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

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Message-ID: <20201112215025.GA55620@ubuntu-m3-large-x86>
Date:   Thu, 12 Nov 2020 14:50:25 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Ard Biesheuvel <ardb@...nel.org>,
        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, Nov 12, 2020 at 11:24:57PM +0200, Adrian Ratiu 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>

Reviewed-by: Nathan Chancellor <natechancellor@...il.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
>