linux-kernel - Re: [PATCH] ARM: use assembly mnemonics for VFP register access

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Date:   Tue, 25 Feb 2020 12:27:26 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Stefan Agner <stefan@...er.ch>, Arnd Bergmann <arnd@...db.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Jian Cai <jiancai@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: use assembly mnemonics for VFP register access

On Tue, Feb 25, 2020 at 11:33 AM Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
>
> On Tue, 25 Feb 2020 at 20:10, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > Ah, this is only when streaming to assembly. Looks like they have the
> > same encoding, and produce the same disassembly. (Godbolt emits
> > assembly by default, and has the option to compile, then disassemble).
> > If I take my case from godbolt above:
> >
> > ➜  /tmp arm-linux-gnueabihf-gcc -O2 -c x.c
> > ➜  /tmp llvm-objdump -dr x.o
> >
> > x.o: file format elf32-arm-little
> >
> >
> > Disassembly of section .text:
> >
> > 00000000 bar:
> >        0: f1 ee 10 0a                  vmrs r0, fpscr
> >        4: 70 47                        bx lr
> >        6: 00 bf                        nop
> >
> > 00000008 baz:
> >        8: f1 ee 10 0a                  vmrs r0, fpscr
> >        c: 70 47                        bx lr
> >        e: 00 bf                        nop
> >
> > So indeed a similar encoding exists for the two different assembler
> > instructions.
>
> Does that hold for ARM (A32) instructions as well?

TIL -mthumb is the default for arm-linux-gnueabihf-gcc -O2.

➜  /tmp arm-linux-gnueabihf-gcc -O2 -c x.c -marm
➜  /tmp llvm-objdump -dr x.o

x.o: file format elf32-arm-little


Disassembly of section .text:

00000000 bar:
       0: 10 0a f1 ee                  vmrs r0, fpscr
       4: 1e ff 2f e1                  bx lr

00000008 baz:
       8: 10 0a f1 ee                  vmrs r0, fpscr
       c: 1e ff 2f e1                  bx lr

^ Just to show the matching encoding.
-- 
Thanks,
~Nick Desaulniers