RE: [PATCH v2 4/6] bitops: define const_*() versions of the non-atomics

David Laight
2022-06-15T15:52:25Z

From: Alexander Lobakin
> Sent: 15 June 2022 14:55
...
> > > +/**
> > > + * const_test_bit - Determine whether a bit is set
> > > + * @nr: bit number to test
> > > + * @addr: Address to start counting from
> > > + *
> > > + * A version of generic_test_bit() which discards the `volatile` qualifier to
> > > + * allow the compiler to optimize code harder. Non-atomic and to be used only
> > > + * for testing compile-time constants, e.g. from the corresponding macro, or
> > > + * when you really know what you are doing.
> >
> > Not sure I understand the last sentence... Can you please rephrase?
>
> I basically want to tell that there potentinally might be cases for
> using those outside of the actual macros from 6/6. But it might be
> redundant at all to mention this.

I bet that is a function has:
long bitmask;
...
if (test_bit(&bitmask, 12))
then the 'volatile' forces the compiler to actually write the
value out to memory (stack) instead of doing a register op.

OTOH such code should be using &.

David

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