On Sat, Sep 24, 2022 at 02:26:52AM +1000, Nicholas Piggin wrote:
> I still don't see what clauses guarantees asm("%0" ::"r"(foo)) to give
> 13. It doesn't say access via inline assembly is special,But it is. It is for all register variables, local and global. I agree this isn't documented clearly. For local register variables this is the *only* thing guaranteed; for global register vars there is more (it changes the ABI, there are safe/restore effects, that kind of thing). Never it is guaranteed that all accesses through this variable will use the register directly: this fundamentally cannot work on all archs, and also not at -O0. More in general it doesn't work if some basic optimisations are not done, be it because of a compiler deficiency, or a straight out bug, or maybe it is a conscious choice in some cases. > I think if it was obviously guaranteed then this might be marginally > better than explicit r13 in the asm > > asm volatile( > "stb %0,%2(%1)" > : > : "r" (mask), > "r" (local_paca), > "i" (offsetof(struct paca_struct, irq_soft_mask)) > : "memory"); (Please use "n" instead of "i". Doesn't matter here, but it does in many other places.) Segher
