linux-kernel - Re: [PATCH 0/3] ARM: make use of UAL VFP mnemonics when possible

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Message-ID: <DBBPR08MB4823E090A8C9C0B48CB35603F8F20@DBBPR08MB4823.eurprd08.prod.outlook.com>
Date:   Sat, 21 Mar 2020 15:04:14 +0000
From:   Peter Smith <Peter.Smith@....com>
To:     "stefan@...er.ch" <stefan@...er.ch>
CC:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Jian Cai <jiancai@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        Manoj Gupta <manojgupta@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Robin Murphy <Robin.Murphy@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        nd <nd@....com>
Subject: Re: [PATCH 0/3] ARM: make use of UAL VFP mnemonics when possible

> To build the kernel with Clang's integrated assembler the VFP code needs
> to make use of the unified assembler language (UAL) VFP mnemonics.
> 
> At first I tried to get rid of the co-processor instructions to access
> the floating point unit along with the macros completely. However, due
> to missing FPINST/FPINST2 argument support in older binutils versions we
> have to keep them around. Once we drop support for binutils 2.24 and
> older, the move to UAL VFP mnemonics will be straight forward with this
> changes applied.
>
> Tested using Clang with integrated assembler as well as external
> (binutils assembler), various gcc/binutils version down to 4.7/2.23.
> Disassembled and compared the object files in arch/arm/vfp/ to make
> sure this changes leads to the same code. Besides different inlining
> behavior I was not able to spot a difference.
>

>From the perspective of an Arm toolchain developer perspective the
substitutions in this patch series look correct to me.

Some references I found helpful:

The v8-A Arm Architecture Reference Manual chapter Legacy Instruction Syntax for
AArch32 Instruction Sets. Table K6-2 Pre-UAL instruction syntax for A32
floating-point instructions

FMSR/FMRS is the pre-UAL name for VMOV (between general-purpose register and
single-precision)
FMDRR/FMRRD is the pre-UAL name for VMOV (between two general-purpose
registers and a doubleword floating-point register)
FLDMIAD is the pre-UAL name for VLDMIA
FSTMIAD is the pre-UAL name for VSTMIA

FLDMIAX and FSTMIAX has no UAL equivalent and is deprecated in ARMv6 and above,
it is equivalent to pre-UAL FLDMIAD/FSTMIAD except that the imm8 field is set
to twice the number of doubleword registers + 1, instead of twice the number of
doubleword registers. This description is taken from A8.8.50 F*, former
Floating-point instruction mnemonics in the v7-A Arm Architecture reference
manual.

The mrrc/mcrr and mcr/mcr correspond to a VMOV instruction. The mrrc/mcrr and
mcr/mcr set opc2 to #4 when accessing registers 16 to 31 as the instructions
can only refer to 16 coprocessor registers. The same bit (7) in the VMOV
corresponds to N, with the register n = UInt(N:Vn) so VMOV can refer to 32
registers.

Ref: Arm V8-A https://static.docs.arm.com/ddi0487/fa/DDI0487F_a_armv8_arm.pdf
Ref: Arm V7-A https://static.docs.arm.com/ddi0406/c/DDI0406C_C_arm_architecture_reference_manual.pdf

Hope this helps move this forward

Peter

> This replaces (and extends) my earlier patch "ARM: use assembly mnemonics
> for VFP register access"
> http://lore.kernel.org/r/8bb16ac4b15a7e28a8e819ef9aae20bfc3f75fbc.1582266841.git.stefan@agner.ch
>
> --
> Stefan
>
> Stefan Agner (3):
>   ARM: use .fpu assembler directives instead of assembler arguments
>   ARM: use VFP assembler mnemonics in register load/store macros
>   ARM: use VFP assembler mnemonics if available
>
>  arch/arm/include/asm/vfp.h       |  2 ++
>  arch/arm/include/asm/vfpmacros.h | 31 ++++++++++++++++++++++---------
>  arch/arm/vfp/Makefile            |  5 ++++-
>  arch/arm/vfp/vfphw.S             | 31 ++++++++++++++++++++-----------
>  arch/arm/vfp/vfpinstr.h          | 23 +++++++++++++++++++----
>  5 files changed, 67 insertions(+), 25 deletions(-)
> 
> -- 
> 2.25.1