diff options
| -rw-r--r-- | lib/compiler/src/beam_block.erl | 83 |
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. |