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

From: Nick Desaulniers <ndesaulniers@google.com>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Stefan Agner <stefan@agner.ch>, Arnd Bergmann <arnd@arndb.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Jian Cai <jiancai@google.com>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Manoj Gupta <manojgupta@google.com>,
	Russell King <linux@armlinux.org.uk>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] ARM: use assembly mnemonics for VFP register access
Date: Tue, 25 Feb 2020 12:27:26 -0800	[thread overview]
Message-ID: <CAKwvOd=9WaeVjvgkkLf5scFaNTpx28d4FAse62vv4X_mEwqRJA@mail.gmail.com> (raw)
In-Reply-To: <CAKv+Gu881ZSwvuACmsbBnpfdeJpNYsEQxLSoepJBbZ=O6D6Rcg@mail.gmail.com>

On Tue, Feb 25, 2020 at 11:33 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> On Tue, 25 Feb 2020 at 20:10, Nick Desaulniers <ndesaulniers@google.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