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
