beam_bsm: Improve handling of bs_start_match2 instructions

The handling of bs_start_match2 was both too conservative and too
careless. It was too conservative in that would not do the
optimization if the were copies of the match state in other
registers. It was careless in that it did not consider the
failure branch.

Reorganize the code and fix both these issues. Add a test case
to test that the failure branch is considered.

1 files changed, 28 insertions, 2 deletions

diff --git a/lib/compiler/test/bs_match_SUITE.erl b/lib/compiler/test/bs_match_SUITE.erl
index e8f5c55c1a..d63d2235d7 100644
--- a/lib/compiler/test/bs_match_SUITE.erl
+++ b/lib/compiler/test/bs_match_SUITE.erl
@@ -33,7 +33,7 @@
 	 matching_meets_construction/1,simon/1,matching_and_andalso/1,
 	 otp_7188/1,otp_7233/1,otp_7240/1,otp_7498/1,
 	 match_string/1,zero_width/1,bad_size/1,haystack/1,
-	 cover_beam_bool/1,matched_out_size/1]).
+	 cover_beam_bool/1,matched_out_size/1,follow_fail_branch/1]).
 
 -export([coverage_id/1,coverage_external_ignore/2]).
 
@@ -57,7 +57,7 @@ groups() ->
        matching_meets_construction,simon,
        matching_and_andalso,otp_7188,otp_7233,otp_7240,
        otp_7498,match_string,zero_width,bad_size,haystack,
-       cover_beam_bool,matched_out_size]}].
+       cover_beam_bool,matched_out_size,follow_fail_branch]}].
 
 
 init_per_suite(Config) ->
@@ -1108,6 +1108,32 @@ mos_bin(<<L,Bin:L/binary,"abcdefghij">>) ->
     L = byte_size(Bin),
     Bin.
 
+follow_fail_branch(_) ->
+    42 = ffb_1(<<0,1>>, <<0>>),
+    8 = ffb_1(<<0,1>>, [a]),
+    42 = ffb_2(<<0,1>>, <<0>>, 17),
+    8 = ffb_2(<<0,1>>, [a], 0),
+    ok.
+
+ffb_1(<<_,T/bitstring>>, List) ->
+    case List of
+	<<_>> ->
+	    42;
+	[_|_] ->
+	    %% The fail branch of the bs_start_match2 instruction
+	    %% pointing to here would be ignored, making the compiler
+	    %% incorrectly assume that the delayed sub-binary
+	    %% optimization was safe.
+	    bit_size(T)
+    end.
+
+ffb_2(<<_,T/bitstring>>, List, A) ->
+    case List of
+	<<_>> when A =:= 17 -> 42;
+	[_|_] -> bit_size(T)
+    end.
+
+
 check(F, R) ->
     R = F().