diff options
Diffstat (limited to 'lib/compiler/src/beam_block.erl')
| -rw-r--r-- | lib/compiler/src/beam_block.erl | 98 |
1 files changed, 63 insertions, 35 deletions
diff --git a/lib/compiler/src/beam_block.erl b/lib/compiler/src/beam_block.erl index 85d332c56e..ec41925beb 100644 --- a/lib/compiler/src/beam_block.erl +++ b/lib/compiler/src/beam_block.erl @@ -159,14 +159,43 @@ find_fixpoint(OptFun, Is0) -> end. %% move_allocates(Is0) -> Is -%% Move allocate instructions upwards in the instruction stream, in the -%% hope of getting more possibilities for optimizing away moves later. +%% Move allocate instructions upwards in the instruction stream +%% (within the same block), in the hope of getting more possibilities +%% for optimizing away moves later. %% -%% NOTE: Moving allocation instructions is only safe because it is done -%% immediately after code generation so that we KNOW that if {x,X} is -%% initialized, all x registers with lower numbers are also initialized. -%% That assumption may not be true after other optimizations, such as -%% the beam_utils:live_opt/1 optimization. +%% For example, we can transform the following instructions: +%% +%% get_tuple_element x(1) Element => x(2) +%% allocate_zero StackSize 3 %% x(0), x(1), x(2) are live +%% +%% to the following instructions: +%% +%% allocate_zero StackSize 2 %% x(0) and x(1) are live +%% get_tuple_element x(1) Element => x(2) +%% +%% NOTE: Since the beam_reorder pass has been run, it is no longer +%% safe to assume that if x(N) is initialized, then all lower-numbered +%% x registers are also initialized. +%% +%% For example, in general it is not safe to transform the following +%% instructions: +%% +%% get_tuple_element x(0) Element => x(1) +%% allocate_zero StackSize 3 %x(0), x(1), x(2) are live +%% +%% to the following instructions: +%% +%% allocate_zero StackSize 3 +%% get_tuple_element x(0) Element => x(1) +%% +%% The transformation is safe if and only if x(1) has been +%% initialized previously. Unfortunately, beam_reorder may have moved +%% a get_tuple_element instruction so that x(1) is not always +%% initialized when this code is reached. To find whether or not x(1) +%% is initialized, we would need to analyze all code preceding these +%% two instructions (across branches). Since we currently don't have +%% any practical mechanism for doing that, we will have to +%% conservatively assume that the transformation is unsafe. move_allocates([{block,Bl0}|Is]) -> Bl = move_allocates_1(reverse(Bl0), []), @@ -175,27 +204,19 @@ move_allocates([I|Is]) -> [I|move_allocates(Is)]; move_allocates([]) -> []. -move_allocates_1([{set,[],[],{alloc,_,_}=Alloc}|Is0], Acc0) -> - {Is,Acc} = move_allocates_2(Alloc, Is0, Acc0), - move_allocates_1(Is, Acc); +move_allocates_1([I|Is], [{set,[],[],{alloc,Live0,Info}}|Acc]=Acc0) -> + case {alloc_may_pass(I),alloc_live_regs(I, Live0)} of + {false,_} -> + move_allocates_1(Is, [I|Acc0]); + {true,not_possible} -> + move_allocates_1(Is, [I|Acc0]); + {true,Live} when is_integer(Live) -> + A = {set,[],[],{alloc,Live,Info}}, + move_allocates_1(Is, [A,I|Acc]) + end; move_allocates_1([I|Is], Acc) -> move_allocates_1(Is, [I|Acc]); -move_allocates_1([], Is) -> Is. - -move_allocates_2({alloc,Live,Info}, [{set,[],[],{alloc,Live0,Info0}}|Is], Acc) -> - Live = Live0, % Assertion. - Alloc = {alloc,Live,combine_alloc(Info0, Info)}, - move_allocates_2(Alloc, Is, Acc); -move_allocates_2({alloc,Live,Info}=Alloc0, [I|Is]=Is0, Acc) -> - case alloc_may_pass(I) of - false -> - {Is0,[{set,[],[],Alloc0}|Acc]}; - true -> - Alloc = {alloc,alloc_live_regs(I, Live),Info}, - move_allocates_2(Alloc, Is, [I|Acc]) - end; -move_allocates_2(Alloc, [], Acc) -> - {[],[{set,[],[],Alloc}|Acc]}. +move_allocates_1([], Acc) -> Acc. alloc_may_pass({set,_,_,{alloc,_,_}}) -> false; alloc_may_pass({set,_,_,{set_tuple_element,_}}) -> false; @@ -204,9 +225,6 @@ alloc_may_pass({set,_,_,put_list}) -> false; alloc_may_pass({set,_,_,put}) -> false; alloc_may_pass({set,_,_,_}) -> true. -combine_alloc({_,Ns,Nh1,Init}, {_,nostack,Nh2,[]}) -> - {zero,Ns,beam_utils:combine_heap_needs(Nh1, Nh2),Init}. - %% opt([Instruction]) -> [Instruction] %% Optimize the instruction stream inside a basic block. @@ -393,10 +411,19 @@ eliminate_use_of_from_reg([I]=Is, From, _To, Acc) -> %% opt_alloc(Instructions) -> Instructions' %% Optimises all allocate instructions. +opt_alloc([{set,[],[],{alloc,Live0,Info0}}, + {set,[],[],{alloc,Live,Info}}|Is]) -> + Live = Live0, %Assertion. + Alloc = combine_alloc(Info0, Info), + I = {set,[],[],{alloc,Live,Alloc}}, + opt_alloc([I|Is]); opt_alloc([{set,[],[],{alloc,R,{_,Ns,Nh,[]}}}|Is]) -> [{set,[],[],opt_alloc(Is, Ns, Nh, R)}|Is]; opt_alloc([I|Is]) -> [I|opt_alloc(Is)]; opt_alloc([]) -> []. + +combine_alloc({_,Ns,Nh1,Init}, {_,nostack,Nh2,[]}) -> + {zero,Ns,beam_utils:combine_heap_needs(Nh1, Nh2),Init}. %% opt_alloc(Instructions, FrameSize, HeapNeed, LivingRegs) -> [Instr] %% Generates the optimal sequence of instructions for @@ -445,13 +472,14 @@ count_ones(Bits, Acc) -> alloc_live_regs({set,Ds,Ss,_}, Regs0) -> Rset = x_live(Ss, x_dead(Ds, (1 bsl Regs0)-1)), - live_regs(Rset). + live_regs(0, Rset). -live_regs(Regs) -> - live_regs_1(0, Regs). - -live_regs_1(N, 0) -> N; -live_regs_1(N, Regs) -> live_regs_1(N+1, Regs bsr 1). +live_regs(N, 0) -> + N; +live_regs(N, Regs) when Regs band 1 =:= 1 -> + live_regs(N+1, Regs bsr 1); +live_regs(_, _) -> + not_possible. x_dead([{x,N}|Rs], Regs) -> x_dead(Rs, Regs band (bnot (1 bsl N))); x_dead([_|Rs], Regs) -> x_dead(Rs, Regs); |