Merge branch 'bjorn/erts/beam_load'

* bjorn/erts/beam_load:
  Eliminate unnecessary renaming of bs_put_utf16/3
  Don't let the loader do the compiler's job
  Remove unused variables after code generation
  Avoid rebuilding unchanged instructions
  Introduce a 'rename' instruction
  Simplify window management for the transformation engine
  Eliminate allocation of variables in transform_engine()
  Refactor calls to transform_engine()
  ops.tab: Remove useless transformation

5 files changed, 270 insertions, 212 deletions

diff --git a/erts/emulator/beam/beam_emu.c b/erts/emulator/beam/beam_emu.c
index a390422040..09a41f2b56 100644
--- a/erts/emulator/beam/beam_emu.c
+++ b/erts/emulator/beam/beam_emu.c
@@ -4102,7 +4102,7 @@ do {						\
      StoreBifResult(1, result);
  }
 
- OpCase(i_bs_put_utf16_jIs): {
+ OpCase(bs_put_utf16_jIs): {
      Eterm arg;
 
      GetArg1(2, arg);
diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index 16cbdbffea..a98900460e 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -2028,42 +2028,47 @@ load_code(LoaderState* stp)
 	ASSERT(arity == last_op->arity);
 
     do_transform:
-	if (stp->genop == NULL) {
-	    last_op_next = NULL;
-	    goto get_next_instr;
-	}
-
+	ASSERT(stp->genop != NULL);
 	if (gen_opc[stp->genop->op].transform != -1) {
-	    int need;
-	    tmp_op = stp->genop;
-
-	    for (need = gen_opc[stp->genop->op].min_window-1; need > 0; need--) {
-		if (tmp_op == NULL) {
-		    goto get_next_instr;
-		}
-		tmp_op = tmp_op->next;
+	    if (stp->genop->next == NULL) {
+		/*
+		 * Simple heuristic: Most transformations requires
+		 * at least two instructions, so make sure that
+		 * there are. That will reduce the number of
+		 * TE_SHORT_WINDOWs.
+		 */
+		goto get_next_instr;
 	    }
 	    switch (transform_engine(stp)) {
 	    case TE_FAIL:
-		last_op_next = NULL;
-		last_op = NULL;
+		/*
+		 * No transformation found. stp->genop != NULL and
+		 * last_op_next is still valid. Go ahead and load
+		 * the instruction.
+		 */
 		break;
 	    case TE_OK:
+		/*
+		 * Some transformation was applied. last_op_next is
+		 * no longer valid and stp->genop may be NULL.
+		 * Try to transform again.
+		 */
+		if (stp->genop == NULL) {
+		    last_op_next = &stp->genop;
+		    goto get_next_instr;
+		}
 		last_op_next = NULL;
-		last_op = NULL;
 		goto do_transform;
 	    case TE_SHORT_WINDOW:
-		last_op_next = NULL;
-		last_op = NULL;
+		/*
+		 * No transformation applied. stp->genop != NULL and
+		 * last_op_next is still valid. Fetch a new instruction
+		 * before trying the transformation again.
+		 */
 		goto get_next_instr;
 	    }
 	}
 
-	if (stp->genop == NULL) {
-	    last_op_next = NULL;
-	    goto get_next_instr;
-	}
-
 	/*
 	 * From the collected generic instruction, find the specific
 	 * instruction.
@@ -2584,7 +2589,10 @@ load_code(LoaderState* stp)
 	{
 	    GenOp* next = stp->genop->next;
 	    FREE_GENOP(stp, stp->genop);
-	    stp->genop = next;
+	    if ((stp->genop = next) == NULL) {
+		last_op_next = &stp->genop;
+		goto get_next_instr;
+	    }
 	    goto do_transform;
 	}
     }
@@ -2728,13 +2736,6 @@ mixed_types(LoaderState* stp, GenOpArg Size, GenOpArg* Rest)
 }
 
