[PATCH, v4] Fix PR51513, switch statement with default case containing __builtin_unreachable leads to wild branch

[Peter Bergner <bergner at vnet dot ibm dot com>]

Here is the updated patch to use gimple_seq_unreachable_p() which scans the
sequence backwards so it bails out earlier in the common case (ie, no call
to __builtin_unreachable).  As discussed in the previous thread, we remove
case statement labels from the jump table that lead to unreachable blocks,
which leads to fewer compare/branches in the decision tree case and
possibly a smaller jump table in the jump table case.  Unreachable default
case statements are only handled here when generating a jump table, since
the current code for decision trees seems to prefer the status quo.  We can
revisit this later if someone finds a test case that would benefit from
handling it for decision trees too.

This passes bootstrap and regtesting on powerpc64le-linux and x86_64-linux
with no regressions.  Ok for trunk now?

Peter

gcc/
	* tree-cfg.c (gimple_seq_unreachable_p): New function.
	(assert_unreachable_fallthru_edge_p): Use it.
	(group_case_labels_stmt): Likewise.
	* tree-cfg.h: Prototype it.
	* stmt.c: Include cfghooks.h and tree-cfg.h.
	(emit_case_dispatch_table) <gap_label>: New local variable.
	Use it to fill dispatch table gaps.
	Test for default_label before updating probabilities.
	(expand_case) <default_label>: Remove unneeded initialization.
	Test for unreachable default case statement and remove its edge.
	Set default_label accordingly.
	* tree-ssa-ccp.c (optimize_unreachable): Update comment.

gcc/testsuite/
	* gcc.target/powerpc/pr51513.c: New test.
	* gcc.dg/predict-13.c: Replace __builtin_unreachable() with
	__builtin_abort().
	* gcc.dg/predict-14.c: Likewise.

Index: gcc/tree-cfg.c
===================================================================
--- gcc/tree-cfg.c	(revision 247291)
+++ gcc/tree-cfg.c	(working copy)
@@ -452,6 +452,31 @@ computed_goto_p (gimple *t)
 	  && TREE_CODE (gimple_goto_dest (t)) != LABEL_DECL);
 }
 
+/* Returns true if the sequence of statements STMTS only contains
+   a call to __builtin_unreachable ().  */
+
+bool
+gimple_seq_unreachable_p (gimple_seq stmts)
+{
+  if (stmts == NULL)
+    return false;
+
+  gimple_stmt_iterator gsi = gsi_last (stmts);
+
+  if (!gimple_call_builtin_p (gsi_stmt (gsi), BUILT_IN_UNREACHABLE))
+    return false;
+
+  for (gsi_prev (&gsi); !gsi_end_p (gsi); gsi_prev (&gsi))
+    {
+      gimple *stmt = gsi_stmt (gsi);
+      if (gimple_code (stmt) != GIMPLE_LABEL
+	  && !is_gimple_debug (stmt)
+	  && !gimple_clobber_p (stmt))
+      return false;
+    }
+  return true;
+}
+
 /* Returns true for edge E where e->src ends with a GIMPLE_COND and
    the other edge points to a bb with just __builtin_unreachable ().
    I.e. return true for C->M edge in:
@@ -476,22 +501,7 @@ assert_unreachable_fallthru_edge_p (edge
       if (other_bb == e->dest)
 	other_bb = EDGE_SUCC (pred_bb, 1)->dest;
       if (EDGE_COUNT (other_bb->succs) == 0)
-	{
-	  gimple_stmt_iterator gsi = gsi_after_labels (other_bb);
-	  gimple *stmt;
-
-	  if (gsi_end_p (gsi))
-	    return false;
-	  stmt = gsi_stmt (gsi);
-	  while (is_gimple_debug (stmt) || gimple_clobber_p (stmt))
-	    {
-	      gsi_next (&gsi);
-	      if (gsi_end_p (gsi))
-		return false;
-	      stmt = gsi_stmt (gsi);
-	    }
-	  return gimple_call_builtin_p (stmt, BUILT_IN_UNREACHABLE);
-	}
+	return gimple_seq_unreachable_p (bb_seq (other_bb));
     }
   return false;
 }
@@ -1668,9 +1678,11 @@ group_case_labels_stmt (gswitch *stmt)
       gcc_assert (base_case);
       base_bb = label_to_block (CASE_LABEL (base_case));
 
