aboutsummaryrefslogtreecommitdiffstats
path: root/lib/compiler
diff options
context:
space:
mode:
Diffstat (limited to 'lib/compiler')
-rw-r--r--lib/compiler/src/beam_block.erl83
1 files changed, 37 insertions, 46 deletions
diff --git a/lib/compiler/src/beam_block.erl b/lib/compiler/src/beam_block.erl
index 4861a2d7ec..58e8f77a2c 100644
--- a/lib/compiler/src/beam_block.erl
+++ b/lib/compiler/src/beam_block.erl
@@ -287,60 +287,51 @@ opt_moves(Ds, Is) ->
%% If there is a {move,Dest,FinalDest} instruction
%% in the instruction stream, remove the move instruction
%% and let FinalDest be the destination.
-%%
-%% For this optimization to be safe, we must be sure that
-%% Dest will not be referenced in any other by other instructions
-%% in the rest of the instruction stream. Not even the indirect
-%% reference by an instruction that may allocate (such as
-%% test_heap/2 or a GC Bif) is allowed.
opt_move(Dest, Is) ->
- opt_move_1(Dest, Is, ?MAXREG, []).
-
-opt_move_1(R, [{set,_,_,{alloc,Live,_}}|_]=Is, SafeRegs, Acc) when Live < SafeRegs ->
- %% Downgrade number of safe regs and rescan the instruction, as it most probably
- %% is a gc_bif instruction.
- opt_move_1(R, Is, Live, Acc);
-opt_move_1(R, [{set,[{x,X}=D],[R],move}|Is], SafeRegs, Acc) ->
- case X < SafeRegs andalso beam_utils:is_killed_block(R, Is) of
- true -> opt_move_2(D, Acc, Is);
- false -> not_possible
- end;
-opt_move_1(R, [{set,[D],[R],move}|Is], _SafeRegs, Acc) ->
+ 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_2(D, Acc, Is);
+ true -> opt_move_rev(D, Acc, Is);
false -> not_possible
end;
-opt_move_1(R, [I|Is], SafeRegs, Acc) ->
- case is_transparent(R, I) of
- false -> not_possible;
- true -> opt_move_1(R, Is, SafeRegs, [I|Acc])
- end.
+opt_move_1({x,_}, [{set,_,_,{alloc,_,_}}|_], _) ->
+ %% The optimization is not possible. If the X register is not
+ %% killed by allocation, the optimization would not be safe.
+ %% If the X register is killed, it means that there cannot
+ %% follow a 'move' instruction with this X register as the
+ %% source.
+ not_possible;
+opt_move_1(R, [{set,_,_,_}=I|Is], Acc) ->
+ %% If the source register is either killed or used by this
+ %% instruction, the optimimization is not possible.
+ case is_killed_or_used(R, I) of
+ true -> not_possible;
+ false -> opt_move_1(R, Is, [I|Acc])
+ end;
+opt_move_1(_, _, _) ->
+ not_possible.
-%% Reverse the instructions, while checking that there are no instructions that
-%% would interfere with using the new destination register chosen.
+%% Reverse the instructions, while checking that there are no
+%% instructions that would interfere with using the new destination
+%% register (D).
-opt_move_2(D, [I|Is], Acc) ->
- case is_transparent(D, I) of
- false -> not_possible;
- true -> opt_move_2(D, Is, [I|Acc])
- end;
-opt_move_2(D, [], Acc) -> {D,Acc}.
-
-%% is_transparent(Register, Instruction) -> true | false
-%% Returns true if Instruction does not in any way references Register
-%% (even indirectly by an allocation instruction).
-%% Returns false if Instruction does reference Register, or we are
-%% not sure.
-
-is_transparent({x,X}, {set,_,_,{alloc,Live,_}}) when X < Live ->
- false;
-is_transparent(R, {set,Ds,Ss,_Op}) ->
- case member(R, Ds) of
- true -> false;
- false -> not member(R, Ss)
+opt_move_rev(D, [I|Is], Acc) ->
+ case is_killed_or_used(D, I) of
+ true -> not_possible;
+ false -> opt_move_rev(D, Is, [I|Acc])
end;
-is_transparent(_, _) -> false.
+opt_move_rev(D, [], Acc) -> {D,Acc}.
+
+%% is_killed_or_used(Register, {set,_,_,_}) -> bool()
+%% Test whether the register is used by the instruction.
+
+is_killed_or_used(R, {set,Ss,Ds,_}) ->
+ member(R, Ds) orelse member(R, Ss).
%% opt_alloc(Instructions) -> Instructions'
%% Optimises all allocate instructions.