On Thu, 12 Feb 2015, Richard Henderson wrote: > When we fixed PR54005, Hm, there's confusion. When was this fixed? (Not fixed AFAICT.) Maybe you mean PR54004, but there was no note there either. Or maybe there's a typo and you meant some other PR and that PR54005 is supposedly fixed by this patch (committed as r221701)
...but it doesn't seem right: you use a specific object when deducing the alignment for the fake-pointer, so it's used anyway and is_lock_free must not be object-specific (despite its name) and only type-specific as mandated by the standard (see PR). To wit, deduce from the known-alignment of _Tp, not known-alignment of _M_i. Or is this what happens; they're the same? Why then use __alignof(_M_i) (the object-alignment) instead of _S_alignment (the deduced alas insufficiently increased type-alignment)? > making sure that atomic_is_lock_free returns the same > value for all objects of a given type, (That would work but it doesn't seem to be the case.) > we probably should have changed the > interface so that we would pass size and alignment rather than size and object > pointer. > > Instead, we decided that passing null for the object pointer would be > sufficient. But as this PR shows, we really do need to take alignment into > account. Regarding what's actually needed, alignment of an atomic type should always be *forced to be at least the natural alignment of of the object* (with non-power-of-two sized-objects rounded up) and until then atomic types won't work for targets where the non-atomic equivalents have less alignment (as straddling a page-boundary won't be lock-less-atomic anywhere where straddling a page-boundary may cause a non-atomic-access...) So, not target-specific except for targets that require even higher-than-natural alignment. > The following patch constructs a fake object pointer that is maximally > misaligned. (With regards to the known object alignment of the _M_i object.) > This allows the interface to both the builtin and to libatomic to > remain unchanged. Which probably makes this back-portable to maintenance > releases as well. > > I believe that for all of our current systems, size_t == uintptr_t, so the > reinterpret_cast ought not generate warnings. > > The test case is problematic, as there's currently no good place to put it. > The libstdc++ testsuite doesn't have the libatomic library path configured, > and > the libatomic testsuite doesn't have the libstdc++ include paths configured. > Yet another example where we really need an install tree for testing. > Thoughts? brgds, H-P
