From: Eli Zaretskii <eliz@gnu.org>
To: emacs-devel@gnu.org
Subject: Packing of union fields and bool_bf
Date: Sat, 13 Oct 2018 18:15:25 +0300 [thread overview]
Message-ID: <838t31vpwy.fsf@gnu.org> (raw)
While working on bug#33017, I found out something unexpected: using
bool_bf in union members bloats the union.
A case in point is this part of 'struct glyph' (I removed the comments
for brevity):
/* A union of sub-structures for different glyph types. */
union
{
unsigned ch;
struct
{
bool_bf automatic : 1;
unsigned id : 31;
} cmp;
int img_id;
#ifdef HAVE_XWIDGETS
struct xwidget *xwidget;
#endif
struct
{
unsigned height : 16;
unsigned ascent : 16;
}
stretch;
struct
{
unsigned method : 2;
bool_bf for_no_font : 1;
unsigned len : 4;
unsigned ch : 25;
} glyphless;
unsigned val;
} u;
Here's how this is compiled with 32-bit MinGW GCC 7.3.0, according to
GDB:
(gdb) ptype /o desired_glyph->u
/* offset | size */ type = union {
/* 4 */ unsigned int ch;
/* 8 */ struct {
/* 0: 7 | 1 */ bool_bf automatic : 1;
/* XXX 7-bit hole */
/* XXX 3-byte hole */
/* 4: 1 | 4 */ unsigned int id : 31;
/* XXX 1-bit padding */
/* total size (bytes): 8 */
} cmp;
/* 4 */ int img_id;
/* 4 */ struct {
/* 0:16 | 4 */ unsigned int height : 16;
/* 0: 0 | 4 */ unsigned int ascent : 16;
/* total size (bytes): 4 */
} stretch;
/* 12 */ struct {
/* 0:30 | 4 */ unsigned int method : 2;
/* XXX 6-bit hole */
/* XXX 3-byte hole */
/* 4: 7 | 1 */ bool_bf for_no_font : 1;
/* XXX 7-bit hole */
/* XXX 3-byte hole */
/* 8:28 | 4 */ unsigned int len : 4;
/* 8: 3 | 4 */ unsigned int ch : 25;
/* XXX 3-bit padding */
/* total size (bytes): 12 */
} glyphless;
/* 4 */ unsigned int val;
/* total size (bytes): 12 */
}
Note the difference between handling 'unsigned int' fields and
'bool_bf' fields: the former are nicely packed, whereas the latter are
not. Which causes the size of union to be 12 bytes, instead of the
expected 4. The 'bool' type is 1-byte wide on this platform.
Is this expected? Should we continue using bool_bf in these cases?
next reply other threads:[~2018-10-13 15:15 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-13 15:15 Eli Zaretskii [this message]
2018-10-13 15:58 ` Packing of union fields and bool_bf Tom Tromey
2018-10-13 16:22 ` Paul Eggert
2018-10-13 16:54 ` Eli Zaretskii
2018-10-13 16:43 ` Eli Zaretskii
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