Re: [PATCH] ARM: Ensure that NEON code always compiles with Clang

[Nick Desaulniers <ndesaulniers@google.com>]

On Sat, Dec 15, 2018 at 1:23 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> While building arm32 allyesconfig, I ran into the following errors:
>
>   arch/arm/lib/xor-neon.c:17:2: error: You should compile this file with
>   '-mfloat-abi=softfp -mfpu=neon'
>
>   In file included from lib/raid6/neon1.c:27:
>   /home/nathan/cbl/prebuilt/lib/clang/8.0.0/include/arm_neon.h:28:2:
>   error: "NEON support not enabled"
>
> Building V=1 showed NEON_FLAGS getting passed along to Clang but
> __ARM_NEON__ was not getting defined. Ultimately, it boils down to Clang
> only defining __ARM_NEON__ when targeting armv7, rather than armv6k,
> which is the '-march' value for allyesconfig.
>
> From lib/Basic/Targets/ARM.cpp in the Clang source:
>
>   // This only gets set when Neon instructions are actually available, unlike
>   // the VFP define, hence the soft float and arch check. This is subtly
>   // different from gcc, we follow the intent which was that it should be set
>   // when Neon instructions are actually available.
>   if ((FPU & NeonFPU) && !SoftFloat && ArchVersion >= 7) {
>     Builder.defineMacro("__ARM_NEON", "1");
>     Builder.defineMacro("__ARM_NEON__");
>     // current AArch32 NEON implementations do not support double-precision
>     // floating-point even when it is present in VFP.
>     Builder.defineMacro("__ARM_NEON_FP",
>                         "0x" + Twine::utohexstr(HW_FP & ~HW_FP_DP));
>   }
>
> Ard Biesheuvel recommended explicitly adding '-march=armv7-a' at the
> beginning of the NEON_FLAGS definitions so that __ARM_NEON__ always gets
> definined by Clang. This doesn't functionally change anything because
> that code will only run where NEON is supported, which is implicitly
> armv7.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/287
> Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Nathan,
Thanks for sending the patch, and thanks to Ard for the suggestion.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  Documentation/arm/kernel_mode_neon.txt | 4 ++--
>  arch/arm/lib/Makefile                  | 2 +-
>  arch/arm/lib/xor-neon.c                | 2 +-
>  lib/raid6/Makefile                     | 2 +-
>  4 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/arm/kernel_mode_neon.txt b/Documentation/arm/kernel_mode_neon.txt
> index 525452726d31..b9e060c5b61e 100644
> --- a/Documentation/arm/kernel_mode_neon.txt
> +++ b/Documentation/arm/kernel_mode_neon.txt
> @@ -6,7 +6,7 @@ TL;DR summary
>  * Use only NEON instructions, or VFP instructions that don't rely on support
>    code
>  * Isolate your NEON code in a separate compilation unit, and compile it with
> -  '-mfpu=neon -mfloat-abi=softfp'
> +  '-march=armv7-a -mfpu=neon -mfloat-abi=softfp'
>  * Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
>    NEON code
>  * Don't sleep in your NEON code, and be aware that it will be executed with
> @@ -87,7 +87,7 @@ instructions appearing in unexpected places if no special care is taken.
>  Therefore, the recommended and only supported way of using NEON/VFP in the
>  kernel is by adhering to the following rules:
>  * isolate the NEON code in a separate compilation unit and compile it with
> -  '-mfpu=neon -mfloat-abi=softfp';
> +  '-march=armv7-a -mfpu=neon -mfloat-abi=softfp';
>  * issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
>    into the unit containing the NEON code from a compilation unit which is *not*
>    built with the GCC flag '-mfpu=neon' set.
> diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
> index ad25fd1872c7..0bff0176db2c 100644
> --- a/arch/arm/lib/Makefile
> +++ b/arch/arm/lib/Makefile
> @@ -39,7 +39,7 @@ $(obj)/csumpartialcopy.o:     $(obj)/csumpartialcopygeneric.S
>  $(obj)/csumpartialcopyuser.o:  $(obj)/csumpartialcopygeneric.S
>
>  ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
> -  NEON_FLAGS                   := -mfloat-abi=softfp -mfpu=neon
> +  NEON_FLAGS                   := -march=armv7-a -mfloat-abi=softfp -mfpu=neon
>    CFLAGS_xor-neon.o            += $(NEON_FLAGS)
>    obj-$(CONFIG_XOR_BLOCKS)     += xor-neon.o
>  endif
> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> index a6741a895189..4600b62d845f 100644
> --- a/arch/arm/lib/xor-neon.c
> +++ b/arch/arm/lib/xor-neon.c
> @@ -14,7 +14,7 @@
>  MODULE_LICENSE("GPL");
>
>  #ifndef __ARM_NEON__
> -#error You should compile this file with '-mfloat-abi=softfp -mfpu=neon'
> +#error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
>  #endif
>
>  /*
> diff --git a/lib/raid6/Makefile b/lib/raid6/Makefile
> index 2f8b61dfd9b0..bfec7c87c61e 100644
> --- a/lib/raid6/Makefile
> +++ b/lib/raid6/Makefile
> @@ -25,7 +25,7 @@ endif
>  ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
>  NEON_FLAGS := -ffreestanding
>  ifeq ($(ARCH),arm)
> -NEON_FLAGS += -mfloat-abi=softfp -mfpu=neon
> +NEON_FLAGS += -march=armv7-a -mfloat-abi=softfp -mfpu=neon
>  endif
>  CFLAGS_recov_neon_inner.o += $(NEON_FLAGS)
>  ifeq ($(ARCH),arm64)
> --
> 2.20.1
>


-- 
Thanks,
~Nick Desaulniers