dwarf2out: Decrease dw_loc_descr_node and dw_attr_struct struct sizes [PR119711]

As noted by Richi on a large testcase, there are unnecessary paddings
in some heavily used dwarf2out.{h,cc} structures on 64-bit hosts.

struct dw_val_node {
        enum dw_val_class          val_class;            /*     0     4 */

        /* XXX 4 bytes hole, try to pack */

        struct addr_table_entry *  val_entry;            /*     8     8 */
        union dw_val_struct_union  v;                    /*    16    16 */

        /* size: 32, cachelines: 1, members: 3 */
        /* sum members: 28, holes: 1, sum holes: 4 */
        /* last cacheline: 32 bytes */
};
struct dw_loc_descr_node {
        dw_loc_descr_ref           dw_loc_next;          /*     0     8 */
        enum dwarf_location_atom   dw_loc_opc:8;         /*     8: 0  4 */
        unsigned int               dtprel:1;             /*     8: 8  4 */
        unsigned int               frame_offset_rel:1;   /*     8: 9  4 */

        /* XXX 22 bits hole, try to pack */

        int                        dw_loc_addr;          /*    12     4 */
        struct dw_val_node         dw_loc_oprnd1;        /*    16    32 */
        struct dw_val_node         dw_loc_oprnd2;        /*    48    32 */

        /* size: 80, cachelines: 2, members: 7 */
        /* sum members: 76 */
        /* sum bitfield members: 10 bits, bit holes: 1, sum bit holes: 22 bits */
        /* last cacheline: 16 bytes */
};
struct dw_attr_struct {
        enum dwarf_attribute       dw_attr;              /*     0     4 */

        /* XXX 4 bytes hole, try to pack */

        struct dw_val_node         dw_attr_val;          /*     8    32 */

        /* size: 40, cachelines: 1, members: 2 */
        /* sum members: 36, holes: 1, sum holes: 4 */
        /* last cacheline: 40 bytes */
};

The following patch is an (not very clean admittedly) attempt to decrease
size of dw_loc_descr_node from 80 bytes to 72 and (more importantly)
dw_attr_struct from 40 bytes to 32 by moving the dw_attr member from
dw_attr_struct into dw_attr_val's padding and similarly move
dw_loc_opc/dtprel/frame_offset_rel members into dw_loc_oprnd1 padding
and dw_loc_addr into dw_loc_oprnd2 padding.
All we need to ensure is that nothing tries to copy whole dw_val_node
structs unless it is copied as part of whole dw_loc_descr_node or
dw_attr_struct copy.

To verify that wasn't the case, I've temporarily added a deleted copy ctor
to dw_val_node and then looked at all the errors/warnings caused by that,
and those were just from memcpy/memmove or structure assignments of whole
dw_loc_descr_node/dw_attr_struct.

2025-04-24  Jakub Jelinek  <jakub@redhat.com>

	PR debug/119711
	* dwarf2out.h (struct dw_val_node): Add u member.
	(struct dw_loc_descr_node): Remove dw_loc_opc, dtprel,
	frame_offset_rel and dw_loc_addr members.
	(dw_loc_opc, dw_loc_dtprel, dw_loc_frame_offset_rel, dw_loc_addr):
	Define.
	(struct dw_attr_struct): Remove dw_attr member.
	(dw_attr): Define.
	* dwarf2out.cc (loc_descr_equal_p_1): Use dw_loc_dtprel instead of
	dtprel.
	(output_loc_operands, new_addr_loc_descr, loc_checksum,
	loc_checksum_ordered): Likewise.
	(resolve_args_picking_1): Use dw_loc_frame_offset_rel instead of
	frame_offset_rel.
	(loc_list_from_tree_1): Likewise.
	(resolve_addr_in_expr): Use dw_loc_dtprel instead of dtprel.
	(copy_deref_exprloc): Copy val_class, val_entry and v members
	instead of whole dw_loc_oprnd1 and dw_loc_oprnd2.
	(optimize_string_length): Copy val_class, val_entry and v members
	instead of whole dw_attr_val.
	(hash_loc_operands): Use dw_loc_dtprel instead of dtprel.
	(compare_loc_operands, compare_locs): Likewise.

