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2019-02-21sys_core_fold: Remove an unsafe optimizationBjörn Gustavsson
`sys_core_fold` has an optimization of repeated pattern matching. For example, when a record is matched the first time, the pattern is remembered. When the same record is matched again, the matching does not need to be repeated, but the variables bound in the first matching can be re-used. It turns out that that there is a name capture problem when the old inliner is used. The old inliner is used when explicitly inling certain functions, and by the compiler test suites for testing the compiler. The name capture problem could be eliminated by more aggressive variable renaming when inlining. But, fortunately, given the new SSA passes, this optimization is no longer as essential as it used to be. Removing the optimization turns out to be mostly benefical, leading to a smaller stack frame in many cases. Also remove the optimizations of `element/2`, `is_record/3`, and `setelement/3` from `sys_core_fold`. Because matched patterns are no longer remembered, those optimizations can very rarely be applied any more. (Those same optimizations are already done in `beam_ssa_type`.)
2019-01-31Fix internal consistency failure caused by beam_exceptBjörn Gustavsson
Fix a bug where the number of live registers in a `bs_get_tail` instruction was too low. Consider this example: -export([bs_get_tail/2]). bs_get_tail(Bin, Config) -> bs_get_tail_1(Bin, 0, 0, Config). bs_get_tail_1(<<_:32, Rest/binary>>, Z1, Z2, F1) -> {Rest,Z1,Z2,F1}. `beam_validator` would emit the following diagnostics: t: function bs_get_tail_1/4+2: Internal consistency check failed - please report this bug. Instruction: {func_info,{atom,t},{atom,bs_get_tail_1},4} Error: {uninitialized_reg,{x,3}}: Here is the part of the code that generates the `function_clause` exception before the optimization: {test_heap,6,4}. {put_list,{x,3},nil,{x,2}}. {put_list,{integer,0},{x,2},{x,2}}. {put_list,{integer,0},{x,2},{x,2}}. {bs_set_position,{x,1},{x,0}}. {bs_get_tail,{x,1},{x,0},3}. %3 live registers. {test_heap,2,3}. {put_list,{x,0},{x,2},{x,1}}. {move,{atom,function_clause},{x,0}}. {line,[{location,"t.erl",8}]}. {call_ext_only,2,{extfunc,erlang,error,2}}. The `bs_get_tail` instruction expects that 3 registers will be live at this point. `beam_except` rewrites the code like this: {bs_set_position,{x,1},{x,0}}. {bs_get_tail,{x,1},{x,0},3}. %Still 3. Too low. {move,{integer,0},{x,1}}. {move,{integer,0},{x,2}}. {jump,{f,3}}. Now the number of live registers in `bs_get_tail` is too low, because the `{x,3}` register will become undefined. This commit adds code to update the number of live registers in the `bs_get_tail` instruction, producing this code: {bs_set_position,{x,1},{x,0}}. {bs_get_tail,{x,1},{x,0},4}. %Adjusted to 4. {move,{integer,0},{x,1}}. {move,{integer,0},{x,2}}. {jump,{f,3}}.
2019-01-29Enhance optimization of function_clause exceptionsBjörn Gustavsson
There is an optimization for reducing the number of instructions needed to generate a `function_clause`. After the latest improvements of the type optimization pass, that optimization is not always applied. Here is an example: -export([foo/3]). foo(X, Y, Z) -> bar(a, X, Y, Z). bar(a, X, Y, Z) when is_tuple(X) -> {X,Y,Z}. Note that the compiler internally adds a clause to each function to generate a `function_clause` exception. Thus: bar(a, X, Y, Z) when is_tuple(X) -> {X,Y,Z}; bar(A1, A2, A3, A4) -> erlang:error(function_clause, [A1,A2,A3,A4]). Optimizations will rewrite the code basically like this: bar(_, X, Y, Z) when is_tuple(X) -> {X,Y,Z}; bar(_, A2, A3, A4) -> erlang:error(function_clause, [a,A2,A3,A4]). Note the `a` as the first element of the list of arguments. It will prevent the optimization of the `function_clause` exception. The BEAM code for `bar/4` looks like this: {function, bar, 4, 4}. {label,3}. {line,[{location,"t.erl",8}]}. {func_info,{atom,t},{atom,bar},4}. {label,4}. {'%',{type_info,{x,0},{atom,a}}}. {test,is_tuple,{f,5},[{x,1}]}. {test_heap,4,4}. {put_tuple2,{x,0},{list,[{x,1},{x,2},{x,3}]}}. return. {label,5}. {test_heap,8,4}. {put_list,{x,3},nil,{x,0}}. {put_list,{x,2},{x,0},{x,0}}. {put_list,{x,1},{x,0},{x,0}}. {put_list,{atom,a},{x,0},{x,1}}. {move,{atom,function_clause},{x,0}}. {line,[{location,"t.erl",8}]}. {call_ext,2,{extfunc,erlang,error,2}}. The code after label 5 is the clause that generates the `function_clause` exception. This commit generalizes the optimization so that it can be applied for this function: {function, bar, 4, 4}. {label,3}. {line,[{location,"t.erl",8}]}. {func_info,{atom,t},{atom,bar},4}. {label,4}. {'%',{type_info,{x,0},{atom,a}}}. {test,is_tuple,{f,5},[{x,1}]}. {test_heap,4,4}. {put_tuple2,{x,0},{list,[{x,1},{x,2},{x,3}]}}. return. {label,5}. {move,{atom,a},{x,0}}. {jump,{f,3}}. For this particular function, it would be safe to omit the `move` instruction before the `{jump,{f,3}}` instruction, but it would not be safe in general to omit `move` instructions.
