Re: [RFC] LKMM: Add volatile_if()

Rasmus Villemoes
2021-06-06T23:39:24Z

On 07/06/2021 00.38, Linus Torvalds wrote:

> Example: variable_test_bit(), which generates a "bt" instruction, does
>
> : "m" (*(unsigned long *)addr), "Ir" (nr) : "memory");
>
> and the memory clobber is obviously wrong: 'bt' only *reads* memory,
> but since the whole reason we use it is that it's not just that word
> at address 'addr', in order to make sure that any previous writes are
> actually stable in memory, we use that "memory" clobber.
>
> It would be much nicer to have a "memory read" marker instead, to let
> the compiler know "I need to have done all pending writes to memory,
> but I can still cache read values over this op because it doesn't
> _change_ memory".
>
> Anybody have ideas or suggestions for something like that?

The obvious thing is to try and mark the function as pure. But when
applied to a static inline, gcc seems to read the contents and say "nah,
you have something here that declares itself to possibly write to
memory". Replacing with a call to an extern function marked pure does
indeed cause gcc to cache the value of y*z, so in theory this should be
possible, if one could convince gcc to "trust me, this really is a pure
function".

https://godbolt.org/z/s4546K6Pj

Rasmus