-      /* Discard cases that have the same destination as the
-	 default case.  */
-      if (base_bb == default_bb)
+      /* Discard cases that have the same destination as the default case
+	 or if their destination block is unreachable.  */
+      if (base_bb == default_bb
+	  || (EDGE_COUNT (base_bb->succs) == 0
+	      && gimple_seq_unreachable_p (bb_seq (base_bb))))
 	{
 	  gimple_switch_set_label (stmt, i, NULL_TREE);
 	  i++;
Index: gcc/tree-cfg.h
===================================================================
--- gcc/tree-cfg.h	(revision 247291)
+++ gcc/tree-cfg.h	(working copy)
@@ -56,6 +56,7 @@ extern bool is_ctrl_stmt (gimple *);
 extern bool is_ctrl_altering_stmt (gimple *);
 extern bool simple_goto_p (gimple *);
 extern bool stmt_ends_bb_p (gimple *);
+extern bool gimple_seq_unreachable_p (gimple_seq);
 extern bool assert_unreachable_fallthru_edge_p (edge);
 extern void delete_tree_cfg_annotations (function *);
 extern gphi *get_virtual_phi (basic_block);
Index: gcc/stmt.c
===================================================================
--- gcc/stmt.c	(revision 247291)
+++ gcc/stmt.c	(working copy)
@@ -30,6 +30,7 @@ along with GCC; see the file COPYING3.
 #include "rtl.h"
 #include "tree.h"
 #include "gimple.h"
+#include "cfghooks.h"
 #include "predict.h"
 #include "alloc-pool.h"
 #include "memmodel.h"
@@ -49,6 +50,7 @@ along with GCC; see the file COPYING3.
 #include "expr.h"
 #include "langhooks.h"
 #include "cfganal.h"
+#include "tree-cfg.h"
 #include "params.h"
 #include "dumpfile.h"
 #include "builtins.h"
@@ -1007,20 +1009,21 @@ emit_case_dispatch_table (tree index_exp
 	  = gen_rtx_LABEL_REF (Pmode, label_rtx (n->code_label));
     }
 
-  /* Fill in the gaps with the default.  We may have gaps at
-     the beginning if we tried to avoid the minval subtraction,
-     so substitute some label even if the default label was
-     deemed unreachable.  */
-  if (!default_label)
-    default_label = fallback_label;
+  /* The dispatch table may contain gaps, including at the beginning of
+     the table if we tried to avoid the minval subtraction.  We fill the
+     dispatch table slots associated with the gaps with the default case label.
+     However, in the event the default case is unreachable, we then use
+     any label from one of the case statements.  */
+  rtx gap_label = (default_label) ? default_label : fallback_label;
+
   for (i = 0; i < ncases; i++)
     if (labelvec[i] == 0)
       {
-        has_gaps = true;
-        labelvec[i] = gen_rtx_LABEL_REF (Pmode, default_label);
+	has_gaps = true;
+	labelvec[i] = gen_rtx_LABEL_REF (Pmode, gap_label);
       }
 
