linux-kernel - Re: [PATCH v2] Make iwmmxt.S support Clang's integrated assembler

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Message-ID: <CAKwvOd=pHgT3LsjYH10eXQjLPtiOKDj-8nJwjQ=NMSFLTG1xAg@mail.gmail.com>
Date:   Mon, 9 Nov 2020 17:55:41 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Jian Cai <jiancai@...gle.com>, Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Russell King <linux@...linux.org.uk>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] Make iwmmxt.S support Clang's integrated assembler

On Sat, Nov 7, 2020 at 12:29 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Sat, 7 Nov 2020 at 01:11, Jian Cai <jiancai@...gle.com> wrote:
> >
> > This patch replaces 6 IWMMXT instructions Clang's integrated assembler
> > does not support in iwmmxt.S using macros, while making sure GNU
> > assembler still emit the same instructions. This should be easier than
> > providing full IWMMXT support in Clang.
> >
> > "Intel Wireless MMX Technology - Developer Guide - August, 2002" should
> > be referenced for the encoding schemes of these extensions.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/975
> >
> > Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> > Signed-off-by: Jian Cai <jiancai@...gle.com>
>
> Please make sure you test this carefully on BE32, as the instruction
> byte order used by .inst is LE IIRC

Unless that was a recent-ish change in GAS, it looks like I get the
same disassembly/encodings before/after this patch for
CONFIG_BIG_ENDIAN with GNU as.

$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make CC=clang -j71 defconfig
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make CC=clang -j71 menuconfig
<enable CPU_BIG_ENDIAN>
$ grep BIG_ENDIAN .config
CONFIG_CPU_BIG_ENDIAN=y
CONFIG_ARCH_SUPPORTS_BIG_ENDIAN=y
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make CC=clang -j71
arch/arm/kernel/iwmmxt.o
$ arm-linux-gnueabi-objdump -dr arch/arm/kernel/iwmmxt.o > gas_before.txt
$ b4 am https://lore.kernel.org/lkml/20201107001056.225807-1-jiancai@google.com/
-o - | git am
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make CC=clang -j71
arch/arm/kernel/iwmmxt.o
$ arm-linux-gnueabi-objdump -dr arch/arm/kernel/iwmmxt.o > gas_after.txt
$ diff -u gas_before.txt gas_after.txt
$ echo $?
0

(Orthogonal, it looks like llvm-objdump has issues decoding
elf32-bigarm that we'll need to fix.)

Either way the patch LGTM for IWMMXT, thanks Jian for the patch and
Ard for the suggestions.

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
-- 
Thanks,
~Nick Desaulniers