[PATCH] Shrink size of dwarf2_per_cu_data

[Tom Tromey <tom@tromey.com>]

I noticed some holes in struct dwarf2_per_cu_data.  This patch
rearranges the type slightly, and shrinks the size of some fields.
This reduces it from 136 bytes to 112 bytes (on x86-64).

I also reduced the size of the DWARF "version" fields in a couple of
spots.  It seemed needless to use a short to hold a value that ranges
from 2 to 5, and this also helped the goal of shrinking
dwarf2_per_cu_data.

gdb/ChangeLog
2020-11-03  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct dwarf2_per_cu_data) <dwarf_version>: Now
	unsigned char.
	(struct dwarf2_per_cu_data): Rearrange.
	* dwarf2/comp-unit.h (struct comp_unit_head) <version>: Now
	unsigned char.
	(struct comp_unit_head): Rearrange.
	* dwarf2/comp-unit.c (read_comp_unit_head): Update.
---
 gdb/ChangeLog          | 10 ++++++++++
 gdb/dwarf2/comp-unit.c |  7 ++++---
 gdb/dwarf2/comp-unit.h | 15 +++++++--------
 gdb/dwarf2/read.h      | 26 +++++++++++++-------------
 4 files changed, 34 insertions(+), 24 deletions(-)

diff --git a/gdb/dwarf2/comp-unit.c b/gdb/dwarf2/comp-unit.c
index 7ddc893aac6..05cf3f1c6a8 100644
--- a/gdb/dwarf2/comp-unit.c
+++ b/gdb/dwarf2/comp-unit.c
@@ -75,11 +75,12 @@ read_comp_unit_head (struct comp_unit_head *cu_header,
   cu_header->initial_length_size = bytes_read;
   cu_header->offset_size = (bytes_read == 4) ? 4 : 8;
   info_ptr += bytes_read;
-  cu_header->version = read_2_bytes (abfd, info_ptr);
-  if (cu_header->version < 2 || cu_header->version > 5)
+  unsigned version = read_2_bytes (abfd, info_ptr);
+  if (version < 2 || version > 5)
     error (_("Dwarf Error: wrong version in compilation unit header "
 	   "(is %d, should be 2, 3, 4 or 5) [in module %s]"),
-	   cu_header->version, filename);
+	   version, filename);
+  cu_header->version = version;
   info_ptr += 2;
   if (cu_header->version < 5)
     switch (section_kind)
diff --git a/gdb/dwarf2/comp-unit.h b/gdb/dwarf2/comp-unit.h
index 35bca8f8e9d..09ffac8d2da 100644
--- a/gdb/dwarf2/comp-unit.h
+++ b/gdb/dwarf2/comp-unit.h
@@ -35,7 +35,7 @@
 struct comp_unit_head
 {
   unsigned int length;
-  short version;
+  unsigned char version;
   unsigned char addr_size;
   unsigned char signed_addr_p;
   sect_offset abbrev_sect_off;
@@ -48,14 +48,16 @@ struct comp_unit_head
 
   enum dwarf_unit_type unit_type;
 
-  /* Offset to the first byte of this compilation unit header in the
-     .debug_info section, for resolving relative reference dies.  */
-  sect_offset sect_off;
-
   /* Offset to first die in this cu from the start of the cu.
      This will be the first byte following the compilation unit header.  */
   cu_offset first_die_cu_offset;
 
+  /* Offset to the first byte of this compilation unit header in the
+     .debug_info section, for resolving relative reference dies.  */
+  sect_offset sect_off;
+
+  /* For types, offset in the type's DIE of the type defined by this TU.  */
+  cu_offset type_cu_offset_in_tu;
 
   /* 64-bit signature of this unit. For type units, it denotes the signature of
      the type (DW_UT_type in DWARF 4, additionally DW_UT_split_type in DWARF 5).
@@ -63,9 +65,6 @@ struct comp_unit_head
      DW_UT_skeleton or DW_UT_split_compile.  */
   ULONGEST signature;
 
-  /* For types, offset in the type's DIE of the type defined by this TU.  */
-  cu_offset type_cu_offset_in_tu;
-
   /* Return the total length of the CU described by this header.  */
   unsigned int get_length () const
   {
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index a0d76f349e8..f358c562fdf 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -433,7 +433,7 @@ struct dwarf2_per_cu_data
   unsigned int length;
 
   /* DWARF standard version this data has been read from (such as 4 or 5).  */
-  short dwarf_version;
+  unsigned char dwarf_version;
 
   /* Flag indicating this compilation unit will be read in before
      any of the current compilation units are processed.  */
@@ -469,6 +469,18 @@ struct dwarf2_per_cu_data
      This flag is only valid if is_debug_types is true.  */
   unsigned int tu_read : 1;
 
+  /* True if HEADER has been read in.
+
+     Don't access this field directly.  It should be private, but we can't make
+     it private at the moment.  */
+  mutable bool m_header_read_in : 1;
+
+  /* The unit type of this CU.  */
+  ENUM_BITFIELD (dwarf_unit_type) unit_type : 8;
+
+  /* The language of this CU.  */
+  ENUM_BITFIELD (language) lang : LANGUAGE_BITS;
+
   /* Our index in the unshared "symtabs" vector.  */
   unsigned index;
 
@@ -477,12 +489,6 @@ struct dwarf2_per_cu_data
      not the DWO file.  */
   struct dwarf2_section_info *section;
 
-  /* The unit type of this CU.  */
-  enum dwarf_unit_type unit_type;
-
-  /* The language of this CU.  */
-  enum language lang;
-
   /* Backlink to the owner of this.  */
   dwarf2_per_bfd *per_bfd;
 
@@ -495,12 +501,6 @@ struct dwarf2_per_cu_data
      should be private, but we can't make it private at the moment.  */
   mutable comp_unit_head m_header;
 
-  /* True if HEADER has been read in.
-
-     Don't access this field directly.  It should be private, but we can't make
-     it private at the moment.  */
-  mutable bool m_header_read_in;
-
   /* When dwarf2_per_bfd::using_index is true, the 'quick' field
      is active.  Otherwise, the 'psymtab' field is active.  */
   union
-- 
2.17.2