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    SubjectRe: [PATCH] ARM: Ensure that NEON code always compiles with Clang
    On Mon, Dec 17, 2018 at 01:23:52PM -0500, Nicolas Pitre wrote:
    > On Sat, 15 Dec 2018, Nathan Chancellor 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>
    >
    > Did you test that this doesn't create issues with gcc e.g. complaints
    > from the linker that objects have incompatible architecture
    > specifications or similar annoyance? This already happened in the past
    > but I forget the exact scenario. If you already did, or after you do
    > validate with gcc as well, then you may add:
    >
    > Acked-by: Nicolas Pitre <nico@linaro.org>
    >
    >

    Hi Nicolas,

    I was 99% sure that I checked GCC before sending this but I just did
    another run to confirm and everything links successfully. We still use
    binutils for assembling/linking the kernel so I assume that I would have
    seen a warning from ld.bfd even with Clang.

    Thank you for the review!
    Nathan

    >
    > > ---
    > > 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
    > >
    > >

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