 static int
-same_label(LoaderState* stp, GenOpArg Target, GenOpArg Label)
-{
-    return Target.type = TAG_f && Label.type == TAG_u &&
-	Target.val == Label.val;
-}
-
-static int
 is_killed_apply(LoaderState* stp, GenOpArg Reg, GenOpArg Live)
 {
     return Reg.type == TAG_x && Live.type == TAG_u &&
@@ -4805,31 +4806,25 @@ transform_engine(LoaderState* st)
     Uint op;
     int ap;			/* Current argument. */
     Uint* restart;		/* Where to restart if current match fails. */
-    GenOpArg def_vars[TE_MAX_VARS]; /* Default buffer for variables. */
-    GenOpArg* var = def_vars;
-    int num_vars = 0;
+    GenOpArg var[TE_MAX_VARS];	/* Buffer for variables. */
+    GenOpArg* rest_args = NULL;
+    int num_rest_args = 0;
     int i;			/* General index. */
     Uint mask;
     GenOp* instr;
+    GenOp* first = st->genop;
+    GenOp* keep = NULL;
     Uint* pc;
-    int rval;
     static Uint restart_fail[1] = {TOP_fail};
 
-    ASSERT(gen_opc[st->genop->op].transform != -1);
-    pc = op_transform + gen_opc[st->genop->op].transform;
-    restart = pc;
+    ASSERT(gen_opc[first->op].transform != -1);
+    restart = op_transform + gen_opc[first->op].transform;
 
  restart:
-    if (var != def_vars) {
-	erts_free(ERTS_ALC_T_LOADER_TMP, (void *) var);
-	var = def_vars;
-    }
     ASSERT(restart != NULL);
     pc = restart;
     ASSERT(*pc < NUM_TOPS);	/* Valid instruction? */
-    instr = st->genop;
-
-#define RETURN(r) rval = (r); goto do_return;
+    instr = first;
 
 #ifdef DEBUG
     restart = NULL;
@@ -4847,7 +4842,7 @@ transform_engine(LoaderState* st)
 		 * We'll need at least one more instruction to decide whether
 		 * this combination matches or not.
 		 */
-		RETURN(TE_SHORT_WINDOW);
+		return TE_SHORT_WINDOW;
 	    }
 	    if (*pc++ != instr->op)
 		goto restart;
@@ -5009,19 +5004,9 @@ transform_engine(LoaderState* st)
 #if defined(TOP_rest_args)
 	case TOP_rest_args:
 	    {
-		int n = *pc++;
 		int formal_arity = gen_opc[instr->op].arity;
-		int j = formal_arity;
-
-		num_vars = n + (instr->arity - formal_arity);
-		var = erts_alloc(ERTS_ALC_T_LOADER_TMP,
-				 num_vars * sizeof(GenOpArg));
-		for (i = 0; i < n; i++) {
-		    var[i] = def_vars[i];
-		}
-		while (i < num_vars) {
-		    var[i++] = instr->a[j++];
-		}
+		num_rest_args = instr->arity - formal_arity;
+		rest_args = instr->a + formal_arity;
 	    }
 	    break;
 #endif
@@ -5030,21 +5015,22 @@ transform_engine(LoaderState* st)
 	    break;
 	case TOP_commit:
 	    instr = instr->next; /* The next_instr was optimized away. */
-
-	    /*
-	     * The left-hand side of this transformation matched.
-	     * Delete all matched instructions.
-	     */
-	    while (st->genop != instr) {
-		GenOp* next = st->genop->next;
-		FREE_GENOP(st, st->genop);
-		st->genop = next;
-	    }
+	    keep = instr;
+	    st->genop = instr;
 #ifdef DEBUG
 	    instr = 0;
 #endif
 	    break;
-
+#if defined(TOP_keep)
+	case TOP_keep:
+	    /* Keep the current instruction unchanged. */
+	    keep = instr;
+	    st->genop = instr;
+#ifdef DEBUG
+	    instr = 0;
+#endif
+	    break;
+#endif
 #if defined(TOP_call_end)
 	case TOP_call_end:
 	    {
@@ -5069,22 +5055,19 @@ transform_engine(LoaderState* st)
 		    lastp = &((*lastp)->next);
 		}
 		 
