[erlang-questions] naive execution of guards

[Ulf Wiger (TN/EAB)]

I was curious to see whether the compiler would optimize away repeated
use of the length/1 guard, e.g in the following code:

check_arity(_, L, Min, Max)
  when is_list(L), length(L) =< Min, length(L) >= Max ->
    true;

but compiling with erlc -S indicates that it doesn't:

{function, check_arity, 4, 8}.
  {label,7}.
    {func_info,{atom,test},{atom,check_arity},4}.
  {label,8}.
    {bif,is_list,{f,9},[{x,1}],{x,4}}.
    {gc_bif,length,{f,9},5,[{x,1}],{x,5}}.
    {bif,'=<',{f,9},[{x,5},{x,2}],{x,5}}.
    {gc_bif,length,{f,9},6,[{x,1}],{x,6}}.
    {bif,'>=',{f,9},[{x,6},{x,3}],{x,6}}.
    {bif,'and',{f,9},[{x,5},{x,6}],{x,5}}.
    {bif,'and',{f,9},[{x,4},{x,5}],{x,4}}.
    {test,is_eq_exact,{f,9},[{x,4},{atom,true}]}.
    {move,{atom,true},{x,0}}.
    return.

I guess there are two reasons for doing the optimization:
- one less register to handle
- length/1 is an O(N) bif

I'm sure it's tricky, and I don't know if the above reasons are strong
enough.

BR,
Ulf W
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