sys_core_fold: Fix name capture problem

sys_core_fold could do unsafe transformations on the
code from the old inliner (invoked using the compiler
option `{inline,[{F/A}]}` to request inlining of specific
functions).

To explain the bug, let's first look at an example that
sys_core_fold handles correctly. Consider this code:

    'foo'/2 =
        fun (Arg1,Arg2) ->
          let <B> = Arg2
          in let <A,B> = <B,Arg1>
             in {A,B}

In this example, the lets can be completely eliminated,
since the arguments for the lets are variables (as opposed
to expressions). Since the variable B is rebound in the
inner let, `sys_core_fold` must take special care when
doing the substitutions.

Here is the correct result:

    'foo'/2 =
        fun (Arg1, Arg2) ->
          {Arg2,Arg1}

Consider a slight modifictation of the example:

    'bar'/2 =
        fun (Arg1,Arg2) ->
            let <B> = [Arg2]
            in let <A,B> = <B,[Arg1]>
               in {A,B}

Here some of the arguments for the lets are expressions, so
the lets must be kept. sys_core_fold does not handle this
example correctly:

    'bar'/2 =
        fun (Arg1,Arg2) ->
          let <B> = [Arg2]
    	  in let <B> = [Arg1]
    	     in {B,B}

In the inner let, the variable A has been eliminated and
replaced with the variable B in the body (the first B in
the tuple). Since the B in the outer let is never used,
the outer let will be eliminated, giving:

    'bar'/2 =
        fun (Arg1,Arg2) ->
    	  let <B> = [Arg1]
    	  in {B,B}

To handle this example correctly, sys_core_fold must
rename the variable B in the inner let like this to
avoid capturing B:

    'bar'/2 =
       fun (Arg1,Arg2) ->
         let <B> = [Arg2]
         in let <NewName> = [Arg1]
            in {B,NewName}

(Note: The `v3_kernel` pass alreday handles those examples correctly
in case `sys_core_fold` has been disabled.)

0 files changed, 0 insertions, 0 deletions