-		instr = instr->next; /* The next_instr was optimized away. */
-
-		/*
-		 * The left-hand side of this transformation matched.
-		 * Delete all matched instructions.
-		 */
-		while (st->genop != instr) {
-		    GenOp* next = st->genop->next;
-		    FREE_GENOP(st, st->genop);
-		    st->genop = next;
-		}
-		*lastp = st->genop;
+		keep = instr->next; /* The next_instr was optimized away. */
+		*lastp = keep;
 		st->genop = new_instr;
 	    }
-	    RETURN(TE_OK);
+	    /* FALLTHROUGH */
 #endif
+	case TOP_end:
+	    while (first != keep) {
+		GenOp* next = first->next;
+		FREE_GENOP(st, first);
+		first = next;
+	    }
+	    return TE_OK;
 	case TOP_new_instr:
 	    /*
 	     * Note that the instructions are generated in reverse order.
@@ -5096,6 +5079,12 @@ transform_engine(LoaderState* st)
 	    instr->arity = gen_opc[op].arity;
 	    ap = 0;
 	    break;
+#ifdef TOP_rename
+	case TOP_rename:
+	    instr->op = op = *pc++;
+	    instr->arity = gen_opc[op].arity;
+	    return TE_OK;
+#endif
 	case TOP_store_type:
 	    i = *pc++;
 	    instr->a[ap].type = i;
@@ -5115,14 +5104,10 @@ transform_engine(LoaderState* st)
 #if defined(TOP_store_rest_args)
 	case TOP_store_rest_args:
 	    {
-		int n = *pc++;
-		int num_extra = num_vars - n;
-
-		ASSERT(n <= num_vars);
-		GENOP_ARITY(instr, instr->arity+num_extra);
+		GENOP_ARITY(instr, instr->arity+num_rest_args);
 		memcpy(instr->a, instr->def_args, ap*sizeof(GenOpArg));
-		memcpy(instr->a+ap, var+n, num_extra*sizeof(GenOpArg));
-		ap += num_extra;
+		memcpy(instr->a+ap, rest_args, num_rest_args*sizeof(GenOpArg));
+		ap += num_rest_args;
 	    }
 	    break;
 #endif
@@ -5134,21 +5119,12 @@ transform_engine(LoaderState* st)
 	case TOP_try_me_else_fail:
 	    restart = restart_fail;
 	    break;
-	case TOP_end:
-	    RETURN(TE_OK);
 	case TOP_fail:
-	    RETURN(TE_FAIL);
+	    return TE_FAIL;
 	default:
 	    ASSERT(0);
 	}
     }
-#undef RETURN
-
- do_return:
-    if (var != def_vars) {
-	erts_free(ERTS_ALC_T_LOADER_TMP, (void *) var);
-    }
-    return rval;
 }
 
 static void
diff --git a/erts/emulator/beam/beam_load.h b/erts/emulator/beam/beam_load.h
index 22ab71c868..68f4b96893 100644
--- a/erts/emulator/beam/beam_load.h
+++ b/erts/emulator/beam/beam_load.h
@@ -33,7 +33,6 @@ typedef struct gen_op_entry {
    int specific;
    int num_specific;
    int transform;
-   int min_window;
 } GenOpEntry;
 
 extern GenOpEntry gen_opc[];
diff --git a/erts/emulator/beam/ops.tab b/erts/emulator/beam/ops.tab
index 772460c177..15f27835a8 100644
--- a/erts/emulator/beam/ops.tab
+++ b/erts/emulator/beam/ops.tab
@@ -181,11 +181,6 @@ i_jump_on_val_zero y f I
 i_jump_on_val x f I I
 i_jump_on_val y f I I
 
-jump Target | label Lbl | same_label(Target, Lbl) => label Lbl
-
-is_ne_exact L1 S1 S2 | jump Fail | label L2 | same_label(L1, L2) => \
-  is_eq_exact Fail S1 S2 | label L2
-
 %macro: get_list GetList -pack
 get_list x x x
 get_list x x y
@@ -1355,9 +1350,7 @@ bs_put_utf8 Fail u Src=s => i_bs_put_utf8 Fail Src
 
 i_bs_put_utf8 j s
 
-bs_put_utf16 Fail Flags=u Src=s => i_bs_put_utf16 Fail Flags Src
-
-i_bs_put_utf16 j I s
+bs_put_utf16 j I s
 
 bs_put_utf32 Fail=j Flags=u Src=s => \
    i_bs_validate_unicode Fail Src | bs_put_integer Fail i=32 u=1 Flags Src
@@ -1539,7 +1532,6 @@ gen_minus p Live Reg=d Int=i Dst | negation_is_small(Int) => \
 # GCing arithmetic instructions.
 #
 
