[erlang-bugs] Compiler bug in sys_core_fold.erl

[Bjorn Gustavsson]

2008/10/8 Robert Virding <rvirding@REDACTED>

> I am running R12B-4 and have generated code with Lisp Flavoured Erlang.
>
> There is a bug in eval_element/3 which is called from fold_call/4 which
> tries to safely evaluate calls at compile time. Eval_element tries to
> evaluate calls to element/2. It also tries to check if the call is
> guaranteed to fail in which case it replaces the original call with a call
> to erlang:error(badarg). It is this check which is causing the bug as it is
> so tuned to code generated by the vanilla erlang compiler that it does not
> accept a nested call in the element. This is the bug.
>
> I would also argue that such a substitution has very little noticeable
> effect as any evaluation of the arguments to element/2 have been broken out
> and will already be evaluated before the code ever reaches the erlang:error
> call. It, in fact, saves very little.
>

The optimization in itself may not be worthwhile (because the code being
optimized is probably wrong), but there will also be a warning about a call
to element/2 that will always fail. The warning could be useful.

I have updated eval_element/2 to be more careful.


-- 
Björn Gustavsson, Erlang/OTP, Ericsson AB
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