Re: [9fans] structure allocation.

[Ronald G Minnich <rminnich@lanl.gov>]

geoff@collyer.net wrote:
> I suspect that the difference in time to access unaligned packed data
> vs. aligned data is irrelevant when compared with the general
> complexity of running a modern host adaptor.
>

It can get worse. Clever use of align and packed resulted, in Xen, in
longword accesses which were odd-byte-aligned half the time, odd-short
aligned 1/4 the time; i.e., misaligned 3/4 of the time. All this from a
goal to be efficient! I kind of wonder how much this packed/aligned
mania results in unaligned accesses in the end ...

This did get fixed. But it's a real warning against the kind of
cleverness that people resort to with gcc nowadays -- just wander
through the linux kernel sometime -- grep unlikely if you wish.

> It sounds like the folks who designed the dac960 either didn't think
> much about how drivers would access it, or they were hog wild over
> gcc's packed data attribute (does microsoft's compiler have something
> similar?).
>

Packed has been around in compilers for realtime and embedded for 30
years -- dare I admit to using it at some time? Nah. You were none of
you born then anyway, so you can't prove a thing. That code is on a
magtape in a landfill somewhere in Pennsylvania.

ron