-gen_plus Fail Live Y=y X=x Dst => i_plus Fail Live X Y Dst
 gen_plus Fail Live S1 S2 Dst => i_plus Fail Live S1 S2 Dst
 
 gen_minus Fail Live S1 S2 Dst => i_minus Fail Live S1 S2 Dst
diff --git a/erts/emulator/utils/beam_makeops b/erts/emulator/utils/beam_makeops
index f805e7cc64..2e7073a8f0 100755
--- a/erts/emulator/utils/beam_makeops
+++ b/erts/emulator/utils/beam_makeops
@@ -113,7 +113,6 @@ my @if_line;
 #
 my $te_max_vars = 0;		# Max number of variables ever needed.
 my %gen_transform;
-my %min_window;
 my %match_engine_ops;		# All opcodes for the match engine.
 my %gen_transform_offset;
 my @transformations;
@@ -382,7 +381,6 @@ while (<>) {
 	    $gen_arity{$name} = $arity;
 	    $gen_to_spec{"$name/$arity"} = undef;
 	    $num_specific{"$name/$arity"} = 0;
-	    $min_window{"$name/$arity"} = 255;
 	    $obsolete[$op_num] = defined $obsolete;
 	} else {		# Unnumbered generic operation.
 	    push(@unnumbered_generic, [$name, $arity]);
@@ -440,7 +438,6 @@ $num_file_opcodes = @gen_opname;
 	$gen_arity{$name} = $arity;
 	$gen_to_spec{"$name/$arity"} = undef;
 	$num_specific{"$name/$arity"} = 0;
-	$min_window{"$name/$arity"} = 255;
     }
 }
 
@@ -607,7 +604,7 @@ sub emulator_output {
 		$is_transformed{$name,$arity} or
 		error("instruction $key has no specific instruction");
 	    $spec_op = -1 unless defined $spec_op;
-	    &init_item($name, $arity, $spec_op, $num_specific, $tr, $min_window{$key});
+	    &init_item($name, $arity, $spec_op, $num_specific, $tr);
 	}
     }
     print "};\n";
@@ -1405,8 +1402,7 @@ sub tr_gen {
 
     foreach $ref (@g) {
 	my($line, $orig_transform, $from_ref, $to_ref) = @$ref;
-	my $used_ref = used_vars($from_ref, $to_ref);
-	my $so_far = tr_gen_from($line, $used_ref, @$from_ref);
+	my $so_far = tr_gen_from($line, @$from_ref);
 	tr_gen_to($line, $orig_transform, $so_far, @$to_ref);
     }
 
@@ -1457,58 +1453,14 @@ sub tr_gen {
     print "};\n\n";
 }
 
-sub used_vars {
-    my($from_ref,$to_ref) = @_;
-    my %used;
-    my %seen;
-
-    foreach my $ref (@$from_ref) {
-	my($name,$arity,@ops) = @$ref;
-	if ($name =~ /^[.]/) {
-	    foreach my $var (@ops) {
-		$used{$var} = 1;
-	    }
-	} else {
-	    # Any variable that is used at least twice on the
-	    # left-hand side is used. (E.g. "move R R".)
-	    foreach my $op (@ops) {
-		my($var, $type, $type_val) = @$op;
-		next if $var eq '';
-		$used{$var} = 1 if $seen{$var};
-		$seen{$var} = 1;
-	    }
-	}
-    }
-
-    foreach my $ref (@$to_ref) {
-	my($name, $arity, @ops) = @$ref;
-	if ($name =~ /^[.]/) {
-	    foreach my $var (@ops) {
-		$used{$var} = 1;
-	    }
-	} else {
-	    foreach my $op (@ops) {
-		my($var, $type, $type_val) = @$op;
-		next if $var eq '';
-		$used{$var} = 1;
-	    }
-	}
-    }
-    \%used;
-}
-
 sub tr_gen_from {
-    my($line,$used_ref,@tr) = @_;
+    my($line,@tr) = @_;
     my(%var) = ();
     my(%var_type);
     my($var_num) = 0;
     my(@code);
-    my($min_window) = 0;
-    my(@fix_rest_args);
-    my(@fix_pred_funcs);
     my($op, $ref);		# Loop variables.
     my $where = "left side of transformation in line $line: ";
-    my %var_used = %$used_ref;
     my $may_fail = 0;
     my $is_first = 1;
 
