Error compiling files that require __ARM_NEON__ during allyesconfig build

[nathanchance]

When building an arm32 allyesconfig build, this error pops up because __ARM_NEON__ is not defined:

arch/arm/lib/xor-neon.c:17:2: error: You should compile this file with '-mfloat-abi=softfp -mfpu=neon'
#error You should compile this file with '-mfloat-abi=softfp -mfpu=neon'
 ^

These flags are very clearly supplied to Clang:

/usr/bin/ccache clang -Wp,-MD,arch/arm/lib/.xor-neon.o.d  -nostdinc -isystem /home/nathan/cbl/prebuilt/lib/clang/8.0.0/include -I./arch/arm/include -I./arch/arm/include/generated  -I./include -I./arch/arm/include/uapi -I./arch/arm/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__ -mlittle-endian -Qunused-arguments -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 --target=arm-linux-gnueabi --prefix=/home/nathan/cbl/prebuilt/bin/ --gcc-toolchain=/home/nathan/cbl/prebuilt -no-integrated-as -fno-PIE -fno-dwarf2-cfi-asm -mabi=aapcs-linux -mfpu=vfp -funwind-tables -marm -Wa,-mno-warn-deprecated -D__LINUX_ARM_ARCH__=6 -march=armv6k -mtune=arm1136j-s -msoft-float -Uarm -fno-delete-null-pointer-checks -O2 -Wframe-larger-than=1024 -fstack-protector-strong -Wno-format-invalid-specifier -Wno-gnu -Wno-address-of-packed-member -Wno-tautological-compare -mno-global-merge -Wno-unused-const-variable -fno-inline-functions -pg -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -fno-strict-overflow -fno-merge-all-constants -fno-stack-check -Werror=implicit-int -Werror=strict-prototypes -Werror=date-time -Werror=incompatible-pointer-types -Wno-initializer-overrides -Wno-unused-value -Wno-format -Wno-sign-compare -Wno-format-zero-length -Wno-uninitialized -mfloat-abi=softfp -mfpu=neon   -fsanitize-coverage=trace-pc -fsanitize-coverage=trace-cmp    -DKBUILD_BASENAME='"xor_neon"' -DKBUILD_MODNAME='"xor_neon"' -c -o arch/arm/lib/xor-neon.o arch/arm/lib/xor-neon.c

However, it looks like the -march=armv6k is probably the problematic flag, according to lib/Basic/Targets/ARM.cpp in Clang (link):

  // 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));
  }

No idea how to reconcile this.

[nathanchance]

I'm sure this is the same issue:

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"
#error "NEON support not enabled"
 ^

/usr/bin/ccache clang -Wp,-MD,lib/raid6/.neon1.o.d  -nostdinc -isystem /home/nathan/cbl/prebuilt/lib/clang/8.0.0/include -I./arch/arm/include -I./arch/arm/include/generated  -I./include -I./arch/arm/include/uapi -I./arch/arm/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -incl
ude ./include/linux/compiler_types.h -D__KERNEL__ -mlittle-endian -Qunused-arguments -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 --target=arm-linux-gnueabi --prefix=/home/nathan/cbl/prebuilt/bin/ --gcc-toolchain=
/home/nathan/cbl/prebuilt -no-integrated-as -fno-PIE -fno-dwarf2-cfi-asm -mabi=aapcs-linux -mfpu=vfp -funwind-tables -marm -Wa,-mno-warn-deprecated -D__LINUX_ARM_ARCH__=6 -march=armv6k -mtune=arm1136j-s -msoft-float -Uarm -fno-delete-null-pointer-checks -O2 -Wframe-larger-than=1024 -fstack-protector-strong -Wno-format-invalid-specif
ier -Wno-gnu -Wno-address-of-packed-member -Wno-tautological-compare -mno-global-merge -Wno-unused-const-variable -fno-inline-functions -pg -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -fno-strict-overflow -fno-merge-all-constants -fno-stack-check -Werror=implicit-int -Werror=strict-prototypes -Werror=date-time -Werror=inco
mpatible-pointer-types -Wno-initializer-overrides -Wno-unused-value -Wno-format -Wno-sign-compare -Wno-format-zero-length -Wno-uninitialized -ffreestanding -mfloat-abi=softfp -mfpu=neon   -fsanitize-coverage=trace-pc -fsanitize-coverage=trace-cmp    -DKBUILD_BASENAME='"neon1"' -DKBUILD_MODNAME='"raid6_pq"' -c -o lib/raid6/neon1.o li
b/raid6/neon1.c

[nickdesaulniers]

Which config(s) flip this?

[nathanchance]

CONFIG_XOR_BLOCKS for the first one and CONFIG_RAID6_PQ for the second (which gets selected by a few various drivers).

CONFIG_KERNEL_MODE_NEON is required, which is on in multi_v7_defconfig.

[vo4]

allyesconfig has these configs enabled:
CONFIG_CPU_32v6=y CONFIG_CPU_32v6K=y CONFIG_CPU_32v7=y
They specify which ISA to use
https://github.com/torvalds/linux/blob/master/arch/arm/Makefile#L62
I'm not sure how well-defined it is to set multiple of those. We could remove CONFIG_CPU_32v6 and CONFIG_CPU_32v6K out of the config or change the Makefile so that in case multiple ISAs are specified highest version is set. WDYT?

[nickdesaulniers]

I'm not sure how well-defined it is to set multiple of those.

I'm not sure that make allyesconfig is selecting all of those, otherwise the command line flags @nathanchance posted in his first comment would have multiple -march= options, yet I only see -march=armv6k. I'll bet that Kconfig already marks them as mutually exclusive.

#error You should compile this file with '-mfloat-abi=softfp -mfpu=neon'

Well that seems very GCC specific, and at odds with Clang:

  if ((FPU & NeonFPU) && !SoftFloat && ArchVersion >= 7) {

specifically SoftFloat which I'll guess is set by -mfloat-abi=softfp. So either Clang is wrong (the comment explicitly says it tries to be different from GCC). Note that Clang also expect armv7, which is maybe too conservative? I have a meeting tomorrow morning with some LLVM folks @ ARM who might be able to clarify. cc @kbeyls @smithp35

Side note, all of those cc-options can probably be removed. What year is it?! -march=armv7-a is supported by arm gcc 4.5.4, and the kernel supports minimally gcc 4.6.

[tpimh]

I'm sure selecting multiple ISAs should be invalid. I will check the Makefile to see how it selects appropriate -march= option.

[smithp35]

From a purely architectural perspective neon (referred to as Advanced SIMD) in the architecture is an optional extension for Arm v7-A and Arm v7-R, and (less optionally) in Arm v8. It is not defined for Arm v6 which is probably why gcc is giving an error message for trying to use Neon with architecture v6k (something like arm1176jzf-s).

SoftFloat will come from -mfloat-abi=soft (use no hardware floating point), this is different to -mfloat-abi=softfp (use hardware floating point, but use the software floating point calling convention).

The only subtle difference I could find with clang is that __ARM_NEON, ARM_NEON and __ARM_NEON_FP are defined for clang with -march=armv7-a even without -mfpu=neon, whereas gcc needs -mfpu=neon.

Neither gcc or clang will enable neon for Arm v6k.

[tpimh]

Yes, from what I can find, the following is true: ARMv8 – NEON is mandatory, ARMv7 – NEON is optional (but usually implemented), ARMv6 – NEON is impossible. Thus config parameter should have similar behavior: force NEON=y, if arch is ARMv8, force NEON=n if arch is ARMv6 and provide choice if arch is ARMv7. Selecting multiple arches in one kernel build should not be possible.