init_code(code,tmp)
 			assert(FALSE); /* non-reducible instruction */
 	}
 	PREV(l->l_next) = l;
+
 	/* Now insert the code at the end of the header block */
 	p = &code->co_loop->LP_INSTR;
-	if (*p == (line_p) 0 || (PREV((*p)) == 0 && INSTR((*p)) == op_bra)) {
+	if (INSTR((*p)) == op_bra) {
-		/* LP_INSTR points to last instruction of header block,
+		/* Add code before branching to the loop. */
-		 * so if it is 0, the header block is empty yet.
-		 */
-		code->co_loop->LP_HEADER->b_start =
-		  add_code(code->co_loop->LP_HEADER->b_start, code->co_lfirst);
-	} else if (INSTR((*p)) == op_bra) {
 		add_code(PREV((*p)), code->co_lfirst);
+	} else {
+		/* Add code before falling into the loop. */
+		add_code(*p, code->co_lfirst);
+		while (l->l_next)
+			l = l->l_next;
+		*p = l; /* new last instruction */
 	}
-	else	add_code(*p, code->co_lfirst);
-	while (l->l_next) l = l->l_next;
-	*p = l; /* new last instruction */
 }
 
 STATIC void incr_code(code,tmp)
@@ -453,43 +450,7 @@ STATIC code_p available(c,vars)
 	return (code_p) 0;
 }
 
-STATIC void fix_header(lp)
+STATIC void reduce(code_p code, lset vars)
-	loop_p lp;
-{
-	/* Check if a header block was added, and if so, add a branch to
-	 * the entry block.
-	 * If it was added, it was added to the end of the procedure, so
-	 * move the END pseudo.
-	 */
-	bblock_p b = curproc->p_start;
-
-	if (lp->LP_HEADER->b_next == 0) {
-		line_p l = last_instr(lp->LP_HEADER);
-		line_p e;
-
-		assert(l != 0);
-		if (INSTR(l) != op_bra) {
-			line_p j = newline(OPINSTRLAB);
-
-			assert(INSTR(lp->lp_entry->b_start) == op_lab);
-			INSTRLAB(j) = INSTRLAB(lp->lp_entry->b_start);
-			j->l_instr = op_bra;
-			DLINK(l, j);
-			l = j;
-		}
-
-		while (b->b_next != lp->LP_HEADER) b = b->b_next;
-		e = last_instr(b);
-		assert(INSTR(e) == ps_end);
-		assert(PREV(e) != 0);
-		PREV(e)->l_next = 0;
-		DLINK(l, e);
-	}
-}
-
-STATIC void reduce(code,vars)
-	code_p code;
-	lset   vars;
 {
 	/* Perform the actual transformations. The code on the left
 	 * gets transformed into the code on the right. Note that
@@ -538,7 +499,6 @@ STATIC void reduce(code,vars)
 		incr_code(code,tmp); /* emit code to increment temp. local */
 		OUTTRACE("emitted increment code",0);
 		Ladd(code,&avail);
-		fix_header(code->co_loop);
 	}
 }
 

util/ego/sr/sr_xform.c

@@ -138,9 +138,7 @@ STATIC void adjust_jump(newtarg,oldtarg,c)
 }
 
 
-void
+void make_header(loop_p lp)
-make_header(lp)
-	loop_p lp;
 {
 	/* Make sure that the loop has a header block, i.e. a block
 	 * has the loop entry block as its only successor and
@@ -149,6 +147,7 @@ make_header(lp)
 	 */
 
 	bblock_p b,c,entry;
+	line_p branch,last;
 	Lindex i,next;
 
 	if (lp->LP_HEADER != (bblock_p) 0) return;
@@ -160,6 +159,17 @@ make_header(lp)
 	b = freshblock(); /* new block with new b_id */
 	entry = lp->lp_entry;
 
+	/* In the header, add a branch from to the entry block. */
+	branch = newline(OPINSTRLAB);
+	assert(INSTR(entry->b_start) == op_lab);
+	INSTRLAB(branch) = INSTRLAB(entry->b_start);
+	branch->l_instr = op_bra;
+	b->b_start = branch;
+
+	/* Plan to insert code before the branch. */
+	assert(lp->LP_INSTR == 0);
+	lp->LP_INSTR = branch;
+
 	/* update succ/pred. Also take care that any jump from outside
 	 * the loop to the entry block now goes to b.
 	 */
@@ -180,14 +190,24 @@ make_header(lp)
 			adjust_jump(b,entry,c);
 		}
 	}
-	assert(lp->LP_INSTR == 0);
+
-	lp->LP_INSTR = b->b_start;
 	Ladd(b,&entry->b_pred);
 	Ladd(entry,&b->b_succ);
+
 	/* put header block at end of procedure */
-	for (c = curproc->p_start; c->b_next != 0; c = c->b_next);
+	for (c = curproc->p_start; c->b_next != 0; c = c->b_next)
+		continue;
 	c->b_next = b;
 	/* b->b_next = 0; */
+
+	/* move the END pseudo to header block */
+	last = last_instr(c);
+	assert(INSTR(last) == ps_end);
+	assert(PREV(last) != (line_p) 0);
+	PREV(last)->l_next = 0;
+	DLINK(branch, last);
+
+	/* fix loops and dominance */
 	copy_loops(b,entry,lp);
 	b->b_idom = entry->b_idom;
 	entry->b_idom = b;