--- beam_bsm.erl.orig	2008-08-21 12:22:09.945529616 -0400
+++ beam_bsm.erl	2008-08-21 12:22:09.917526638 -0400
@@ -63,6 +63,7 @@
 	{f,					%Gbtrees for all functions.
 	 index,					%{Label,Code} index (for liveness).
 	 ok_br,					%Labels that are OK.
+	 must_not_save,				%Must not save position when optimizing (reaches bs_context_to_binary)
 	 need_save				%Must save position when optmizing.
 	}).
 
@@ -158,9 +159,12 @@
 btb_reaches_match(Is, RegList, D0) ->
     try
 	Regs = btb_regs_from_list(RegList),
-	D = D0#btb{ok_br=gb_sets:empty(),need_save=false},
-	#btb{need_save=NeedSave} = btb_reaches_match_1(Is, Regs, D),
-	{ok,NeedSave}
+	D = D0#btb{ok_br=gb_sets:empty(),must_not_save=false,need_save=false},
+	#btb{must_not_save=MustNotSave,need_save=NeedSave} = btb_reaches_match_1(Is, Regs, D),
+	case MustNotSave and NeedSave of
+	    true -> btb_error(must_and_must_not_save);
+	    _    -> {ok,NeedSave}
+	end
     catch
 	throw:{error,_}=Error -> Error
     end.
@@ -313,7 +317,7 @@
 	    CtxRegs = btb_context_regs(Regs),
 	    case btb_are_all_killed(CtxRegs, Is, D) of
 		false -> btb_error({CtxRegs,not_all_killed_after,I});
-		true -> D
+		true -> D#btb{must_not_save=true}
 	    end
     end;
 btb_reaches_match_2([{badmatch,Src}=I|_], Regs, D) ->
@@ -400,10 +404,10 @@
 	false ->
 	    %% New branch. Try it.
 	    Is = fetch_code_at(Lbl, Li),
-	    #btb{ok_br=Br} = btb_reaches_match_1(Is, Regs, D),
+	    #btb{ok_br=Br,must_not_save=MustNotSave,need_save=NeedSave} = btb_reaches_match_1(Is, Regs, D),
 
 	    %% Since we got back, this branch is OK.
-	    D#btb{ok_br=gb_sets:insert(Lbl, Br)}
+	    D#btb{ok_br=gb_sets:insert(Lbl, Br),must_not_save=MustNotSave,need_save=NeedSave}
     end.
 
 btb_reaches_match_block([{set,Ds,Ss,{alloc,Live,_}}=I|Is], Regs0) ->
@@ -526,7 +530,7 @@
 btb_context_regs_1(Regs, N, Tag, Acc) ->
     btb_context_regs_1(Regs bsr 1, N+1, Tag, Acc).
 
-%% btb_index([Function]) -> GbTree({EntryLabel,{Register,NumberOfSlots}})
+%% btb_index([Function]) -> GbTree({EntryLabel,{Register,NumberOfSlots,MustNotSave,NeedsSave}})
 %%  Build an index of functions that accept a match context instead of
 %%  a binary.