hipe_rtl: drop alub dest when unused

3 files changed, 43 insertions, 11 deletions

diff --git a/lib/hipe/icode/hipe_icode.erl b/lib/hipe/icode/hipe_icode.erl
index 78508dff22..dce2fcb392 100644
--- a/lib/hipe/icode/hipe_icode.erl
+++ b/lib/hipe/icode/hipe_icode.erl
@@ -594,6 +594,7 @@
 	 uses/1,
 	 defines/1,
 	 is_safe/1,
+	 reduce_unused/1,
 	 strip_comments/1,
 	 subst/2,
 	 subst_uses/2,
@@ -1765,6 +1766,18 @@ is_safe(Instr) ->
     #icode_end_try{} -> false
   end.
 
+%% @doc Produces a simplified instruction sequence that is equivalent to [Instr]
+%% under the assumption that all results of Instr are unused, or 'false' if
+%% there is no such sequence (other than [Instr] itself).
+
+-spec reduce_unused(icode_instr()) -> false | [icode_instr()].
+
+reduce_unused(Instr) ->
+  case is_safe(Instr) of
+    true -> [];
+    false -> false
+  end.
+
 %%-----------------------------------------------------------------------
 
 -spec highest_var(icode_instrs()) -> non_neg_integer().
diff --git a/lib/hipe/rtl/hipe_rtl.erl b/lib/hipe/rtl/hipe_rtl.erl
index e4e857ec43..d39969a0ed 100644
--- a/lib/hipe/rtl/hipe_rtl.erl
+++ b/lib/hipe/rtl/hipe_rtl.erl
@@ -334,6 +334,7 @@
 	 defines/1,
 	 redirect_jmp/3,
 	 is_safe/1,
+	 reduce_unused/1,
 	 %% highest_var/1,
 	 pp/1,
 	 pp/2,
@@ -1314,6 +1315,24 @@ is_safe(Instr) ->
     #switch{} -> false %% Maybe this is safe...
   end.
 
+%% @spec reduce_unused(rtl_instruction())
+%%           -> false | [rtl_instruction()].
+%%
+%% @doc Produces a simplified instruction sequence that is equivalent to [Instr]
+%% under the assumption that all results of Instr are unused, or 'false' if
+%% there is no such sequence (other than [Instr] itself).
+
+reduce_unused(Instr) ->
+  case Instr of
+    #alub{dst=Dst} when Dst =/= [] ->
+      [Instr#alub{dst=[]}];
+    _ ->
+      case is_safe(Instr) of
+	true -> [];
+	false -> false
+      end
+  end.
+
 %%
 %% True if argument is an alu-operator
 %%
diff --git a/lib/hipe/ssa/hipe_ssa.inc b/lib/hipe/ssa/hipe_ssa.inc
index 83ab320306..b511bb6f25 100644
--- a/lib/hipe/ssa/hipe_ssa.inc
+++ b/lib/hipe/ssa/hipe_ssa.inc
@@ -1,4 +1,4 @@
-%% -*- erlang-indent-level: 2 -*-
+%% -*- mode: erlang; erlang-indent-level: 2 -*-
 %%
 %% %CopyrightBegin%
 %% 
@@ -943,9 +943,9 @@ do_code([Instr|Instrs], LiveOut, Changed, Acc) ->
     false ->
       do_code(Instrs, LiveIn, Changed, [Instr|Acc]);
     true ->
-      case ?CODE:is_safe(Instr) of
+      case ?CODE:is_call(Instr) of
 	true ->
-	  case ?CODE:is_call(Instr) of
+	  case ?CODE:is_safe(Instr) of
 	    true ->
 	      case ?CODE:call_continuation(Instr) of
 		[] ->
@@ -955,11 +955,6 @@ do_code([Instr|Instrs], LiveOut, Changed, Acc) ->
 		  do_code(Instrs, LiveOut, true, [NewInstr|Acc])
 	      end;
 	    false ->
-	      do_code(Instrs, LiveOut, true, Acc)
-	  end;
-	false -> %% not a safe instruction - cannot be removed
-	  case ?CODE:is_call(Instr) of
-	    true ->
 	      case ?CODE:call_dstlist(Instr) of
 	        [] ->  %% result was not used anyway; no change
 		  do_code(Instrs, LiveIn, Changed, [Instr|Acc]);
@@ -968,9 +963,14 @@ do_code([Instr|Instrs], LiveOut, Changed, Acc) ->
 		  do_code(Instrs, LiveIn, true, [NewInstr|Acc]);
 		[_|_] ->  %% calls with multiple dests are left untouched
 		  do_code(Instrs, LiveIn, Changed, [Instr|Acc])
-	      end;
-	    false ->
-	      do_code(Instrs, LiveIn, Changed, [Instr|Acc])
+	      end
+	  end;
+	false ->
+	  case ?CODE:reduce_unused(Instr) of
+	    false -> % not a safe instruction - cannot be removed
+	      do_code(Instrs, LiveIn, Changed, [Instr|Acc]);
+	    Replacement ->
+	      do_code(lists:reverse(Replacement, Instrs), LiveOut, true, Acc)
 	  end
       end
   end;