[PATCH 2/3] cfgcleanup: Fold jumps and conditional branches with returns

[Segher Boessenkool]

This patch makes cfgcleanup optimize jumps to returns.  There are three
cases this handles:

-- A jump to a return; this is simplified to just that return.
-- A conditional branch to a return; simplified to a conditional return.
-- A conditional branch that falls through to a return.  This is simplified
   to a conditional return (with the condition inverted), falling through
   to a jump to the original destination.  That jump can then be optimized
   further, as usual.

This handles all cases the current function.c does, and a few it misses.


2016-05-03  Segher Boessenkool  <segher@kernel.crashing.org>

	* cfgcleanup.c (bb_is_just_return): New function.
	(try_optimize_cfg): Simplify jumps to return, branches to return,
	and branches around return.

---
 gcc/cfgcleanup.c | 123 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 123 insertions(+)

diff --git a/gcc/cfgcleanup.c b/gcc/cfgcleanup.c
index 19583a7..4f87811 100644
--- a/gcc/cfgcleanup.c
+++ b/gcc/cfgcleanup.c
@@ -2606,6 +2606,35 @@ trivially_empty_bb_p (basic_block bb)
     }
 }
 
+/* Return true if BB contains just a return and possibly a USE of the
+   return value.  Fill in *RET and *USE with the return and use insns
+   if any found, otherwise NULL.  */
+
+static bool
+bb_is_just_return (basic_block bb, rtx_insn **ret, rtx_insn **use)
+{
+  *ret = *use = NULL;
+  rtx_insn *insn;
+
+  if (bb == EXIT_BLOCK_PTR_FOR_FN (cfun))
+    return false;
+
+  FOR_BB_INSNS (bb, insn)
+    if (NONDEBUG_INSN_P (insn))
+      {
+	if (!*ret && ANY_RETURN_P (PATTERN (insn)))
+	  *ret = insn;
+	else if (!*ret && !*use && GET_CODE (PATTERN (insn)) == USE
+	    && REG_P (XEXP (PATTERN (insn), 0))
+	    && REG_FUNCTION_VALUE_P (XEXP (PATTERN (insn), 0)))
+	  *use = insn;
+	else
+	  return false;
+      }
+
+  return !!*ret;
+}
+
 /* Do simple CFG optimizations - basic block merging, simplifying of jump
    instructions etc.  Return nonzero if changes were made.  */
 
@@ -2792,6 +2821,100 @@ try_optimize_cfg (int mode)
 		      }
 		}
 
+	      /* Try to change a branch to a return to just that return.  */
+	      rtx_insn *ret, *use;
+	      if (single_succ_p (b)
+		  && onlyjump_p (BB_END (b))
+		  && bb_is_just_return (single_succ (b), &ret, &use))
+		{
+		  if (redirect_jump (as_a <rtx_jump_insn *> (BB_END (b)),
+				     PATTERN (ret), 0))
+		    {
+		      if (use)
+			emit_insn_before (copy_insn (PATTERN (use)),
+					  BB_END (b));
+		      if (dump_file)
+			fprintf (dump_file, "Changed jump %d->%d to return.\n",
+					    b->index, single_succ (b)->index);
+		      redirect_edge_succ (single_succ_edge (b),
+					  EXIT_BLOCK_PTR_FOR_FN (cfun));
+		      single_succ_edge (b)->flags &= ~EDGE_CROSSING;
+		      changed_here = true;
+		    }
+		}
+
+	      /* Try to change a conditional branch to a return to the
+		 respective conditional return.  */
+	      if (EDGE_COUNT (b->succs) == 2
+		  && any_condjump_p (BB_END (b))
+		  && bb_is_just_return (BRANCH_EDGE (b)->dest, &ret, &use))
+		{
+		  if (redirect_jump (as_a <rtx_jump_insn *> (BB_END (b)),
+				     PATTERN (ret), 0))
+		    {
+		      if (use)
+			emit_insn_before (copy_insn (PATTERN (use)),
+					  BB_END (b));
+		      if (dump_file)
+			fprintf (dump_file, "Changed conditional jump %d->%d "
+					    "to conditional return.\n",
+					    b->index,
+					    BRANCH_EDGE (b)->dest->index);
+		      redirect_edge_succ (BRANCH_EDGE (b),
+					  EXIT_BLOCK_PTR_FOR_FN (cfun));
+		      BRANCH_EDGE (b)->flags &= ~EDGE_CROSSING;
+		      changed_here = true;
+		    }
+		}
+
+	      /* Try to flip a conditional branch that falls through to
+		 a return so that it becomes a conditional return and a
+		 new jump to the original branch target.  */
+	      if (EDGE_COUNT (b->succs) == 2
+		  && any_condjump_p (BB_END (b))
+		  && bb_is_just_return (FALLTHRU_EDGE (b)->dest, &ret, &use))
+		{
+		  if (invert_jump (as_a <rtx_jump_insn *> (BB_END (b)),
+				   JUMP_LABEL (BB_END (b)), 0))
+		    {
+		      basic_block new_ft = BRANCH_EDGE (b)->dest;
+		      if (redirect_jump (as_a <rtx_jump_insn *> (BB_END (b)),
+					 PATTERN (ret), 0))
+			{
+			  if (use)
+			    emit_insn_before (copy_insn (PATTERN (use)),
+					      BB_END (b));
+			  if (dump_file)
+			    fprintf (dump_file, "Changed conditional jump "
+						"%d->%d to conditional return, "
+						"adding fall-through jump.\n",
+						b->index,
+						BRANCH_EDGE (b)->dest->index);
+			  redirect_edge_succ (BRANCH_EDGE (b),
+					      EXIT_BLOCK_PTR_FOR_FN (cfun));
+			  BRANCH_EDGE (b)->flags &= ~EDGE_CROSSING;
+			  std::swap (BRANCH_EDGE (b)->probability,
+				     FALLTHRU_EDGE (b)->probability);
+			  update_br_prob_note (b);
+			  basic_block jb = force_nonfallthru (FALLTHRU_EDGE (b));
+			  notice_new_block (jb);
+			  if (!redirect_jump (as_a <rtx_jump_insn *> (BB_END (jb)),
+					      block_label (new_ft), 0))
+			    gcc_unreachable ();
+			  redirect_edge_succ (single_succ_edge (jb), new_ft);
+			  changed_here = true;
+			}
+		      else
+			{
+			  /* Invert the jump back to what it was.  This should
+			     never fail.  */
+			  if (!invert_jump (as_a <rtx_jump_insn *> (BB_END (b)),
+					    JUMP_LABEL (BB_END (b)), 0))
+			    gcc_unreachable ();
+			}
+		    }
+		}
+
 	      /* Simplify branch over branch.  */
 	      if ((mode & CLEANUP_EXPENSIVE)
 		   && !(mode & CLEANUP_CFGLAYOUT)
-- 
1.9.3