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authorBjörn Gustavsson <[email protected]>2018-11-09 06:58:33 +0100
committerBjörn Gustavsson <[email protected]>2018-11-28 11:36:54 +0100
commit2e59e2ab69d7f50c8a03b1c89528f8cf38a3a8b4 (patch)
tree2428080d1fdf80942fe4d6b4562c1f5107c0f465 /lib/compiler/src
parent0798674578374938136bc7d3c92416d156492c3e (diff)
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beam_jump: Eliminate jumps to return instructions
Eliminate a jump to a return sequence, replacing the jump with the return sequence. This optimization always save execution time and may also save code space.
Diffstat (limited to 'lib/compiler/src')
-rw-r--r--lib/compiler/src/beam_jump.erl40
1 files changed, 39 insertions, 1 deletions
diff --git a/lib/compiler/src/beam_jump.erl b/lib/compiler/src/beam_jump.erl
index d53dc61612..57329338bf 100644
--- a/lib/compiler/src/beam_jump.erl
+++ b/lib/compiler/src/beam_jump.erl
@@ -101,6 +101,10 @@
%%% always keep the label. (beam_clean will remove any unused
%%% labels.)
%%%
+%%% (7) Replace a jump to a return instruction with a return instruction.
+%%% Similarly, replace a jump to deallocate + return with those
+%%% instructions.
+%%%
%%% Note: This modules depends on (almost) all branches and jumps only
%%% going forward, so that we can remove instructions (including definition
%%% of labels) after any label that has not been referenced by the code
@@ -150,7 +154,8 @@ function({function,Name,Arity,CLabel,Asm0}, Lc0) ->
Asm3 = share(Asm2),
Asm4 = move(Asm3),
Asm5 = opt(Asm4, CLabel),
- Asm = remove_unused_labels(Asm5),
+ Asm6 = unshare(Asm5),
+ Asm = remove_unused_labels(Asm6),
{{function,Name,Arity,CLabel,Asm},Lc}
catch
Class:Error:Stack ->
@@ -624,6 +629,39 @@ drop_upto_label([{label,_}|_]=Is) -> Is;
drop_upto_label([_|Is]) -> drop_upto_label(Is);
drop_upto_label([]) -> [].
+%% unshare([Instruction]) -> [Instruction].
+%% Replace a jump to a return sequence (a `return` instruction
+%% optionally preced by a `deallocate` instruction) with the return
+%% sequence. This always saves execution time and may also save code
+%% space (depending on the architecture). Eliminating `jump`
+%% instructions also gives beam_trim more opportunities to trim the
+%% stack.
+
+unshare(Is) ->
+ Short = unshare_collect_short(Is, #{}),
+ unshare_short(Is, Short).
+
+unshare_collect_short([{label,L},return|Is], Map) ->
+ unshare_collect_short(Is, Map#{L=>[return]});
+unshare_collect_short([{label,L},{deallocate,_}=D,return|Is], Map) ->
+ %% `deallocate` and `return` are combined into one instruction by
+ %% the loader.
+ unshare_collect_short(Is, Map#{L=>[D,return]});
+unshare_collect_short([_|Is], Map) ->
+ unshare_collect_short(Is, Map);
+unshare_collect_short([], Map) -> Map.
+
+unshare_short([{jump,{f,F}}=I|Is], Map) ->
+ case Map of
+ #{F:=Seq} ->
+ Seq ++ unshare_short(Is, Map);
+ #{} ->
+ [I|unshare_short(Is, Map)]
+ end;
+unshare_short([I|Is], Map) ->
+ [I|unshare_short(Is, Map)];
+unshare_short([], _Map) -> [].
+
%% ulbl(Instruction, UsedCerlSet) -> UsedCerlSet'
%% Update the cerl_set UsedCerlSet with any function-local labels
%% (i.e. not with labels in call instructions) referenced by