2 files changed, 50 insertions, 25 deletions

diff --git a/gcc/dwarf2out.cc b/gcc/dwarf2out.cc
index 69e9d77..34ffeed 100644
--- a/gcc/dwarf2out.cc
+++ b/gcc/dwarf2out.cc
@@ -1536,7 +1536,7 @@ loc_descr_equal_p_1 (dw_loc_descr_ref a, dw_loc_descr_ref b)
   /* ??? This is only ever set for DW_OP_constNu, for N equal to the
      address size, but since we always allocate cleared storage it
      should be zero for other types of locations.  */
-  if (a->dtprel != b->dtprel)
+  if (a->dw_loc_dtprel != b->dw_loc_dtprel)
     return false;
 
   return (dw_val_equal_p (&a->dw_loc_oprnd1, &b->dw_loc_oprnd1)
@@ -2115,7 +2115,7 @@ output_loc_operands (dw_loc_descr_ref loc, int for_eh_or_skip)
       dw2_asm_output_data (2, val1->v.val_int, NULL);
       break;
     case DW_OP_const4u:
-      if (loc->dtprel)
+      if (loc->dw_loc_dtprel)
 	{
 	  gcc_assert (targetm.asm_out.output_dwarf_dtprel);
 	  targetm.asm_out.output_dwarf_dtprel (asm_out_file, 4,
@@ -2128,7 +2128,7 @@ output_loc_operands (dw_loc_descr_ref loc, int for_eh_or_skip)
       dw2_asm_output_data (4, val1->v.val_int, NULL);
       break;
     case DW_OP_const8u:
-      if (loc->dtprel)
+      if (loc->dw_loc_dtprel)
 	{
 	  gcc_assert (targetm.asm_out.output_dwarf_dtprel);
 	  targetm.asm_out.output_dwarf_dtprel (asm_out_file, 8,
@@ -2323,7 +2323,7 @@ output_loc_operands (dw_loc_descr_ref loc, int for_eh_or_skip)
       break;
 
     case DW_OP_addr:
-      if (loc->dtprel)
+      if (loc->dw_loc_dtprel)
 	{
 	  if (targetm.asm_out.output_dwarf_dtprel)
 	    {
@@ -4028,7 +4028,7 @@ new_addr_loc_descr (rtx addr, enum dtprel_bool dtprel)
 
   ref->dw_loc_oprnd1.val_class = dw_val_class_addr;
   ref->dw_loc_oprnd1.v.val_addr = addr;
-  ref->dtprel = dtprel;
+  ref->dw_loc_dtprel = dtprel;
   if (dwarf_split_debug_info)
     ref->dw_loc_oprnd1.val_entry
       = add_addr_table_entry (addr,
@@ -7036,7 +7036,7 @@ loc_checksum (dw_loc_descr_ref loc, struct md5_ctx *ctx)
   inchash::hash hstate;
   hashval_t hash;
 
-  tem = (loc->dtprel << 8) | ((unsigned int) loc->dw_loc_opc);
+  tem = (loc->dw_loc_dtprel << 8) | ((unsigned int) loc->dw_loc_opc);
   CHECKSUM (tem);
   hash_loc_operands (loc, hstate);
   hash = hstate.end();
@@ -7259,7 +7259,7 @@ loc_checksum_ordered (dw_loc_descr_ref loc, struct md5_ctx *ctx)
       inchash::hash hstate;
       hashval_t hash;
 
-      CHECKSUM_ULEB128 (loc->dtprel);
+      CHECKSUM_ULEB128 (loc->dw_loc_dtprel);
       CHECKSUM_ULEB128 (loc->dw_loc_opc);
       hash_loc_operands (loc, hstate);
       hash = hstate.end ();
@@ -18310,7 +18310,7 @@ resolve_args_picking_1 (dw_loc_descr_ref loc, unsigned initial_frame_offset,
 
