From f4c7a59d96b110f79cc0fcc16286ece770a166a3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 19 Feb 2013 15:00:23 +0100
Subject: asn1_constructed_per: Optimize decoding of CHOICE

Optimize the decoding of CHOICE. Most important is to inline decoding
of the extension bit (if present) and decoding of the choice index
to give the BEAM compiler more opportunities for optimization.

We will also change the structure of the generated code. The current
code uses a flattened case for both the root and extension alternatives:

  case Choice + NumRootChoices * Ext of
     %% Root alternatives.
     0 - ...;
     :
     LastRootAlternative -> ...;
     %% Extension alternatives.
     LastRootAlternative+1 -> ...;
     :
     %% Unknown extension.
     _ -> ...;
  end

We will instead generate nested cases:

  case Ext of
    0 ->
      case Choice of
        %% Root alternatives.
        0 - ...;
        :
        LastRootAlternative -> ...
       end;
    1 ->
       %% Extension alternatives.
       <Decode the open type here>
       case Choice of
         0 -> ...;
         :
	 LastExtensionAlternative -> ...;
         %% Unknown extension.
         _ -> ...;
       end
    end

Nested cases should be slightly faster. For decoding of the extensions,
it also makes it possible to hoist the decoding of the open type up
from each case to before the case switching on the extension index,
thus reducing the size of the generated code.

We will also do another change to the structure. Currently, the
big flat clase is wrapped in code that repackages the return values:

  {Alt,{Value,RemainingEncodedData}} =
     case Choice + NumRootChoices * Ext of
       :
     end,
  {{Value,Alt},RemainingEncodedData}.

We still need to do the repackaging, but we can push it down to
the case arm for decoding each alternative. In many cases, that
will give the BEAM compiler the opportunity to avoid building the
temporary tuples.
---
 lib/asn1/src/asn1rtt_uper.erl | 46 +------------------------------------------
 1 file changed, 1 insertion(+), 45 deletions(-)

(limited to 'lib/asn1/src/asn1rtt_uper.erl')

diff --git a/lib/asn1/src/asn1rtt_uper.erl b/lib/asn1/src/asn1rtt_uper.erl
index 8efe9a7b0f..e55535860f 100644
--- a/lib/asn1/src/asn1rtt_uper.erl
+++ b/lib/asn1/src/asn1rtt_uper.erl
@@ -21,7 +21,7 @@
 
 -export([setext/1, fixoptionals/3,
 	 fixextensions/2,
-	 skipextensions/3, getbit/1, getchoice/3 ]).
+	 skipextensions/3]).
 -export([set_choice/3, encode_integer/2, encode_integer/3]).
 -export([encode_small_number/1, encode_constrained_number/2,
 	 encode_boolean/1,
@@ -123,29 +123,6 @@ skipextensions(Bytes0, Nr, ExtensionBitstr) when is_bitstring(ExtensionBitstr) -
     end.
 
 
-getchoice(Bytes,1,0) -> % only 1 alternative is not encoded
-    {0,Bytes};
-getchoice(Bytes,_,1) ->
-    decode_small_number(Bytes);
-getchoice(Bytes,NumChoices,0) ->
-    decode_constrained_number(Bytes,{0,NumChoices-1}).
-
-
-getbit(Buffer) ->
-    <<B:1,Rest/bitstring>> = Buffer,
-    {B,Rest}.
-
-getbits(Buffer, Num) when is_bitstring(Buffer) ->
-    <<Bs:Num,Rest/bitstring>> = Buffer,
-    {Bs,Rest}.
-
-
-%% Pick the first Num octets.
-%% Returns octets as an integer with bit significance as in buffer.
-getoctets(Buffer, Num) when is_bitstring(Buffer) ->
-    <<Val:Num/integer-unit:8,RestBitStr/bitstring>> = Buffer,
-    {Val,RestBitStr}.
-
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %% set_choice(Alt,Choices,Altnum) -> ListofBitSettings
 %% Alt = atom()
@@ -265,15 +242,6 @@ encode_small_number(Val) when Val < 64 ->
 encode_small_number(Val) ->
     [<<1:1>>|encode_semi_constrained_number(0, Val)].
 
-decode_small_number(Bytes) ->
-    {Bit,Bytes2} = getbit(Bytes),
-    case Bit of
-	0 ->
-	    getbits(Bytes2,6);
-	1 ->
-	    decode_semi_constrained_number(Bytes2)
-    end.
-
 %% X.691:10.7 Encoding of a semi-constrained whole number
 encode_semi_constrained_number(Lb, Val) ->
     %% encoding in minimum number of octets preceeded by a length
@@ -289,11 +257,6 @@ encode_semi_constrained_number(Lb, Val) ->
 	    [encode_length(Size),Bin]
     end.
 
-decode_semi_constrained_number(Bytes) ->
-    {Len,Bytes2} = decode_length(Bytes),
-    {V,Bytes3} = getoctets(Bytes2,Len),
-    {V,Bytes3}.
-
 encode_constrained_number({Lb,Ub}, Val) when Val >= Lb, Ub >= Val ->
     Range = Ub - Lb + 1,
     Val2 = Val - Lb,
@@ -302,13 +265,6 @@ encode_constrained_number({Lb,Ub}, Val) when Val >= Lb, Ub >= Val ->
 encode_constrained_number(Range,Val) ->
     exit({error,{asn1,{integer_range,Range,value,Val}}}).
 
-
-decode_constrained_number(Buffer, {Lb,Ub}) ->
-    Range = Ub - Lb + 1,
-    NumBits = num_bits(Range),
-    {Val,Remain} = getbits(Buffer,NumBits),
-    {Val+Lb,Remain}.
-
 %% X.691:10.8 Encoding of an unconstrained whole number
 
 encode_unconstrained_number(Val) when Val >= 0 ->
-- 
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