Hello!
I recently discovered a problem that affects my entire code base. I
often do things like this:
volatile unsigned char state;
...
state |= (state >> 1) & DIRTY_FLAG;
Since ISR's might be trying to set DIRTY_FLAG at the same time, it is
very important that the final "|=" instruction happens atomically,
like with a "BIS.B foo, &state" instruction. This was my original
expectation, but the compiler apparently generates code like this
instead:
MOV.B &state, R15
MOV.B &state, R14
RRUM #0x0001, R15
AND.B #0x0002, R15
BIS.B R14, R15
MOV.B R15, &state
As you can see, there is a window of about three instructions where
changes from an ISR would be lost. I could surround this code with
__eint() / __dint() calls, but that seems overkill. I really want the
compiler to generate code like this:
MOV.B &state, R15
RRUM #0x0001, R15
AND.B #0x0002, R15
BIS.B R15, &state
Besides being smaller and faster, this actually does the right thing.
So, what do I do? Short-term options include rewriting the critical
bits in assembly and/or inserting the __eint() / __dint() calls, but I
am really looking for a more elegant long-term solution. Fixing the
code generator would be one option, but adding set of intrinsics like
__atomic_add() would also be nice, since those would document the
intention as well.
-William
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