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SubjectRe: [PATCH v2] powerpc: slightly improve cache helpers
On Mon, Jul 22, 2019 at 08:15:14PM +1000, Michael Ellerman wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote:
> >> 0000017c clear_user_page:
> >> 17c: 94 21 ff f0 stwu 1, -16(1)
> >> 180: 38 80 00 80 li 4, 128
> >> 184: 38 63 ff e0 addi 3, 3, -32
> >> 188: 7c 89 03 a6 mtctr 4
> >> 18c: 38 81 00 0f addi 4, 1, 15
> >> 190: 8c c3 00 20 lbzu 6, 32(3)
> >> 194: 98 c1 00 0f stb 6, 15(1)
> >> 198: 7c 00 27 ec dcbz 0, 4
> >> 19c: 42 00 ff f4 bdnz .+65524
> >
> > Uh, yeah, well, I have no idea what clang tried here, but that won't
> > work. It's copying a byte from each target cache line to the stack,
> > and then does clears the cache line containing that byte on the stack.
>
> So it seems like this is a clang bug.
>
> None of the distros we support use clang, but we would still like to
> keep it working if we can.

Which version? Which versions *are* broken?

> Looking at the original patch, the only upside is that the compiler
> can use both RA and RB to compute the address, rather than us forcing RA
> to 0.
>
> But at least with my compiler here (GCC 8 vintage) I don't actually see
> GCC ever using both GPRs even with the patch. Or at least, there's no
> difference before/after the patch as far as I can see.

The benefit is small, certainly.

> So my inclination is to revert the original patch. We can try again in a
> few years :D
>
> Thoughts?

I think you should give the clang people time to figure out what is
going on.


Segher

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