@@ -1530,8 +1482,20 @@ sub tr_gen_from {
 	    my $var;
 	    my(@args);
 
-	    push(@fix_pred_funcs, scalar(@code));
-	    push(@code, [$name, @ops]);
+	    foreach $var (@ops) {
+		error($where, "variable '$var' unbound")
+		    unless defined $var{$var};
+		if ($var_type{$var} eq 'scalar') {
+		    push(@args, "var[$var{$var}]");
+		} else {
+		    push(@args, "rest_args");
+		}
+	    }
+	    my $pi = tr_next_index(\@pred_table, \%pred_table, $name, @args);
+	    my $op = make_op("$name()", 'pred', $pi);
+	    my @slots = grep(/^\d+/, map { $var{$_} } @ops);
+	    op_slot_usage($op, @slots);
+	    push(@code, $op);
 	    next;
 	}
 
@@ -1544,7 +1508,6 @@ sub tr_gen_from {
 	$opnum = $gen_opnum{$name,$arity};
 
 	push(@code, make_op("$name/$arity", 'next_instr', $opnum));
-	$min_window++;
 	foreach $op (@ops) {
 	    my($var, $type, $type_val, $cond, $val) = @$op;
 	    my $ignored_var = "$var (ignored)";
@@ -1593,15 +1556,21 @@ sub tr_gen_from {
 		if (defined $var{$var}) {
 		    $ignored_var = '';
 		    $may_fail = 1;
-		    push(@code, &make_op($var, 'is_same_var', $var{$var}));
+		    my $op = make_op($var, 'is_same_var', $var{$var});
+		    op_slot_usage($op, $var{$var});
+		    push(@code, $op);
 		} elsif ($type eq '*') {
-		    #
-		    # Reserve a hole for a 'rest_args' instruction.
-		    #
+		    foreach my $type (values %var_type) {
+			error("only one use of a '*' variable is " .
+			      "allowed on the left hand side of " .
+			      "a transformation")
+			    if $type eq 'array';
+		    }
 		    $ignored_var = '';
-		    push(@fix_rest_args, scalar(@code));
-		    push(@code, $var);
-		} elsif ($var_used{$var}) {
+		    $var{$var} = 'unnumbered';
+		    $var_type{$var} = 'array';
+		    push(@code, make_op($var, 'rest_args'));
+		} else {
 		    $ignored_var = '';
 		    $var_type{$var} = 'scalar';
 		    $var{$var} = $var_num;
@@ -1629,46 +1598,14 @@ sub tr_gen_from {
     #
     push(@code, make_op($may_fail ? '' : 'always reached', 'commit'));
 
-    #
-    # If there is an rest_args instruction, we must insert its correct
-    # variable number (higher than any other).
-    #
-    my $index;
-    &error("only one use of a '*' variable is allowed on the left hand side of a transformation")
-	if @fix_rest_args > 1;
-    foreach $index (@fix_rest_args) {
-	my $var = $code[$index];
-	$var{$var} = $var_num++;
-	$var_type{$var} = 'array';
-	splice(@code, $index, 1, &make_op($var, 'rest_args', $var{$var}));
-    }
-
-    foreach $index (@fix_pred_funcs) {
-	my($name, @ops) = @{$code[$index]};
-	my(@args);
-	my $var;
-
-	foreach $var (@ops) {
-	    &error($where, "variable '$var' unbound")
-		unless defined $var{$var};
-	    if ($var_type{$var} eq 'scalar') {
-		push(@args, "var[$var{$var}]");
-	    } else {
-		push(@args, "var+$var{$var}");
-	    }
-	}
-	my $pi = tr_next_index(\@pred_table, \%pred_table, $name, @args);
-	splice(@code, $index, 1, make_op("$name()", 'pred', $pi));
-    }
-
     $te_max_vars = $var_num
 	if $te_max_vars < $var_num;
-    [$min_window, \%var, \%var_type, \@code];
+    [\%var, \%var_type, \@code];
 }
 
