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117f140c46
The IPC::Message header is surrounded by:
#pragma pack(push,2)
...
#pragma pack(pop)
which (at least on GCC) specifies that structure members defined lexically
within the pragma should be two-byte aligned, rather than the ABI's declared
alignment.
But for IPC::Message::Header, this is a silly requirement, as everything there
is four bytes; there's no reason to pack the members any tighter. And packing
tighter means that strict alignment platforms (like ARM) need to use more
complex code for something as simple as storing to one of the members--like
when we set a message's request ID, over and over and over. The current code
for setting a message's request ID on ARM looks like:
264: 6863 ldr r3, [r4, #4]
266: 696a ldr r2, [r5, #20]
268: 809a strh r2, [r3, #4]
26a: 0c12 lsrs r2, r2, #16
26c: 80da strh r2, [r3, #6]
With the patch, it looks like:
264: 6863 ldr r3, [r4, #4]
266: 696a ldr r2, [r5, #20]
268: 605a str r2, [r3, #4]
Only four bytes, but multiplied over several hundred set_routing_id calls, it
saves some code size and runtime. I verified that the header's length doesn't
change by looking at debug information.
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