[erlang-questions] bit syntax slower than bit operations

Matthias Radestock matthias@REDACTED
Sat Sep 1 19:13:23 CEST 2007

I stumbled upon some unexpected performance characteristics involving 
bit syntax. After some experimenting, I came up with the following 
example to illustrate the problem:

Say I want to extract bits 2-5 of a single byte of binary data into a 
tuple, and also, conversely, construct a single byte of binary data from 
such a tuple:

decode_compact(<<_:2, B5:1, B4:1, B3:1, B2:1,_:2>>) ->
     {B2, B3, B4, B5}.

encode_compact({B2, B3, B4, B5}) ->
     <<0:2, B5:1, B4:1, B3:1, B2:1, 0:2>>.

Code doesn't get much more compact than that.

Now compare this against the following:

decode_verbose(<<B:8>>) ->
     {(B band 4) bsr 2, (B band 8) bsr 3, (B band 16) bsr 4, (B band 32) 
bsr 5}.

encode_verbose({B2, B3, B4, B5}) ->
     <<((B2 bsl 2) bor (B3 bsl 3) bor (B4 bsl 4) bor (B5 bsl 5)):8>>.

Same result, but far more verbose.

Unfortunately, my tests indicate that the latter version is about twice 
as fast, using R11B-5 under Linux on x86.

Does the bit-syntax compilation produce rather inefficient code?


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