linux-kernel - Re: [PATCH v2 2/2] arm: lib: xor-neon: move pragma options to makefile

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Message-ID: <CAMj1kXEpD1pp5uzOMeSYhgS_dzOysKuPMOMNgivUx58PGUdMJw@mail.gmail.com>
Date:   Fri, 13 Nov 2020 08:50:20 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Arvind Sankar <nivedita@...m.mit.edu>, kernel@...labora.com,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] arm: lib: xor-neon: move pragma options to makefile

On Thu, 12 Nov 2020 at 22:23, Adrian Ratiu <adrian.ratiu@...labora.com> wrote:
>
> Using a pragma like GCC optimize is a bad idea because it tags
> all functions with an __attribute__((optimize)) which replaces
> optimization options rather than appending so could result in
> dropping important flags. Not recommended for production use.
>
> Because these options should always be enabled for this file,
> it's better to set them via command line. tree-vectorize is on
> by default in Clang, but it doesn't hurt to make it explicit.
>
> Suggested-by: Arvind Sankar <nivedita@...m.mit.edu>
> Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Adrian Ratiu <adrian.ratiu@...labora.com>
> ---
>  arch/arm/lib/Makefile   |  2 +-
>  arch/arm/lib/xor-neon.c | 10 ----------
>  2 files changed, 1 insertion(+), 11 deletions(-)
>
> diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
> index 6d2ba454f25b..12d31d1a7630 100644
> --- a/arch/arm/lib/Makefile
> +++ b/arch/arm/lib/Makefile
> @@ -45,6 +45,6 @@ $(obj)/csumpartialcopyuser.o: $(obj)/csumpartialcopygeneric.S
>
>  ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
>    NEON_FLAGS                   := -march=armv7-a -mfloat-abi=softfp -mfpu=neon
> -  CFLAGS_xor-neon.o            += $(NEON_FLAGS)
> +  CFLAGS_xor-neon.o            += $(NEON_FLAGS) -ftree-vectorize -Wno-unused-variable
>    obj-$(CONFIG_XOR_BLOCKS)     += xor-neon.o
>  endif
> diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> index e1e76186ec23..62b493e386c4 100644
> --- a/arch/arm/lib/xor-neon.c
> +++ b/arch/arm/lib/xor-neon.c
> @@ -14,16 +14,6 @@ MODULE_LICENSE("GPL");
>  #error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
>  #endif
>
> -/*
> - * Pull in the reference implementations while instructing GCC (through
> - * -ftree-vectorize) to attempt to exploit implicit parallelism and emit
> - * NEON instructions.
> - */
> -#ifdef CONFIG_CC_IS_GCC
> -#pragma GCC optimize "tree-vectorize"
> -#endif
> -
> -#pragma GCC diagnostic ignored "-Wunused-variable"
>  #include <asm-generic/xor.h>
>
>  struct xor_block_template const xor_block_neon_inner = {
> --
> 2.29.2
>

So what is the status now here? How does putting -ftree-vectorize on
the command line interact with Clang?