linux-kernel - Re: [RFC PATCH 05/12] lib/raid6: Use CC_FLAGS

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Message-ID: <7c40dfe8-f245-413f-a424-bde52ce21b6a@sifive.com>
Date:   Mon, 11 Dec 2023 10:12:27 -0600
From:   Samuel Holland <samuel.holland@...ive.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH 05/12] lib/raid6: Use CC_FLAGS_FPU for NEON CFLAGS

On 2023-12-11 10:07 AM, Christoph Hellwig wrote:
>> +CFLAGS_REMOVE_neon1.o += $(CC_FLAGS_NO_FPU)
>> +CFLAGS_REMOVE_neon2.o += $(CC_FLAGS_NO_FPU)
>> +CFLAGS_REMOVE_neon4.o += $(CC_FLAGS_NO_FPU)
>> +CFLAGS_REMOVE_neon8.o += $(CC_FLAGS_NO_FPU)
> 
> Btw, do we even really need the extra variables for compiler flags
> to remove?  Don't gcc/clang options work so that if you add a
> no-prefixed version of the option later it transparently gets removed?

Unfortunately, not all of the relevant options can be no-prefixed:

$ cat float.c 
int main(void) { volatile float f = 123.456; return f / 10; }
$ aarch64-linux-musl-gcc float.c 
$ aarch64-linux-musl-gcc -mgeneral-regs-only float.c 
float.c: In function 'main':
float.c:1:33: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
    1 | int main(void) { volatile float f = 123.456; return f / 10; }
      |                                 ^
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
    1 | int main(void) { volatile float f = 123.456; return f / 10; }
      |                                                     ~~^~~~
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
float.c:1:55: error: '-mgeneral-regs-only' is incompatible with the use of floating-point types
$ aarch64-linux-musl-gcc -mgeneral-regs-only -mno-general-regs-only float.c 
aarch64-linux-musl-gcc: error: unrecognized command-line option '-mno-general-regs-only'; did you mean '-mgeneral-regs-only'?
$