Speed up beam_ssa_dead when there are many sequential blocks

beam_ssa_dead could be very slow if there were many blocks
connected with unconditional branches (for example, if a block
had contained many `call` instructions and been split by
ssa_opt_split_blocks).

It turns out that `comb_get_sw/3` does an unnecessary (and perhaps
incorrect) recursive call to itself when the terminator for the
block is an unconditional branch. Removing the recursive call does
not disable any optimizations, but will be much faster if there
are many blocks connected with unconditional branches.

Reported-by: Michał Muskała

1 files changed, 3 insertions, 3 deletions

diff --git a/lib/compiler/src/beam_ssa_dead.erl b/lib/compiler/src/beam_ssa_dead.erl
index 09b29025c6..db82aaa8e6 100644
--- a/lib/compiler/src/beam_ssa_dead.erl
+++ b/lib/compiler/src/beam_ssa_dead.erl
@@ -850,7 +850,7 @@ combine_eqs_1([], St) -> St.
 comb_get_sw(L, Blocks) ->
     comb_get_sw(L, true, Blocks).
 
-comb_get_sw(L, Safe0, #st{bs=Blocks,skippable=Skippable}=St) ->
+comb_get_sw(L, Safe0, #st{bs=Blocks,skippable=Skippable}) ->
     #b_blk{is=Is,last=Last} = maps:get(L, Blocks),
     Safe1 = Safe0 andalso is_map_key(L, Skippable),
     case Last of
@@ -865,8 +865,8 @@ comb_get_sw(L, Safe0, #st{bs=Blocks,skippable=Skippable}=St) ->
                 {#b_set{},_} ->
                     none
             end;
-        #b_br{bool=#b_literal{val=true},succ=Succ} ->
-            comb_get_sw(Succ, Safe1, St);
+        #b_br{} ->
+            none;
         #b_switch{arg=#b_var{}=Arg,fail=Fail,list=List} ->
             {none,Safe} = comb_is(Is, none, Safe1),
             {Safe,Arg,L,Fail,List}