 sub tr_gen_to {
     my($line, $orig_transform, $so_far, @tr) = @_;
-    my($min_window, $var_ref, $var_type_ref, $code_ref) = @$so_far;
+    my($var_ref, $var_type_ref, $code_ref) = @$so_far;
     my(%var) = %$var_ref;
     my(%var_type) = %$var_type_ref;
     my(@code) = @$code_ref;
@@ -1697,13 +1634,16 @@ sub tr_gen_to {
 		if ($var_type{$var} eq 'scalar') {
 		    push(@args, "var[$var{$var}]");
 		} else {
-		    push(@args, "var+$var{$var}");
+		    push(@args, "rest_args");
 		}
 	    }
 	    pop(@code);	# Get rid of 'commit' instruction
 	    my $index = tr_next_index(\@call_table, \%call_table,
 				      $name, @args);
-	    push(@code, make_op("$name()", 'call_end', $index));
+	    my $op = make_op("$name()", 'call_end', $index);
+	    my @slots = grep(/^\d+/, map { $var{$_} } @ops);
+	    op_slot_usage($op, @slots);
+	    push(@code, $op);
 	    last;
 	}
 
@@ -1725,11 +1665,13 @@ sub tr_gen_to {
 	    my($var, $type, $type_val) = @$op;
 
 	    if ($type eq '*') {
-		push(@code, make_op($var, 'store_rest_args', $var{$var}));
+		push(@code, make_op($var, 'store_rest_args'));
 	    } elsif ($var ne '') {
 		&error($where, "variable '$var' unbound")
 		    unless defined $var{$var};
-		push(@code, &make_op($var, 'store_var_next_arg', $var{$var}));
+		my $op = make_op($var, 'store_var_next_arg', $var{$var});
+		op_slot_usage($op, $var{$var});
+		push(@code, $op);
 	    } elsif ($type ne '') {
 		push(@code, &make_op('', 'store_type', "TAG_$type"));
 		if ($type_val) {
@@ -1744,6 +1686,10 @@ sub tr_gen_to {
     push(@code, make_op('', 'end'))
 	unless is_instr($code[$#code], 'call_end');
 
+    tr_maybe_keep(\@code);
+    tr_maybe_rename(\@code);
+    tr_remove_unused(\@code);
+
     #
     # Chain together all codes segments having the same first operation.
     #
@@ -1752,8 +1698,6 @@ sub tr_gen_to {
     my($dummy, $arity);
     ($dummy, $op, $arity) = @$first;
     my($comment) = "\n/*\n * Line $line:\n *   $orig_transform\n */\n\n";
-    $min_window{$key} = $min_window
-	if $min_window{$key} > $min_window;
 
     my $prev_last;
     $prev_last = pop(@{$gen_transform{$key}})
@@ -1771,6 +1715,148 @@ sub tr_gen_to {
     push(@{$gen_transform{$key}}, @code),
 }
 
