On Wed, Sep 02, 2020 at 08:19:25PM +0200, Miguel Ojeda wrote: > On Wed, Sep 2, 2020 at 5:33 PM Arvind Sankar <[email protected]> wrote: > > > > + * The compiler should not reorder volatile asm, however older versions of > > GCC > > + * had a bug (which was fixed in 8.1, 7.3 and 6.5) where they could > > sometimes > > + * reorder volatile asm. The write functions are not a problem since they > > have > > + * memory clobbers preventing reordering. To prevent reads from being > > reordered > > + * with respect to writes, use a dummy memory operand. > > I see you added the information to the commit message, but I'd still > reword this to something like:
Ah sorry, I forgot to change the comment in the source as well. > > "The compiler should not reorder volatile asm, however GCC 4.9.x and > 5.x have a bug where they could sometimes reorder volatile asm. The > bug was fixed in 8.1, 7.3 and 6.5. The write functions are not a > problem since they have memory clobbers preventing reordering. To > prevent reads from being reordered with respect to writes, use a dummy > memory operand." > > The important point is that 4.9.x and 5.x *have* the bug and that is > the reason for having the hack. In the old wording it seems like the > bug is no more. Then one wonders why the hack is still there (i.e. > perhaps because we don't trust it, perhaps to support the rest of the > minor versions which are newer, perhaps to avoid regressions, perhaps > only the comment was updated, etc.). > > Cheers, > Miguel