       /* If needed, relocate the picking offset with respect to the frame
 	 offset. */
-      if (l->frame_offset_rel)
+      if (l->dw_loc_frame_offset_rel)
 	{
 	  unsigned HOST_WIDE_INT off;
 	  switch (l->dw_loc_opc)
@@ -18826,7 +18826,7 @@ loc_list_from_tree_1 (tree loc, int want_address,
 	       && want_address == 0)
 	{
 	  ret = new_loc_descr (DW_OP_pick, 0, 0);
-	  ret->frame_offset_rel = 1;
+	  ret->dw_loc_frame_offset_rel = 1;
 	  context->placeholder_seen = true;
 	  break;
 	}
@@ -18993,7 +18993,7 @@ loc_list_from_tree_1 (tree loc, int want_address,
 	  gcc_assert (cursor != NULL_TREE);
 
 	  ret = new_loc_descr (DW_OP_pick, i, 0);
-	  ret->frame_offset_rel = 1;
+	  ret->dw_loc_frame_offset_rel = 1;
 	  break;
 	}
       /* FALLTHRU */
@@ -31061,7 +31061,7 @@ resolve_addr_in_expr (dw_attr_node *a, dw_loc_descr_ref loc)
 	     || loc->dw_loc_opc == DW_OP_addrx)
 	    || ((loc->dw_loc_opc == DW_OP_GNU_const_index
 		 || loc->dw_loc_opc == DW_OP_constx)
-		&& loc->dtprel))
+		&& loc->dw_loc_dtprel))
           {
             rtx rtl = loc->dw_loc_oprnd1.val_entry->addr.rtl;
             if (!resolve_one_addr (&rtl))
@@ -31073,7 +31073,7 @@ resolve_addr_in_expr (dw_attr_node *a, dw_loc_descr_ref loc)
 	break;
       case DW_OP_const4u:
       case DW_OP_const8u:
-	if (loc->dtprel
+	if (loc->dw_loc_dtprel
 	    && !resolve_one_addr (&loc->dw_loc_oprnd1.v.val_addr))
 	  return false;
 	break;
@@ -31359,8 +31359,12 @@ copy_deref_exprloc (dw_loc_descr_ref expr)
   while (expr != l)
     {
       *p = new_loc_descr (expr->dw_loc_opc, 0, 0);
-      (*p)->dw_loc_oprnd1 = expr->dw_loc_oprnd1;
-      (*p)->dw_loc_oprnd2 = expr->dw_loc_oprnd2;
+      (*p)->dw_loc_oprnd1.val_class = expr->dw_loc_oprnd1.val_class;
+      (*p)->dw_loc_oprnd1.val_entry = expr->dw_loc_oprnd1.val_entry;
+      (*p)->dw_loc_oprnd1.v = expr->dw_loc_oprnd1.v;
+      (*p)->dw_loc_oprnd2.val_class = expr->dw_loc_oprnd2.val_class;
+      (*p)->dw_loc_oprnd2.val_entry = expr->dw_loc_oprnd2.val_entry;
+      (*p)->dw_loc_oprnd2.v = expr->dw_loc_oprnd2.v;
       p = &(*p)->dw_loc_next;
       expr = expr->dw_loc_next;
     }
@@ -31451,7 +31455,9 @@ optimize_string_length (dw_attr_node *a)
      copy over the DW_AT_location attribute from die to a.  */
   if (l->dw_loc_next != NULL)
     {
-      a->dw_attr_val = av->dw_attr_val;
+      a->dw_attr_val.val_class = av->dw_attr_val.val_class;
+      a->dw_attr_val.val_entry = av->dw_attr_val.val_entry;
+      a->dw_attr_val.v = av->dw_attr_val.v;
       return 1;
     }
 
