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authorBjörn Gustavsson <[email protected]>2016-10-06 06:01:11 +0200
committerBjörn Gustavsson <[email protected]>2016-10-06 07:10:37 +0200
commit67808ef46f8f429652ddebb81b0b5c3c603f8655 (patch)
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parent05be21eb0ad09fcdd63d955ed1d5b50ed34af925 (diff)
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beam_bsm: Eliminate unsafe optimization
The following code causes a compiler failure: first_after(Data, Offset) -> case byte_size(Data) > Offset of false -> {First, Rest} = {ok, ok}, ok; true -> <<_:Offset/binary, Rest/binary>> = Data, %% 'Rest' saved in y(0) before the call. {First, _} = match_first(Data, Rest), %% When beam_bsm sees the code, the following line %% which uses y(0) has been optimized away. {First, Rest} = {First, Rest}, First end. match_first(_, <<First:1/binary, Rest/binary>>) -> {First, Rest}. Here is the error message from beam_validator: t: function first_after/2+15: Internal consistency check failed - please report this bug. Instruction: {call,2,{f,7}} Error: {multiple_match_contexts,[{x,1},0]}: Basically, what happens is that at time of code generation, the variable 'Rest' is needed after the call to match_first/2 and is therefore saved in y(0). When beam_bsm (a late optimization pass) sees the code, the use of y(0) following the call to match_first/2 has been optimized away. beam_bsm therefore assumes that the delayed sub-binary creation is safe. (Actually, it is safe, but beam_validator does not realize it.) The bug was caused by two separate commits: e199e2471a reduced the number of special cases to handle in BEAM optimization passed by breaking apart the tail-recursive call instructions (call_only and call_last) into separate instructions. Unfortunately, the special handling for tail calls was lost, which resulted in worse code (i.e. the delayed sub-binary creation optimization could not be applied). e1aa422290 tried to compensate, but did so in a way that was not always safe. Teaching beam_validator that this kind of code is safe would be expensive. Instead, we will undo the damage caused by the two commits. Re-introduce the special handling of tail-recursive calls in beam_bsm that was lost in the first commit. (Effectively) revert the change in the second commit. ERL-268
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