b6fba48f0f
sd_bus_message carries an array of header field offsets and a counter,
but their only reader was part of the D-Bus v2/GVariant sealing path
removed in 0dd4876815 ("sd-bus: drop D-Bus version 2 format support").
There are three remaining callers of message_extend_fields(), for
SD_BUS_MESSAGE_HEADER_DESTINATION, _PATH, and _INTERFACE, and all of
them pass false for add_offset. The receive path does not populate the
array either, so nothing reads or writes it any more.
Let's drop the two fields and the now unused add_offset argument and
branch.
This reduces sizeof(sd_bus_message) from 792...
$ gdb -batch build-baseline/test-bus-benchmark \
-ex 'ptype /o struct sd_bus_message'
...
/* 688 | 8 */ usec_t timeout;
/* 696 | 80 */ size_t header_offsets[10];
/* 776 | 4 */ unsigned int n_header_offsets;
/* XXX 4-byte hole */
/* 784 | 8 */ uint64_t read_counter;
/* total size (bytes): 792 */
to 704 bytes:
$ gdb -batch build-patched/test-bus-benchmark \
-ex 'ptype /o struct sd_bus_message'
...
/* 688 | 8 */ usec_t timeout;
/* 696 | 8 */ uint64_t read_counter;
/* total size (bytes): 704 */
On Fedora 43 aarch64 with glibc 2.42, the usable allocation for a
method-call message falls from 808 to 728 bytes, a reduction of 9.9%.
In my tests, retaining 400,000 method-call messages reduces in median
peak RSS from 363M to 332M.
Using `test-bus-benchmark chart direct 500ms` across message sizes from
1 byte to 2 MiB one can also see things are around 1% faster, which is
another nice incidental boost.
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