From 02d6135813dc133e018c161d803a642582aac36f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Wed, 22 Jul 2015 10:20:59 +0200
Subject: beam_block: Improve the move optimizations
Here is an example of a move instruction that could not be optimized
away because the {x,2} register was not killed:
get_tuple_element Reg Pos {x,2}
.
.
.
move {x,2} {y,0}
put_list {x,2} nil Any
We can do the optimization if we replace all occurrences of the {x,2}
register as a source with {y,0}:
get_tuple_element Reg Pos {y,0}
.
.
.
put_list {y,0} nil Dst
---
lib/compiler/src/beam_block.erl | 53 +++++++++++++++++++++++++++++++++++------
1 file changed, 46 insertions(+), 7 deletions(-)
(limited to 'lib/compiler/src')
diff --git a/lib/compiler/src/beam_block.erl b/lib/compiler/src/beam_block.erl
index 58e8f77a2c..af1bf8fd08 100644
--- a/lib/compiler/src/beam_block.erl
+++ b/lib/compiler/src/beam_block.erl
@@ -291,13 +291,12 @@ opt_moves(Ds, Is) ->
opt_move(Dest, Is) ->
opt_move_1(Dest, Is, []).
-opt_move_1(R, [{set,[D],[R],move}|Is], Acc) ->
- %% Check whether this instruction can be safely eliminated.
- %% First check that the source (R) is not used by the
- %% instructions that follow.
- case beam_utils:is_killed_block(R, Is) of
- true -> opt_move_rev(D, Acc, Is);
- false -> not_possible
+opt_move_1(R, [{set,[D],[R],move}|Is0], Acc) ->
+ %% Provided that the source register is killed by instructions
+ %% that follow, the optimization is safe.
+ case eliminate_use_of_from_reg(Is0, R, D, []) of
+ {yes,Is} -> opt_move_rev(D, Acc, Is);
+ no -> not_possible
end;
opt_move_1({x,_}, [{set,_,_,{alloc,_,_}}|_], _) ->
%% The optimization is not possible. If the X register is not
@@ -333,6 +332,46 @@ opt_move_rev(D, [], Acc) -> {D,Acc}.
is_killed_or_used(R, {set,Ss,Ds,_}) ->
member(R, Ds) orelse member(R, Ss).
+%% eliminate_use_of_from_reg([Instruction], FromRegister, ToRegister, Acc) ->
+%% {yes,Is} | no
+%% Eliminate any use of FromRegister in the instruction sequence
+%% by replacing uses of FromRegister with ToRegister. If FromRegister
+%% is referenced by an allocation instruction, return 'no' to indicate
+%% that FromRegister is still used and that the optimization is not
+%% possible.
+
+eliminate_use_of_from_reg([{set,_,_,{alloc,Live,_}}|_]=Is0, {x,X}, _, Acc) ->
+ if
+ X < Live ->
+ no;
+ true ->
+ {yes,reverse(Acc, Is0)}
+ end;
+eliminate_use_of_from_reg([{set,Ds,Ss0,Op}=I0|Is], From, To, Acc) ->
+ I = case member(From, Ss0) of
+ true ->
+ Ss = [case S of
+ From -> To;
+ _ -> S
+ end || S <- Ss0],
+ {set,Ds,Ss,Op};
+ false ->
+ I0
+ end,
+ case member(From, Ds) of
+ true ->
+ {yes,reverse(Acc, [I|Is])};
+ false ->
+ eliminate_use_of_from_reg(Is, From, To, [I|Acc])
+ end;
+eliminate_use_of_from_reg([I]=Is, From, _To, Acc) ->
+ case beam_utils:is_killed_block(From, [I]) of
+ true ->
+ {yes,reverse(Acc, Is)};
+ false ->
+ no
+ end.
+
%% opt_alloc(Instructions) -> Instructions'
%% Optimises all allocate instructions.
--
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