Fix internal consistency failure caused by beam_except

Fix a bug where the number of live registers in a `bs_get_tail`
instruction was too low.

Consider this example:

    -export([bs_get_tail/2]).
    bs_get_tail(Bin, Config) ->
        bs_get_tail_1(Bin, 0, 0, Config).

    bs_get_tail_1(<<_:32, Rest/binary>>, Z1, Z2, F1) ->
        {Rest,Z1,Z2,F1}.

`beam_validator` would emit the following diagnostics:

    t: function bs_get_tail_1/4+2:
      Internal consistency check failed - please report this bug.
      Instruction: {func_info,{atom,t},{atom,bs_get_tail_1},4}
      Error:       {uninitialized_reg,{x,3}}:

Here is the part of the code that generates the `function_clause`
exception before the optimization:

    {test_heap,6,4}.
    {put_list,{x,3},nil,{x,2}}.
    {put_list,{integer,0},{x,2},{x,2}}.
    {put_list,{integer,0},{x,2},{x,2}}.
    {bs_set_position,{x,1},{x,0}}.
    {bs_get_tail,{x,1},{x,0},3}.     %3 live registers.
    {test_heap,2,3}.
    {put_list,{x,0},{x,2},{x,1}}.
    {move,{atom,function_clause},{x,0}}.
    {line,[{location,"t.erl",8}]}.
    {call_ext_only,2,{extfunc,erlang,error,2}}.

The `bs_get_tail` instruction expects that 3 registers will be live
at this point. `beam_except` rewrites the code like this:

    {bs_set_position,{x,1},{x,0}}.
    {bs_get_tail,{x,1},{x,0},3}.    %Still 3. Too low.
    {move,{integer,0},{x,1}}.
    {move,{integer,0},{x,2}}.
    {jump,{f,3}}.

Now the number of live registers in `bs_get_tail` is too low,
because the `{x,3}` register will become undefined.

This commit adds code to update the number of live registers
in the `bs_get_tail` instruction, producing this code:

    {bs_set_position,{x,1},{x,0}}.
    {bs_get_tail,{x,1},{x,0},4}.    %Adjusted to 4.
    {move,{integer,0},{x,1}}.
    {move,{integer,0},{x,2}}.
    {jump,{f,3}}.

1 files changed, 21 insertions, 2 deletions

diff --git a/lib/compiler/src/beam_except.erl b/lib/compiler/src/beam_except.erl
index 254a716b81..09925b2872 100644
--- a/lib/compiler/src/beam_except.erl
+++ b/lib/compiler/src/beam_except.erl
@@ -167,8 +167,10 @@ dig_out_fc_1([{block,Bl}|Is], Regs0, Acc) ->
     dig_out_fc_1(Is, Regs, Acc);
 dig_out_fc_1([{bs_set_position,_,_}=I|Is], Regs, Acc) ->
     dig_out_fc_1(Is, Regs, [I|Acc]);
-dig_out_fc_1([{bs_get_tail,_,_,Live}=I|Is], Regs0, Acc) ->
-    Regs = prune_xregs(Live, Regs0),
+dig_out_fc_1([{bs_get_tail,Src,Dst,Live0}|Is], Regs0, Acc) ->
+    Regs = prune_xregs(Live0, Regs0),
+    Live = dig_out_stack_live(Regs, Live0),
+    I = {bs_get_tail,Src,Dst,Live},
     dig_out_fc_1(Is, Regs, [I|Acc]);
 dig_out_fc_1([_|_], _Regs, _Acc) ->
     {#{},[]};
@@ -189,6 +191,23 @@ dig_out_fc_block([{set,_,_,_}|_], _Regs) ->
     #{};
 dig_out_fc_block([], Regs) -> Regs.
 
+dig_out_stack_live(Regs, Default) ->
+    Reg = {x,2},
+    case Regs of
+        #{Reg:=List} ->
+            dig_out_stack_live_1(List, Default);
+        #{} ->
+            Default
+    end.
+
+dig_out_stack_live_1({cons,{arg,N},T}, Live) ->
+    dig_out_stack_live_1(T, max(N + 1, Live));
+dig_out_stack_live_1({cons,_,T}, Live) ->
+    dig_out_stack_live_1(T, Live);
+dig_out_stack_live_1(nil, Live) ->
+    Live;
+dig_out_stack_live_1(_, Live) -> Live.
+
 prune_xregs(Live, Regs) ->
     maps:filter(fun({x,X}, _) -> X < Live end, Regs).