http://gcc.gnu.org/ml/gcc-patches/2007-04/msg00927.html

Hi,

The attached patch makes sure that we create smaller object code for
simple switch statements. We just make sure to flatten the switch
statement into an if-else chain, basically.

This fixes a size-regression as compared to gcc-3.4, as can be seen
below.

2007-04-15  Bernhard Fischer  <..>

	* stmt.c (expand_case): Do not create a complex binary tree when
	optimizing for size but rather use the simple ordered list.
	(emit_case_nodes): do not emit jumps to the default_label when
	optimizing for size.

Not regtested so far.
Comments?

Attached is the test switch.c mentioned below.

$ for i in 2.95 3.3 3.4 4.0 4.1 4.2.orig-HEAD 4.3.orig-HEAD 4.3-HEAD;do
gcc-$i  -DCHAIN -Os -o switch-CHAIN-$i.o -c switch.c ;done
$ for i in 2.95 3.3 3.4 4.0 4.1 4.2.orig-HEAD 4.3.orig-HEAD 4.3-HEAD;do
gcc-$i  -UCHAIN -Os -o switch-$i.o -c switch.c ;done

$ size switch-*.o
   text	   data	    bss	    dec	    hex	filename
    169	      0	      0	    169	     a9	switch-2.95.o
    115	      0	      0	    115	     73	switch-3.3.o
    103	      0	      0	    103	     67	switch-3.4.o
    124	      0	      0	    124	     7c	switch-4.0.o
    124	      0	      0	    124	     7c	switch-4.1.o
    124	      0	      0	    124	     7c	switch-4.2.orig-HEAD.o
     95	      0	      0	     95	     5f	switch-4.3-HEAD.o
    124	      0	      0	    124	     7c	switch-4.3.orig-HEAD.o
    166	      0	      0	    166	     a6	switch-CHAIN-2.95.o
    111	      0	      0	    111	     6f	switch-CHAIN-3.3.o
     95	      0	      0	     95	     5f	switch-CHAIN-3.4.o
     95	      0	      0	     95	     5f	switch-CHAIN-4.0.o
     95	      0	      0	     95	     5f	switch-CHAIN-4.1.o
     95	      0	      0	     95	     5f	switch-CHAIN-4.2.orig-HEAD.o
     95	      0	      0	     95	     5f	switch-CHAIN-4.3-HEAD.o
     95	      0	      0	     95	     5f	switch-CHAIN-4.3.orig-HEAD.o


Content-Type: text/x-diff; charset=us-ascii
Content-Disposition: attachment; filename="gcc-4.3.gcc-flatten-switch-stmt.00.diff"

Index: gcc-4.2.0/gcc/stmt.c
===================================================================
--- gcc-4.2.0.orig/gcc/stmt.c	(revision 123843)
+++ gcc-4.2.0/gcc/stmt.c	(working copy)
@@ -2517,7 +2517,11 @@ expand_case (tree exp)
 	  use_cost_table
 	    = (TREE_CODE (orig_type) != ENUMERAL_TYPE
 	       && estimate_case_costs (case_list));
-	  balance_case_nodes (&case_list, NULL);
+	  /* When optimizing for size, we want a straight list to avoid
+	     jumps as much as possible. This basically creates an if-else
+	     chain.  */
+	  if (!optimize_size)
+	    balance_case_nodes (&case_list, NULL);
 	  emit_case_nodes (index, case_list, default_label, index_type);
 	  emit_jump (default_label);
 	}
@@ -3075,6 +3079,7 @@ emit_case_nodes (rtx index, case_node_pt
 	    {
 	      if (!node_has_low_bound (node, index_type))
 		{
+		  if (!optimize_size) /* don't jl to the .default_label. */
 		  emit_cmp_and_jump_insns (index,
 					   convert_modes
 					   (mode, imode,


Content-Type: text/x-csrc; charset=us-ascii
Content-Disposition: attachment; filename="switch.c"

int
commutative_tree_code (int code)
{
#define CASE(val, ret) case val:/* __asm__("# val="#val ",ret="#ret);*/ return ret;
#ifndef CHAIN
  switch (code)
    {
# if 1
  CASE(1,3)
  CASE(3,2)
  CASE(5,8)
  CASE(7,1)
  CASE(33,4)
  CASE(44,9)
  CASE(55,10)
  CASE(66,-1)
  CASE(77,99)
  CASE(666,0)
# else
    case 1:
      return 3;
    case 3:
      return 2;
    case 5:
      return 8;
    case 7:
      return 1;
    case 33:
      return 4;
    case 44:
      return 9;
    case 55:
      return 10;
    case 66:
      return -1;
    case 77:
      return 99;
    case 666:
      return 0;
# endif
    default:
      break;
    }
  return 4711;

#else
   if (code == 1)
	return 3;
  else if (code == 3)
	return 2;
  else if (code == 5)
	return 8;
  else if (code == 7)
	return 1;
  else if (code == 33)
	return 4;
  else if (code == 44)
	return 9;
  else if (code == 55)
	return 10;
  else if (code == 66)
	return -1;
  else if (code == 77)
	return 99;
  else if (code == 666)
	return 0;
  else
	return 4711;
#endif
}


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