From 18dc9a8099d1ad8f52b9db7bd251bcbfd632ce5e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 14 May 2019 10:11:31 +0200
Subject: Make the swap instruction known to the compiler

BEAM has had a `swap` instruction for several releases, but it was not
known to the compiler. The loader would translate a sequence of three
`move` instructions to the `swap` instructions, but only when it was
possible to determine that it would be safe.

By making `swap` known to the compiler, it can be applied in more
situations since it is easier for the compiler than for the loader
to ensure that the usage is safe, and the loader shenanigans can be
eliminated.
---
 lib/compiler/src/beam_clean.erl | 52 ++++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 14 deletions(-)

(limited to 'lib/compiler/src/beam_clean.erl')

diff --git a/lib/compiler/src/beam_clean.erl b/lib/compiler/src/beam_clean.erl
index 7299654476..6b2b2ce085 100644
--- a/lib/compiler/src/beam_clean.erl
+++ b/lib/compiler/src/beam_clean.erl
@@ -34,7 +34,8 @@ module({Mod,Exp,Attr,Fs0,_}, Opts) ->
     Used = find_all_used(WorkList, All, cerl_sets:from_list(WorkList)),
     Fs1 = remove_unused(Order, Used, All),
     {Fs2,Lc} = clean_labels(Fs1),
-    Fs = maybe_remove_lines(Fs2, Opts),
+    Fs3 = fix_swap(Fs2, Opts),
+    Fs = maybe_remove_lines(Fs3, Opts),
     {ok,{Mod,Exp,Attr,Fs,Lc}}.
 
 %% Determine the rootset, i.e. exported functions and
@@ -136,6 +137,24 @@ function_replace([{function,Name,Arity,Entry,Asm0}|Fs], Dict, Acc) ->
     function_replace(Fs, Dict, [{function,Name,Arity,Entry,Asm}|Acc]);
 function_replace([], _, Acc) -> Acc.
 
+%%%
+%%% If compatibility with a previous release (OTP 22 or earlier) has
+%%% been requested, replace swap instructions with a sequence of moves.
+%%%
+
+fix_swap(Fs, Opts) ->
+    case proplists:get_bool(no_swap, Opts) of
+        false -> Fs;
+        true -> fold_functions(fun swap_moves/1, Fs)
+    end.
+
+swap_moves([{swap,Reg1,Reg2}|Is]) ->
+    Temp = {x,1022},
+    [{move,Reg1,Temp},{move,Reg2,Reg1},{move,Temp,Reg2}|swap_moves(Is)];
+swap_moves([I|Is]) ->
+    [I|swap_moves(Is)];
+swap_moves([]) -> [].
+
 %%%
 %%% Remove line instructions if requested.
 %%%
@@ -143,25 +162,30 @@ function_replace([], _, Acc) -> Acc.
 maybe_remove_lines(Fs, Opts) ->
     case proplists:get_bool(no_line_info, Opts) of
 	false -> Fs;
-	true -> remove_lines(Fs)
+	true -> fold_functions(fun remove_lines/1, Fs)
     end.
 
-remove_lines([{function,N,A,Lbl,Is0}|T]) ->
-    Is = remove_lines_fun(Is0),
-    [{function,N,A,Lbl,Is}|remove_lines(T)];
-remove_lines([]) -> [].
-
-remove_lines_fun([{line,_}|Is]) ->
-    remove_lines_fun(Is);
-remove_lines_fun([{block,Bl0}|Is]) ->
+remove_lines([{line,_}|Is]) ->
+    remove_lines(Is);
+remove_lines([{block,Bl0}|Is]) ->
     Bl = remove_lines_block(Bl0),
-    [{block,Bl}|remove_lines_fun(Is)];
-remove_lines_fun([I|Is]) ->
-    [I|remove_lines_fun(Is)];
-remove_lines_fun([]) -> [].
+    [{block,Bl}|remove_lines(Is)];
+remove_lines([I|Is]) ->
+    [I|remove_lines(Is)];
+remove_lines([]) -> [].
 
 remove_lines_block([{set,_,_,{line,_}}|Is]) ->
     remove_lines_block(Is);
 remove_lines_block([I|Is]) ->
     [I|remove_lines_block(Is)];
 remove_lines_block([]) -> [].
+
+
+%%%
+%%% Helpers.
+%%%
+
+fold_functions(F, [{function,N,A,Lbl,Is0}|T]) ->
+    Is = F(Is0),
+    [{function,N,A,Lbl,Is}|fold_functions(F, T)];
+fold_functions(_F, []) -> [].
-- 
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