Zbigniew Jędrzejewski-Szmek
519ae503e3
It's the same old story: 'struct sd_bus' is generally instantiated once, so
bitfields, for which we pay with more complicated code in all users of this
struct, are counterproductive. In some progs the structure may be instantiated
a few times, but it's still not worth it because we save a few bytes of memory
in one place and pay for this with many more bytes in the code.
$ size build/libsystemd.so.0.39.0{.orig,}
text data bss dec hex filename
2452757 65376 3768 2521901 267b2d build/libsystemd.so.0.39.0.orig
2451669 65376 3768 2520813 2676ed build/libsystemd.so.0.39.0
$ diff -u <(pahole build/libsystemd.so.0.39.0.orig) <(pahole build/libsystemd.so.0.39.0)
...
- /* size: 1960, cachelines: 31, members: 105 */
- /* sum members: 1944, holes: 3, sum holes: 9 */
- /* sum bitfield members: 25 bits, bit holes: 2, sum bit holes: 31 bits */
+ /* size: 1984, cachelines: 31, members: 105 */
+ /* sum members: 1971, holes: 4, sum holes: 13 */
/* member types with holes: 1, total: 1 */
i.e. 2452757 - 2451669 = 1088 extra bytes of code and slower execution, to save
24 bytes of memory per instance of the struct. (But the number of cachelines
doesn't change, so the smaller struct most likely has no effect on memory
access, and the alignment of the struct most likely means that the memory
saving is illusory too, we just end up with a few bytes of padding after the
struct.)
In the other structs, the alignment prevent the bitfield for having any effect
on memory use, but the compiler would still generate more complicated code,
i.e. we pay something for nothing.
For example:
$ diff -u <(pahole build/libsystemd.so.0.39.0.orig) <(pahole build/libsystemd.so.0.39.0)
...
struct node_callback {
struct node * node; /* 0 8 */
- _Bool is_fallback:1; /* 8: 0 1 */
+ _Bool is_fallback; /* 8 1 */
- /* XXX 7 bits hole, try to pack */
/* XXX 3 bytes hole, try to pack */
unsigned int last_iteration; /* 12 4 */
@@ -455,15 +448,13 @@
struct node_callback * callbacks_prev; /* 32 8 */
/* size: 40, cachelines: 1, members: 6 */
- /* sum members: 36, holes: 1, sum holes: 3 */
- /* sum bitfield members: 1 bits, bit holes: 1, sum bit holes: 7 bits */
+ /* sum members: 37, holes: 1, sum holes: 3 */
/* last cacheline: 40 bytes */
};
I kept the bitfield in sd_bus_slot because it prevents the struct from growing
from 112 to 120 bytes by reducing the alignment requirement for subsequent
fields, and we potentially can have this instantiated many times.
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