aboutsummaryrefslogtreecommitdiffstats
path: root/lib/compiler/src/beam_except.erl
AgeCommit message (Collapse)Author
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.