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

[Nathan Chancellor]

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@xxxxxxxxxx>
> > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> 
> 

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
> > 
> >