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authorBjörn Gustavsson <[email protected]>2015-01-09 12:31:55 +0100
committerBjörn Gustavsson <[email protected]>2015-01-12 09:53:16 +0100
commit5d9aa72a69bd3fd2054558036838fab6434c20df (patch)
treec9986611ddde0f196dc35b9d451b2ce141eb9f13 /lib/compiler
parentc01e015797153b56cd6b32cab94b8b6b30975f7e (diff)
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Update the comments that explain what beam_dead does
Diffstat (limited to 'lib/compiler')
-rw-r--r--lib/compiler/src/beam_dead.erl187
1 files changed, 76 insertions, 111 deletions
diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index 7e582a2b43..7cd07dc3be 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -21,112 +21,10 @@
-export([module/2]).
-%%% The following optimisations are done:
-%%%
-%%% (1) In this code
-%%%
-%%% move DeadValue {x,0}
-%%% jump L2
-%%% .
-%%% .
-%%% .
-%%% L2: move Anything {x,0}
-%%% .
-%%% .
-%%% .
-%%%
-%%% the first assignment to {x,0} has no effect (is dead),
-%%% so it can be removed. Besides removing a move instruction,
-%%% if the move was preceeded by a label, the resulting code
-%%% will look this
-%%%
-%%% L1: jump L2
-%%% .
-%%% .
-%%% .
-%%% L2: move Anything {x,0}
-%%% .
-%%% .
-%%% .
-%%%
-%%% which can be further optimized by the jump optimizer (beam_jump).
-%%%
-%%% (2) In this code
-%%%
-%%% L1: move AtomLiteral {x,0}
-%%% jump L2
-%%% .
-%%% .
-%%% .
-%%% L2: test is_atom FailLabel {x,0}
-%%% select_val {x,0}, FailLabel [... AtomLiteral => L3...]
-%%% .
-%%% .
-%%% .
-%%% L3: ...
-%%%
-%%% FailLabel: ...
-%%%
-%%% the first code fragment can be changed to
-%%%
-%%% L1: move AtomLiteral {x,0}
-%%% jump L3
-%%%
-%%% If the literal is not included in the table of literals in the
-%%% select_val instruction, the first code fragment will instead be
-%%% rewritten as:
-%%%
-%%% L1: move AtomLiteral {x,0}
-%%% jump FailLabel
-%%%
-%%% The move instruction will be removed by optimization (1) above,
-%%% if the code following the L3 label overwrites {x,0}.
-%%%
-%%% The code following the L2 label will be kept, but it will be removed later
-%%% by the jump optimizer.
-%%%
-%%% (3) In this code
-%%%
-%%% test is_eq_exact ALabel Src Dst
-%%% move Src Dst
-%%%
-%%% the move instruction can be removed.
-%%% Same thing for
-%%%
-%%% test is_nil ALabel Dst
-%%% move [] Dst
-%%%
-%%%
-%%% (4) In this code
-%%%
-%%% select_val {x,Reg}, ALabel [... Literal => L1...]
-%%% .
-%%% .
-%%% .
-%%% L1: move Literal {x,Reg}
-%%%
-%%% we can remove the move instruction.
-%%%
-%%% (5) In the following code
-%%%
-%%% bif '=:=' Fail Src1 Src2 {x,0}
-%%% jump L1
-%%% .
-%%% .
-%%% .
-%%% L1: select_val {x,0}, ALabel [... true => L2..., ...false => L3...]
-%%% .
-%%% .
-%%% .
-%%% L2: .... L3: ....
-%%%
-%%% the first two instructions can be replaced with
-%%%
-%%% test is_eq_exact L3 Src1 Src2
-%%% jump L2
-%%%
-%%% provided that {x,0} is killed at both L2 and L3.
-%%%
+%%% Dead code is code that is executed but has no effect. This
+%%% optimization pass either removes dead code or jumps around it,
+%%% potentially making it unreachable and a target for the
+%%% the beam_jump pass.
-import(lists, [mapfoldl/3,reverse/1]).
@@ -173,7 +71,28 @@ move_move_into_block([I|Is], Acc) ->
move_move_into_block([], Acc) -> reverse(Acc).
%%%
-%%% Scan instructions in execution order and remove dead code.
+%%% Scan instructions in execution order and remove redundant 'move'
+%%% instructions. 'move' instructions are redundant if we know that
+%%% the register already contains the value being assigned, as in the
+%%% following code:
+%%%
+%%% test is_eq_exact SomeLabel Src Dst
+%%% move Src Dst
+%%%
+%%% or in:
+%%%
+%%% test is_nil SomeLabel Dst
+%%% move nil Dst
+%%%
+%%% or in:
+%%%
+%%% select_val Register FailLabel [... Literal => L1...]
+%%% .
+%%% .
+%%% .
+%%% L1: move Literal Register
+%%%
+%%% Also add extra labels to help the second backward pass.
%%%
forward(Is, Lc) ->
@@ -249,8 +168,37 @@ useful_to_insert_label([{test,Op,_,_}|_]) ->
end.
%%%
-%%% Scan instructions in reverse execution order and remove dead code.
+%%% Scan instructions in reverse execution order and try to
+%%% shortcut branch instructions.
%%%
+%%% For example, in this code:
+%%%
+%%% move Literal Register
+%%% jump L1
+%%% .
+%%% .
+%%% .
+%%% L1: test is_{integer,atom} FailLabel Register
+%%% select_val {x,0} FailLabel [... Literal => L2...]
+%%% .
+%%% .
+%%% .
+%%% L2: ...
+%%%
+%%% the 'selectval' instruction will always transfer control to L2,
+%%% so we can just as well jump to L2 directly by rewriting the
+%%% first part of the sequence like this:
+%%%
+%%% move Literal Register
+%%% jump L2
+%%%
+%%% If register Register is killed at label L2, we can remove the
+%%% 'move' instruction, leaving just the 'jump' instruction:
+%%%
+%%% jump L2
+%%%
+%%% These transformations may leave parts of the code unreachable.
+%%% The beam_jump pass will remove the unreachable code.
backward(Is, D) ->
backward(Is, D, []).
@@ -392,6 +340,23 @@ shortcut_boolean_label(To0, Reg, {atom,Bool0}=Lit, D) when is_boolean(Bool0) ->
end;
shortcut_boolean_label(To, _, Bool, _) -> {To,Bool}.
+%% Replace a comparison operator with a test instruction and a jump.
+%% For example, if we have this code:
+%%
+%% bif '=:=' Fail Src1 Src2 {x,0}
+%% jump L1
+%% .
+%% .
+%% .
+%% L1: select_val {x,0} FailLabel [... true => L2..., ...false => L3...]
+%%
+%% the first two instructions can be replaced with
+%%
+%% test is_eq_exact L3 Src1 Src2
+%% jump L2
+%%
+%% provided that {x,0} is killed at both L2 and L3.
+
replace_comp_op(To, Reg, Op, Ops, D) ->
False = comp_op_find_shortcut(To, Reg, {atom,false}, D),
True = comp_op_find_shortcut(To, Reg, {atom,true}, D),
@@ -426,9 +391,9 @@ not_possible() -> throw(not_possible).
%%
%% is_eq_exact F1 Reg Lit1 select_val Reg F2 [ Lit1 L1
%% L1: . Lit2 L2 ]
-%% .
-%% . ==>
-%% .
+%% .
+%% . ==>
+%% .
%% F1: is_eq_exact F2 Reg Lit2 F1: is_eq_exact F2 Reg Lit2
%% L2: .... L2:
%%