beam_bool: Fix generation of code that does not validate

The following code (by Simon Cornish)

bad(XDo1, XDo2, Do3) ->
    Do1    = (XDo1 =/= []),
    Do2    = (XDo2 =/= []),
    CH1 = if Do1 == true;
	     Do1 == false,Do2==false,Do3 == blah ->
		  ch1;
	     true ->
		  no
	  end,
    CH2 = if Do1 == true;
	     Do1 == false,Do2==false,Do3 == xx ->
		  ch2;
	     true ->
		  no
	  end,
    {CH1,CH2}.

is optimized by beam_bool even though the optimization is not
safe. The trouble is that an assignment to {y,0} no longer
occurs on all paths leading to its use.

The bug is in dst_regs/2 which is supposed to return a set
of all registers assigned in a code block, but it ignores
registers assigned in 'move' instructions.

Fix the bug by taking 'move' instructions into account. This change
is safe since it can only cause more registers to be added
to the MustBeKilled and MustBeUnused sets in ensure_opt_safe/6,
which means that it can only cause the optimization to be turned
off for code that used to be optimized.

1 files changed, 7 insertions, 5 deletions

diff --git a/lib/compiler/src/beam_bool.erl b/lib/compiler/src/beam_bool.erl
index ffe5cdb501..dcc6ad4c7c 100644
--- a/lib/compiler/src/beam_bool.erl
+++ b/lib/compiler/src/beam_bool.erl
@@ -173,7 +173,7 @@ bopt_block(Reg, Fail, OldIs, [{block,Bl0}|Acc0], St0) ->
 %%  whether the optimized code is guaranteed to work in the same
 %%  way as the original code.
 %%
-%%  Throws an exception if the optmization is not safe.
+%%  Throw an exception if the optimization is not safe.
 %%
 ensure_opt_safe(Bl, NewCode, OldIs, Fail, PreceedingCode, St) ->
     %% Here are the conditions that must be true for the
@@ -190,10 +190,10 @@ ensure_opt_safe(Bl, NewCode, OldIs, Fail, PreceedingCode, St) ->
     %%    by the code that follows.
     %%
     %% 3. Any register that is assigned a value in the optimized
-    %%    code must be UNUSED or KILLED in the following code.
-    %%    (Possible future improvement: Registers that are known
-    %%    to be assigned the SAME value in the original and optimized
-    %%    code don't need to be unused in the following code.)
+    %%    code must be UNUSED or KILLED in the following code
+    %%    (because the register might be assigned the wrong value,
+    %%    and even if the value is right it might no longer be
+    %%    assigned on *all* paths leading to its use).
 
     InitInPreceeding = initialized_regs(PreceedingCode),
 
@@ -310,6 +310,8 @@ dst_regs([{set,[D],_,{bif,_,{f,_}}}|Is], Acc) ->
     dst_regs(Is, [D|Acc]);
 dst_regs([{set,[D],_,{alloc,_,{gc_bif,_,{f,_}}}}|Is], Acc) ->
     dst_regs(Is, [D|Acc]);
+dst_regs([{set,[D],_,move}|Is], Acc) ->
+    dst_regs(Is, [D|Acc]);
 dst_regs([_|Is], Acc) ->
     dst_regs(Is, Acc);
 dst_regs([], Acc) -> ordsets:from_list(Acc).