Fix rare bug in binary matching (again)

2e40d8d1c51a attempted fix a bug in binary matching, but it only
fixed the bug for the minimized test case.

This commit removes the previous fix and fixes the bug in a more
effective way. See the comments in the new code in `sys_core_bsm`
for an explanation.

This commit restores the optimizations in string.erl and dets_v9.erl
that the previous fix disabled.

I have not found any code where this commit will disable optimizations
when they are actually safe. There are some changes to the code
in ssl_cipher.erl in that some bs_start_match2 instruction did not
reuse the binary register for the match context, but the delayed
sub binary optimizations was never applied to the code in the first
place.

https://bugs.erlang.org/browse/ERL-689

1 files changed, 1 insertions, 12 deletions

diff --git a/lib/compiler/src/beam_bsm.erl b/lib/compiler/src/beam_bsm.erl
index abc6e96c85..1c8e0e9854 100644
--- a/lib/compiler/src/beam_bsm.erl
+++ b/lib/compiler/src/beam_bsm.erl
@@ -310,18 +310,7 @@ btb_reaches_match_2([{test,bs_start_match2,{f,F},Live,[Bin,_],Ctx}|Is],
     end;
 btb_reaches_match_2([{test,_,{f,F},Ss}=I|Is], Regs, D0) ->
     btb_ensure_not_used(Ss, I, Regs),
-    D1 = btb_follow_branch(F, Regs, D0),
-    D = case Is of
-            [{bs_context_to_binary,_}|_] ->
-                %% bs_context_to_binary following a test instruction
-                %% probably needs the current position to be saved as
-                %% the new start position, but we can't be sure.
-                %% Therefore, conservatively disable the optimization
-                %% (instead of forcing a saving of the position).
-                D1#btb{must_save=true,must_not_save=true};
-            _ ->
-                D1
-        end,
+    D = btb_follow_branch(F, Regs, D0),
     btb_reaches_match_1(Is, Regs, D);
 btb_reaches_match_2([{test,_,{f,F},_,Ss,_}=I|Is], Regs, D0) ->
     btb_ensure_not_used(Ss, I, Regs),