On Sun, Sep 2, 2012 at 4:05 AM, David Brown <david.br...@hesbynett.no> wrote:
> I think the nearest you could get would be to implement built_in
> functions such as __built_in_bis(dst, src) for an atomic "|=".
This is exactly the right thing. It's pretty common to do code like this:
flags |= (a bunch of code);
The goal is to set a bit in the shared flags register. If an ISR tries
to toggle other bits at the same time, those changes would be
clobbered unless the "|=" operator is atomic. The stuff on the right
doesn't write to the shared location, so it does not need to be
atomic.
My original example contained a red herring, in that it also read from
the shared location:
state |= (state >> 1) & DIRTY_FLAG;
However, as I said before, I do *NOT* expect the right-hand side to be
atomic. Only the "|=" operator needs to be atomic. This particular
example is copying the to the left of DIRTY_FLAG into DIRTY_FLAG. The
bit to the left is stable at this particular point in time, so there
is no need to treat it specially.
Therefore, the following code expresses the intent perfectly:
U8 temp = (state >> 1) & DIRTY_FLAG;
__atomic_bis(state, temp);
Since we don't have __atomic_bis or anything like it, I have been
combing through my codebase and eliminating anything like this. I'm
only about half done.
On Sun, Sep 2, 2012 at 6:43 AM, Przemek Klosowski
<przemek.klosow...@gmail.com> wrote:
> It seems to me that a better approach is to admit the reality that
> it's a hardware specific operation, and express it via the appropriate
> asm(), perhaps with some syntactic sugar of function/library calls.
Inline assembly blocks interact poorly with the surrounding C code, so
the atomic primitives really need to be intrinsic functions.
Atomic intrinsics are pretty common, even if they are
hardware-dependent. GCC has them on other targets, the Microsoft
compiler has them, Glib has them, and the C++11 actually bakes them
right into the standard library. It's weird that mspgcc doesn't have
them, although I understand that our noble maintainer has enough on
his plate as it is.
I wonder if it would be a more efficient use of my time to just add
__atomic_bis to the compiler myself, rather than re-design my entire
firmware.
-William
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