-  if (has_gaps)
+  if (has_gaps && default_label)
     {
       /* There is at least one entry in the jump table that jumps
          to default label. The default label can either be reached
@@ -1115,7 +1118,7 @@ void
 expand_case (gswitch *stmt)
 {
   tree minval = NULL_TREE, maxval = NULL_TREE, range = NULL_TREE;
-  rtx_code_label *default_label = NULL;
+  rtx_code_label *default_label;
   unsigned int count, uniq;
   int i;
   int ncases = gimple_switch_num_labels (stmt);
@@ -1232,9 +1235,21 @@ expand_case (gswitch *stmt)
                              case_list, default_label,
                              default_prob);
   else
-    emit_case_dispatch_table (index_expr, index_type,
-			      case_list, default_label,
-			      minval, maxval, range, bb);
+    {
+      /* If the default case is unreachable, then set default_label to NULL
+	 so that we omit the range check when generating the dispatch table.
+	 We also remove the edge to the unreachable default case.  The block
+	 itself will be automatically removed later.  */
+      if (EDGE_COUNT (default_edge->dest->succs) == 0
+	  && gimple_seq_unreachable_p (bb_seq (default_edge->dest)))
+	{
+	  default_label = NULL;
+	  remove_edge (default_edge);
+	}
+      emit_case_dispatch_table (index_expr, index_type,
+				case_list, default_label,
+				minval, maxval, range, bb);
+    }
 
   reorder_insns (NEXT_INSN (before_case), get_last_insn (), before_case);
 
Index: gcc/tree-ssa-ccp.c
===================================================================
--- gcc/tree-ssa-ccp.c	(revision 247291)
+++ gcc/tree-ssa-ccp.c	(working copy)
@@ -2715,7 +2715,8 @@ optimize_unreachable (gimple_stmt_iterat
 	}
       else
 	{
-	  /* Todo: handle other cases, f.i. switch statement.  */
+	  /* Todo: handle other cases.  Note that unreachable switch case
+	     statements have already been removed.  */
 	  continue;
 	}
 
Index: gcc/testsuite/gcc.target/powerpc/pr51513.c
===================================================================
--- gcc/testsuite/gcc.target/powerpc/pr51513.c	(nonexistent)
+++ gcc/testsuite/gcc.target/powerpc/pr51513.c	(working copy)
@@ -0,0 +1,25 @@
+/* { dg-do compile { target { powerpc*-*-linux* } } } */
+/* { dg-options "-O2 -fjump-tables --param case-values-threshold=1" } */
+/* Verify we created a jump table.  */
+/* { dg-final { scan-assembler-times "mtctr "  1 } } */
+/* { dg-final { scan-assembler-times "bctr" 1 } } */
+/* Verify we eliminated the range check.  */
+/* { dg-final { scan-assembler-not "cmpldi" } } */
+/* { dg-final { scan-assembler-not "cmplwi" } } */
+
+long
+bug (long cond, long v0, long v1, long v2)
+{
+  switch (cond)
+    {
+      case 0:
+	return v0;
+      case 1:
+	return v1;
+      case 2:
+	return v2;
+      default:
+	__builtin_unreachable ();
+    }
+  __builtin_abort ();
+}
Index: gcc/testsuite/gcc.dg/predict-13.c
===================================================================
--- gcc/testsuite/gcc.dg/predict-13.c	(revision 247291)
+++ gcc/testsuite/gcc.dg/predict-13.c	(working copy)
@@ -10,9 +10,9 @@ int main(int argc, char **argv)
     case 2:
       return 2;
     case 3:
-      __builtin_unreachable();
+      __builtin_abort();
     case 4:
-      __builtin_unreachable();
+      __builtin_abort();
     default:
       return 5;
     }
Index: gcc/testsuite/gcc.dg/predict-14.c
===================================================================
--- gcc/testsuite/gcc.dg/predict-14.c	(revision 247291)
+++ gcc/testsuite/gcc.dg/predict-14.c	(working copy)
@@ -6,11 +6,11 @@ int main(int argc, char **argv)
   switch (argc)
     {
     case 1:
-      __builtin_unreachable();
+      __builtin_abort();
     case 4:
-      __builtin_unreachable();
+      __builtin_abort();
     default:
-      __builtin_unreachable();
+      __builtin_abort();
     }
 
   return 10;