@@ -31737,7 +31743,7 @@ hash_loc_operands (dw_loc_descr_ref loc, inchash::hash &hstate)
     {
     case DW_OP_const4u:
     case DW_OP_const8u:
-      if (loc->dtprel)
+      if (loc->dw_loc_dtprel)
 	goto hash_addr;
       /* FALLTHRU */
     case DW_OP_const1u:
@@ -31839,7 +31845,7 @@ hash_loc_operands (dw_loc_descr_ref loc, inchash::hash &hstate)
       break;
     case DW_OP_addr:
     hash_addr:
-      if (loc->dtprel)
+      if (loc->dw_loc_dtprel)
 	{
 	  unsigned char dtprel = 0xd1;
 	  hstate.add_object (dtprel);
@@ -31851,7 +31857,7 @@ hash_loc_operands (dw_loc_descr_ref loc, inchash::hash &hstate)
     case DW_OP_GNU_const_index:
     case DW_OP_constx:
       {
-        if (loc->dtprel)
+	if (loc->dw_loc_dtprel)
           {
             unsigned char dtprel = 0xd1;
 	    hstate.add_object (dtprel);
@@ -31998,7 +32004,7 @@ compare_loc_operands (dw_loc_descr_ref x, dw_loc_descr_ref y)
     {
     case DW_OP_const4u:
     case DW_OP_const8u:
-      if (x->dtprel)
+      if (x->dw_loc_dtprel)
 	goto hash_addr;
       /* FALLTHRU */
     case DW_OP_const1u:
@@ -32162,7 +32168,7 @@ compare_locs (dw_loc_descr_ref x, dw_loc_descr_ref y)
 {
   for (; x != NULL && y != NULL; x = x->dw_loc_next, y = y->dw_loc_next)
     if (x->dw_loc_opc != y->dw_loc_opc
-	|| x->dtprel != y->dtprel
+	|| x->dw_loc_dtprel != y->dw_loc_dtprel
 	|| !compare_loc_operands (x, y))
       break;
   return x == NULL && y == NULL;
diff --git a/gcc/dwarf2out.h b/gcc/dwarf2out.h
index fe50956..a0b0fa4 100644
--- a/gcc/dwarf2out.h
+++ b/gcc/dwarf2out.h
@@ -276,6 +276,25 @@ typedef struct GTY(()) dw_loc_list_struct {
 
 struct GTY(()) dw_val_node {
   enum dw_val_class val_class;
+  /* On 64-bit host, there are 4 bytes of padding between val_class
+     and val_entry.  Reuse the padding for other content of
+     dw_loc_descr_node and dw_attr_struct.  */
+  union dw_val_node_parent
+    {
+      struct dw_val_loc_descr_node
+	{
+	  ENUM_BITFIELD (dwarf_location_atom) dw_loc_opc_v : 8;
+	  /* Used to distinguish DW_OP_addr with a direct symbol relocation
+	     from DW_OP_addr with a dtp-relative symbol relocation.  */
+	  unsigned int dw_loc_dtprel_v : 1;
+	  /* For DW_OP_pick, DW_OP_dup and DW_OP_over operations: true iff.
+	     it targets a DWARF prodecure argument.  In this case, it needs to be
+	     relocated according to the current frame offset.  */
+	  unsigned int dw_loc_frame_offset_rel_v : 1;
+	} u1;
+      int u2;
+      enum dwarf_attribute u3;
+    } GTY((skip)) u;
   struct addr_table_entry * GTY(()) val_entry;
   union dw_val_struct_union
     {
@@ -321,15 +340,15 @@ struct GTY(()) dw_val_node {
 
 struct GTY((chain_next ("%h.dw_loc_next"))) dw_loc_descr_node {
   dw_loc_descr_ref dw_loc_next;
-  ENUM_BITFIELD (dwarf_location_atom) dw_loc_opc : 8;
+#define dw_loc_opc dw_loc_oprnd1.u.u1.dw_loc_opc_v
   /* Used to distinguish DW_OP_addr with a direct symbol relocation
      from DW_OP_addr with a dtp-relative symbol relocation.  */
-  unsigned int dtprel : 1;
+#define dw_loc_dtprel dw_loc_oprnd1.u.u1.dw_loc_dtprel_v
   /* For DW_OP_pick, DW_OP_dup and DW_OP_over operations: true iff.
      it targets a DWARF prodecure argument.  In this case, it needs to be
      relocated according to the current frame offset.  */
-  unsigned int frame_offset_rel : 1;
-  int dw_loc_addr;
+#define dw_loc_frame_offset_rel dw_loc_oprnd1.u.u1.dw_loc_frame_offset_rel_v
+#define dw_loc_addr dw_loc_oprnd2.u.u2
   dw_val_node dw_loc_oprnd1;
   dw_val_node dw_loc_oprnd2;
 };
@@ -493,7 +512,7 @@ void dwarf2out_cc_finalize (void);
    Attributes are typically linked below the DIE they modify.  */
 
 typedef struct GTY(()) dw_attr_struct {
-  enum dwarf_attribute dw_attr;
+#define dw_attr dw_attr_val.u.u3
   dw_val_node dw_attr_val;
 }
 dw_attr_node;