+sub tr_maybe_keep {
+    my($ref) = @_;
+    my @last_instr;
+    my $pos;
+    my $reused_instr;
+
+    for (my $i = 0; $i < @$ref; $i++) {
+	my $instr = $$ref[$i];
+	my($size, $instr_ref, $comment) = @$instr;
+	my($op, @args) = @$instr_ref;
+	if ($op eq 'next_instr') {
+	    @last_instr = ($args[0]);
+	} elsif ($op eq 'set_var_next_arg') {
+	    push @last_instr, $args[0];
+	} elsif ($op eq 'next_arg') {
+	    push @last_instr, 'ignored';
+	} elsif ($op eq 'new_instr') {
+	    unless (defined $pos) {
+		# 'new_instr' immediately after 'commit'.
+		$reused_instr = $args[0];
+		return unless shift(@last_instr) == $reused_instr;
+		$pos = $i - 1;
+	    } else {
+		# Second 'new_instr' after 'commit'. The instructions
+		# from $pos up to and including $i - 1 rebuilds the
+		# existing instruction exactly.
+		my $name = $gen_opname[$reused_instr];
+		my $arity = $gen_arity[$reused_instr];
+		my $reuse = make_op("$name/$arity", 'keep');
+		splice @$ref, $pos, $i-$pos, ($reuse);
+		return;
+	    }
+	} elsif ($op eq 'store_var_next_arg') {
+	    return unless shift(@last_instr) eq $args[0];
+	} elsif (defined $pos) {
+	    return;
+	}
+    }
+}
+
+sub tr_maybe_rename {
+    my($ref) = @_;
+    my $s = 'left';
+    my $a = 0;
+    my $num_args = 0;
+    my $new_instr;
+    my $first;
+    my $i;
+
+    for ($i = 1; $i < @$ref; $i++) {
+	my $instr = $$ref[$i];
+	my($size, $instr_ref, $comment) = @$instr;
+	my($op, @args) = @$instr_ref;
+
+	if ($s eq 'left') {
+	    if ($op eq 'set_var_next_arg') {
+		if ($num_args == $a and $args[0] == $a) {
+		    $num_args++;
+		}
+		$a++;
+	    } elsif ($op eq 'next_arg') {
+		$a++;
+	    } elsif ($op eq 'commit') {
+		$a = 0;
+		$first = $i;
+		$s = 'committed';
+	    } elsif ($op eq 'next_instr') {
+		return;
+	    }
+	} elsif ($s eq 'committed') {
+	    if ($op eq 'new_instr') {
+		$new_instr = $args[0];
+		$a = 0;
+		$s = 'right';
+	    } else {
+		return;
+	    }
+	} elsif ($s eq 'right') {
+	    if ($op eq 'store_var_next_arg' && $args[0] == $a) {
+		$a++;
+	    } elsif ($op eq 'end' && $a <= $num_args) {
+		my $name = $gen_opname[$new_instr];
+		my $arity = $gen_arity[$new_instr];
+		my $new_op = make_op("$name/$arity", 'rename', $new_instr);
+		splice @$ref, $first, $i-$first+1, ($new_op);
+		return;
+	    } else {
+		return;
+	    }
+	}
+    }
+}
+
+sub tr_remove_unused {
+    my($ref) = @_;
+    my %used;
+
+    # Collect all used variables.
+    for my $instr (@$ref) {
+	my $uref = $$instr[3];
+	for my $slot (@$uref) {
+	    $used{$slot} = 1;
+	}
+    }
+
+    # Replace 'set_var_next_arg' with 'next_arg' if the variable
+    # is never used.
+    for my $instr (@$ref) {
+	my($size, $instr_ref, $comment) = @$instr;
+	my($op, @args) = @$instr_ref;
+	if ($op eq 'set_var_next_arg') {
+	    my $var = $args[0];
+	    next if $used{$var};
+	    $instr = make_op("$comment (ignored)", 'next_arg');
+	}
+    }
+
+    # Delete a sequence of 'next_arg' instructions when they are
+    # redundant before instructions such as 'commit'.
+    my @opcode;
+    my %ending = (call_end => 1,
+		  commit => 1,
+		  next_instr => 1,
+		  pred => 1,
+		  rename => 1,
+		  keep => 1);
+    for (my $i = 0; $i < @$ref; $i++) {
+	my $instr = $$ref[$i];
+	my($size, $instr_ref, $comment) = @$instr;
+	my($opcode) = @$instr_ref;
+
+	if ($ending{$opcode}) {
+	    my $first = $i;
+	    $first-- while $first > 0 and $opcode[$first-1] eq 'next_arg';
+	    my $n = $i - $first;
+	    splice @$ref, $first, $n;
+	    $i -= $n;
+	}
+	$opcode[$i] = $opcode;
+    }
+}
+
 sub tr_code_len {
     my($sum) = 0;
     my($ref);
@@ -1783,7 +1869,12 @@ sub tr_code_len {
 
 sub make_op {
     my($comment, @op) = @_;
-    [scalar(@op), [@op], $comment];
+    [scalar(@op), [@op], $comment, []];
+}
+
+sub op_slot_usage {
+    my($op_ref, @slots) = @_;
+    $$op_ref[3] = \@slots;
 }
 
 sub is_instr {