• Nathan Chancellor's avatar
    ARM: 8833/1: Ensure that NEON code always compiles with Clang · de9c0d49
    Nathan Chancellor authored
    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/287Suggested-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
    Signed-off-by: Nathan Chancellor's avatarNathan Chancellor <natechancellor@gmail.com>
    Acked-by: default avatarNicolas Pitre <nico@linaro.org>
    Reviewed-by: default avatarNick Desaulniers <ndesaulniers@google.com>
    Reviewed-by: Stefan Agner's avatarStefan Agner <stefan@agner.ch>
    Signed-off-by: default avatarRussell King <rmk+kernel@armlinux.org.uk>
    de9c0d49
Makefile 5.12 KB