Messages in this thread | | | Date | Fri, 6 Nov 2020 03:14:19 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH 2/2] arm: lib: xor-neon: disable clang vectorization |
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+ 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@collabora.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 :(
Should no other options come to fruition from further discussions, you can carry my tag forward:
Acked-by: Nathan Chancellor <natechancellor@gmail.com>
Hopefully others can comment soon.
> --- > arch/arm/include/asm/xor.h | 3 ++- > arch/arm/lib/Makefile | 3 +++ > arch/arm/lib/xor-neon.c | 4 ++++ > 3 files changed, 9 insertions(+), 1 deletion(-) > > diff --git a/arch/arm/include/asm/xor.h b/arch/arm/include/asm/xor.h > index aefddec79286..49937dafaa71 100644 > --- a/arch/arm/include/asm/xor.h > +++ b/arch/arm/include/asm/xor.h > @@ -141,7 +141,8 @@ static struct xor_block_template xor_block_arm4regs = { > NEON_TEMPLATES; \ > } while (0) > > -#ifdef CONFIG_KERNEL_MODE_NEON > +/* disabled on clang/arm due to https://bugs.llvm.org/show_bug.cgi?id=40976 */ > +#if defined(CONFIG_KERNEL_MODE_NEON) && !defined(CONFIG_CC_IS_CLANG) > > extern struct xor_block_template const xor_block_neon_inner; > > diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile > index 6d2ba454f25b..53f9e7dd9714 100644 > --- a/arch/arm/lib/Makefile > +++ b/arch/arm/lib/Makefile > @@ -43,8 +43,11 @@ endif > $(obj)/csumpartialcopy.o: $(obj)/csumpartialcopygeneric.S > $(obj)/csumpartialcopyuser.o: $(obj)/csumpartialcopygeneric.S > > +# disabled on clang/arm due to https://bugs.llvm.org/show_bug.cgi?id=40976 > +ifndef CONFIG_CC_IS_CLANG > ifeq ($(CONFIG_KERNEL_MODE_NEON),y) > NEON_FLAGS := -march=armv7-a -mfloat-abi=softfp -mfpu=neon > CFLAGS_xor-neon.o += $(NEON_FLAGS) > obj-$(CONFIG_XOR_BLOCKS) += xor-neon.o > endif > +endif > diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c > index e1e76186ec23..84c91c48dfa2 100644 > --- a/arch/arm/lib/xor-neon.c > +++ b/arch/arm/lib/xor-neon.c > @@ -18,6 +18,10 @@ MODULE_LICENSE("GPL"); > * Pull in the reference implementations while instructing GCC (through > * -ftree-vectorize) to attempt to exploit implicit parallelism and emit > * NEON instructions. > + > + * On Clang the loop vectorizer is enabled by default, but due to a bug > + * (https://bugs.llvm.org/show_bug.cgi?id=40976) vectorization is broke > + * so xor-neon is disabled in favor of the default reg implementations. > */ > #ifdef CONFIG_CC_IS_GCC > #pragma GCC optimize "tree-vectorize" > -- > 2.29.0 >
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