On Fri, May 31, 2019 at 8:32 PM Nathan Chancellor
[off-list ref] wrote:

On Fri, May 31, 2019 at 10:05:22AM +0200, Arnd Bergmann wrote:

quoted

If I remember correctly, we also had the same issue with older versions
of clang, possibly even newer ones. Shouldn't we check for a minimum
compiler version when building with clang to ensure that the code is
really vectorized?

       Arnd

Even on tip of tree, it doesn't look like vectorization happens
properly. With -S -Rpass-missed='.*' added to the xor-neon.c command:

/home/nathan/cbl/linux-next/include/asm-generic/xor.h:15:2: remark: the cost-model indicates that interleaving is not beneficial [-Rpass-missed=loop-vectorize]
/home/nathan/cbl/linux-next/include/asm-generic/xor.h:11:1: remark: List vectorization was possible but not beneficial with cost 0 >= 0 [-Rpass-missed=slp-vectorizer]
xor_8regs_2(unsigned long bytes, unsigned long *p1, unsigned long *p2)
^

So right now, it doesn't look like there is a minimum version for clang
and I don't think adding a warning for clang is productive (what is a
user supposed to do?)

I see. If we don't have a vectorized version of the xor module with
clang, I would suggest dropping your patch then, and instead adding
a Kconfig dependency on CC_IS_GCC, until clang is able to do it right.

       Arnd

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