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