diff options
Diffstat (limited to 'lib/compiler/src/beam_dead.erl')
| -rw-r--r-- | lib/compiler/src/beam_dead.erl | 187 |
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: %% |