Improve binary matching of literals

The bs_match_string instruction is used to speed up matching of
binary literals. For example, given this source code:

foo1(<<1,2,3>>) -> ok.

The matching part of the code will look like:

    {test,bs_start_match2,{f,1},1,[{x,0},0],{x,0}}.
    {test,bs_match_string,{f,3},[{x,0},24,{string,[1,2,3]}]}.
    {test,bs_test_tail2,{f,3},[{x,0},0]}.

Nice. However, if we do a simple change to the source code:

foo2(<<1,2,3>>) -> ok;
foo2(<<>>) -> error.

the resulting matching code will look like (sligthly simplified):

    {test,bs_start_match2,{f,4},1,[{x,0},0],{x,0}}.
    {test,bs_get_integer2,{f,7},1,[{x,0},{integer,8},1,Flags],{x,1}}.
    {test,is_eq_exact,{f,8},[{x,1},{integer,1}]}.
    {test,bs_match_string,{f,6},[{x,0},16,{string,[2,3]}]}.
    {test,bs_test_tail2,{f,6},[{x,0},0]}.
    {move,{atom,ok},{x,0}}.
    return.
  {label,6}.
    {bs_restore2,{x,0},{atom,start}}.
  {label,7}.
    {test,bs_test_tail2,{f,8},[{x,0},0]}.

That is, matching of the first byte is not combined into the
bs_match_string instruction that follows.

Fix this problem by allowing a bs_match_string instruction to be
used if all clauses will match either the same integer literal or
the empty binary.

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index 5f12a98f09..cf6272a2ff 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -527,6 +527,8 @@ count_bits_matched([{test,_,_,_,[_,Sz,U,{field_flags,_}],_}|Is], SavePoint, Bits
 	{integer,N} -> count_bits_matched(Is, SavePoint, Bits+N*U);
 	_ -> count_bits_matched(Is, SavePoint, Bits)
     end;
+count_bits_matched([{test,bs_match_string,_,[_,Bits,_]}|Is], SavePoint, Bits0) ->
+    count_bits_matched(Is, SavePoint, Bits0+Bits);
 count_bits_matched([{test,_,_,_}|Is], SavePoint, Bits) ->
     count_bits_matched(Is, SavePoint, Bits);
 count_bits_matched([{bs_save2,Reg,SavePoint}|_], {Reg,SavePoint}, Bits) ->