From 937f527054f13dd524588c064cd5d76e3cfd23eb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 5 Apr 2016 12:56:57 +0200
Subject: Avoid rebuilding unchanged instructions

In transformations such as:

  move S X0=x==0 | line Loc  | call_ext Ar Func => \
  line Loc       | move S X0 | call_ext Ar Func

we can avoid rebuilding the last instruction in the sequence
by introducing a 'keep' instruction.

Currently, there are only 13 transformations that are hit by
this optimization, but most of them are frequently used.
---
 erts/emulator/utils/beam_makeops | 41 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

(limited to 'erts/emulator/utils')

diff --git a/erts/emulator/utils/beam_makeops b/erts/emulator/utils/beam_makeops
index 3d4213d55d..66ffd83ee9 100755
--- a/erts/emulator/utils/beam_makeops
+++ b/erts/emulator/utils/beam_makeops
@@ -1718,6 +1718,7 @@ sub tr_gen_to {
     push(@code, make_op('', 'end'))
 	unless is_instr($code[$#code], 'call_end');
 
+    tr_maybe_keep(\@code);
     tr_maybe_rename(\@code);
 
     #
@@ -1745,6 +1746,46 @@ 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';
-- 
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