2019-01-28beam_except: Eliminate unsafe function_clause translationBjörn Gustavsson
The translation from `error(function_clause, Args)` to a jump to the `func_info` label is not safe if there is a stack frame.
2018-10-30beam_except: Generalize translation to func_info instructionsBjörn Gustavsson
The `beam_except` pass replaces some calls to `erlang:error/1` or `erlang:error/2` with specialized instructions in order to reduce the size of the BEAM code. In functions that do binary matching, `beam_except` would fail to translate the instructions that generate a `function_clause` exception. Here is an example: bsum(<<H:16,T/binary>>, Acc) -> bsum(T, Acc+H); bsum(<<>>, Acc) -> Acc. The BEAM code that generates the `function_clause` exception looks like this: {label,4}. {test_heap,2,3}. {put_list,{x,1},nil,{x,1}}. %% The following two instructions prevents the translation. {bs_set_position,{x,2},{x,0}}. {bs_get_tail,{x,2},{x,0},3}. {test_heap,2,2}. {put_list,{x,0},{x,1},{x,1}}. {move,{atom,function_clause},{x,0}}. {line,...}. {call_ext,2,{extfunc,erlang,error,2}}. Make the translation of `function_clause` exceptions smarter, so that the following code will be produced: {label,4}. {bs_set_position,{x,2},{x,0}}. {bs_get_tail,{x,2},{x,0},3}. {jump,{f,1}}. %Jump to `func_info` instruction. (This issue was noticed when looking at the code generated by https://github.com/tomas-abrahamsson/gpb.)
2018-09-03Introduce a put_tuple2 instructionBjörn Gustavsson
Sometimes when building a tuple, there is no way to avoid an extra `move` instruction. Consider this code: make_tuple(A) -> {ok,A}. The corresponding BEAM code looks like this: {test_heap,3,1}. {put_tuple,2,{x,1}}. {put,{atom,ok}}. {put,{x,0}}. {move,{x,1},{x,0}}. return. To avoid overwriting the source register `{x,0}`, a `move` instruction is necessary. The problem doesn't exist when building a list: %% build_list(A) -> [A]. {test_heap,2,1}. {put_list,{x,0},nil,{x,0}}. return. Introduce a new `put_tuple2` instruction that builds a tuple in a single instruction, so that the `move` instruction can be eliminated: %% make_tuple(A) -> {ok,A}. {test_heap,3,1}. {put_tuple2,{x,0},{list,[{atom,ok},{x,0}]}}. return. Note that the BEAM loader already combines `put_tuple` and `put` instructions into an internal instruction similar to `put_tuple2`. Therefore the introduction of the new instruction will not speed up execution of tuple building itself, but it will be less work for the loader to load the new instruction.
2018-08-17beam_except: Enhance recognition of function_clause exceptionsBjörn Gustavsson
Don't match exact BEAM instructions when trying to recognize function_clause exceptions that should be replaced with a jump to the func_info instruction. Instead, do a symbolic evaluation of the list building code and see if the result is a list of the argument registers. While at it, also teach fix_block_1/2 to completely remove a test_heap instruction before an exception generation instruction.
