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* [9fans] non-interruptible temporary: why do we care?
@ 2021-01-04  3:32 ori
  0 siblings, 0 replies; only message in thread
From: ori @ 2021-01-04  3:32 UTC (permalink / raw)
  To: 9fans

Our compilers will sometimes produce a warning about
a non-interruptible temporary:

        #include <u.h>
        #include <libc.h>
        void
        f(vlong a)
        {
                vlong x, y, z;
                x = y = z = a;
        }

        cpu% 5c -w test.c
        warning: /fd/0:7 non-interruptable temporary
        warning: /fd/0:7 non-interruptable temporary

I know this means that the intermediate values
cannot be assigned atomically, but I don't see
why this rates a warning. From what I understand,
the uninterruptability of the temporaries should
be unobservable as long as our note handlers
save and restore registers correctly.

The only case I can think of is that a note
handler may see "tearing" of a register, but
that doesn't require a non-interruptible
temporary -- this kind of code will suffice:

        vlong x;
        int
        use_x(){ return x; }
        void
        main(void)
        {
                atnotify(use_x, 1);
                while(1)
                        x++;
        }

Why do we warn about non-interruptible
temporaries? What issues am I missing?

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