linux-kernel - Re: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization

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Message-ID: <CAKwvOdkodob0M0r_AK_4nG3atLGMyNENMd6qVAHSPa92Zh7UZA@mail.gmail.com>
Date:   Fri, 6 Nov 2020 11:52:46 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Russell King <linux@...linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        Collabora Kernel ML <kernel@...labora.com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization

On Fri, Nov 6, 2020 at 3:50 AM Adrian Ratiu <adrian.ratiu@...labora.com> wrote:
>
> Hi Nathan,
>
> On Fri, 06 Nov 2020, Nathan Chancellor <natechancellor@...il.com>
> wrote:
> > + Ard, who wrote this code.
> >
> > On Fri, Nov 06, 2020 at 07:14:36AM +0200, Adrian Ratiu wrote:
> >> Due to a Clang bug [1] neon autoloop vectorization does not
> >> happen or happens badly with no gains and considering previous
> >> GCC experiences which generated unoptimized code which was
> >> worse than the default asm implementation, it is safer to
> >> default clang builds to the known good generic implementation.
> >> The kernel currently supports a minimum Clang version of
> >> v10.0.1, see commit 1f7a44f63e6c ("compiler-clang: add build
> >> check for clang 10.0.1").   When the bug gets eventually fixed,
> >> this commit could be reverted or, if the minimum clang version
> >> bump takes a long time, a warning could be added for users to
> >> upgrade their compilers like was done for GCC.   [1]
> >> https://bugs.llvm.org/show_bug.cgi?id=40976  Signed-off-by:
> >> Adrian Ratiu <adrian.ratiu@...labora.com>
> >
> > Thank you for the patch! We are also tracking this here:
> >
> > https://github.com/ClangBuiltLinux/linux/issues/496
> >
> > It was on my TODO to revist getting the warning eliminated,
> > which likely would have involved a patch like this as well.
> >
> > I am curious if it is worth revisting or dusting off Arnd's
> > patch in the LLVM bug tracker first. I have not tried it
> > personally. If that is not a worthwhile option, I am fine with
> > this for now. It would be nice to try and get a fix pinned down
> > on the LLVM side at some point but alas, finite amount of
> > resources and people :(
>
> I tested Arnd's kernel patch from the LLVM bugtracker [1], but
> with the Clang v10.0.1 I still get warnings like the following
> even though the __restrict workaround seems to affect the
> generated instructions:
>
> ./include/asm-generic/xor.h:15:2: remark: the cost-model indicates
> that interleaving is not beneficial [-Rpass-missed=loop-vectorize]
> ./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 *__restrict p1, unsigned long *__restrict p2)

If it's just a matter of overruling the cost model
#pragma clang loop vectorize(enable)

will do the trick.

Indeed,
```
diff --git a/include/asm-generic/xor.h b/include/asm-generic/xor.h
index b62a2a56a4d4..8796955498b7 100644
--- a/include/asm-generic/xor.h
+++ b/include/asm-generic/xor.h
@@ -12,6 +12,7 @@ xor_8regs_2(unsigned long bytes, unsigned long *p1,
unsigned long *p2)
 {
        long lines = bytes / (sizeof (long)) / 8;

+#pragma clang loop vectorize(enable)
        do {
                p1[0] ^= p2[0];
                p1[1] ^= p2[1];
@@ -32,6 +33,7 @@ xor_8regs_3(unsigned long bytes, unsigned long *p1,
unsigned long *p2,
 {
        long lines = bytes / (sizeof (long)) / 8;

+#pragma clang loop vectorize(enable)
        do {
                p1[0] ^= p2[0] ^ p3[0];
                p1[1] ^= p2[1] ^ p3[1];
@@ -53,6 +55,7 @@ xor_8regs_4(unsigned long bytes, unsigned long *p1,
unsigned long *p2,
 {
        long lines = bytes / (sizeof (long)) / 8;

+#pragma clang loop vectorize(enable)
        do {
                p1[0] ^= p2[0] ^ p3[0] ^ p4[0];
                p1[1] ^= p2[1] ^ p3[1] ^ p4[1];
@@ -75,6 +78,7 @@ xor_8regs_5(unsigned long bytes, unsigned long *p1,
unsigned long *p2,
 {
        long lines = bytes / (sizeof (long)) / 8;

+#pragma clang loop vectorize(enable)
        do {
                p1[0] ^= p2[0] ^ p3[0] ^ p4[0] ^ p5[0];
                p1[1] ^= p2[1] ^ p3[1] ^ p4[1] ^ p5[1];
```
seems to generate the vectorized code.

Why don't we find a way to make those pragma's more toolchain
portable, rather than open coding them like I have above rather than
this series?

-- 
Thanks,
~Nick Desaulniers