2018-06-18Update copyright yearHenrik Nord
2017-12-08Use the new syntax for retrieving stack tracesBjörn Gustavsson
2017-01-12beam_except: Add types and specsBjörn Gustavsson
2016-05-25beam_expect: Correctly handle blocks with multiple allocsBjörn Gustavsson
A negative allocation could be calculated if a block had multiple allocations. Make sure to process the block in the right order so that the correct allocation is processed. Also add an assertion. This bug was often not noticed because beam_type usually silently recalculates the allocation amount in test_heap/2 instructions.
2016-03-15update copyright-yearHenrik Nord
2015-06-18Change license text to APLv2Bruce Yinhe
2014-01-24beam_except: Eliminate compiler crashBjörn Gustavsson
Code such as: bar(X) -> case {X+1} of 1 -> ok end. would crash the beam_except pass of the compiler. The reason for the crash is that the '+' operator would add a line/1 instruction that the beam_except pass was not prepared to handle. Reported-by: Erik Søe Sørensen
2013-02-09Fix unsafe optimization of funsBjörn Gustavsson
Commits 53bd4974a101 and 726f6e4c7afe simplified the handling of match_fail (used to generated exceptions such as 'function_clause') by first rewriting them to a call to erlang/error{1,2} and later rewriting them to specialized BEAM instructions (to reduce the code size). There was one flaw, though, which only was exposed when more aggressive optimizations were added in c3b60f86c622. Here is an example to explain it: t(V) -> fun(get) -> V end. The following BEAM code will be initially generated for the fun: {function, '-t/1-fun-0-', 2, 5}. {label,1}. {line,[{location,"t.erl",5}]}. {func_info,{atom,t},{atom,'-t/1-fun-0-'},2}. {label,2}. {test,is_eq_exact,{f,2},[{x,0},{atom,get}]}. {move,{x,1},{x,0}}. return. {label,2}. {test_heap,2,1}. {put_list,{x,0},nil,{x,1}}. {move,{atom,function_clause},{x,0}}. {line,[{location,"t.erl",5}]}. {call_ext_only,2,{extfunc,erlang,error,2}}. Translating back to Erlang code, that would be roughly: '-t/1-fun-0-'(get, V) -> V; '-t/1-fun-0-'(Arg1, _) -> erlang:error(function_clause, [Arg1]). Note that the second argument (the free variable V) is not included in the call to erlang:error/2. The beam_except pass will simplify the code to: {function, '-t/1-fun-0-', 2, 8}. {label,1}. {line,[{location,"t.erl",5}]}. {func_info,{atom,t},{atom,'-t/1-fun-0-'},2}. {label,2}. {test,is_eq_exact,{f,1},[{x,0},{atom,get}]}. {move,{x,1},{x,0}}. return. The code has been shortened by jumping to the func_info/3 instruction. Translating back to Erlang: '-t/1-fun-0-'(get, V) -> V; '-t/1-fun-0-'(Arg1, Arg2) -> erlang:error(function_clause, [Arg1,Arg2]). it is clear that both arguments are now included in the 'function_clause' exception, even though the initially generated code only included the first argument. That is no problem in this particular case, but for some more complex funs, optimizing the first version based on variable usage could make the second version unsafe. I rejected the following potential solutions: - Including the free arguments in the call to erlang:error/2: '-t/1-fun-0-'(get, V) -> V; '-t/1-fun-0-'(Arg1, Arg2) -> erlang:error(function_clause, [Arg1,Arg2]). Unfortunately, that is tricky. The free variables are only known after the second pass in v3_kernel when variable usage has been calculated. We would need to add a third pass (only for funs) that would the free arguments to the second argument for erlang:error/2 *and* update the variable usage information. - Calling beam_except earlier, from within beam_block before any optimizations based on variable usages are done. But means that the problem could reappear in some other form in the future when other updates are done to the code generator and/or optimization passes. The solution I have chosen is to modify beam_except to only replace a call to erlang:error(function_class, Args) if the length of Args is the same as the arity in the func_info/3 instruction. The code will be slightly larger. Also, the free variables for funs and list comprehensions will no longer be included in the function_clause exception (that could be less confusing, but it also means less information during debugging).
2013-01-25Update copyright yearsBjörn-Egil Dahlberg
2012-10-10Break apart tail-recursive call instructionsBjörn Gustavsson
Somewhat reduce the code bloat by eliminating special cases.
2012-01-04Add the beam_except pass to optimize exceptionsBjörn Gustavsson
In order to save space, rewrite suitable calls to erlang:error/{1,2} to special BEAM instructions. This code is probably longer than the code taken out of v3_life and v3_codegen in the previous commit, but it is much easier to understand and maintain since the BEAM assembler format is better understood than the v3_life format.