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Subject: Re: [PATCH 3/5] MdePkg/BaseLib: use normal register init in ARM SetJump implementations
To: Leif Lindholm <leif@nuviainc.com>, devel@edk2.groups.io
Cc: Michael D Kinney <michael.d.kinney@intel.com>,
 Liming Gao <gaoliming@byosoft.com.cn>, Zhiguang Liu <zhiguang.liu@intel.com>
References: <20201001183712.1738-1-leif@nuviainc.com>
 <20201001183712.1738-4-leif@nuviainc.com>
From: "Ard Biesheuvel" <ard.biesheuvel@arm.com>
Message-ID: <efab6bcf-bab6-c5da-ea63-c0d55beaea9b@arm.com>
Date: Tue, 13 Oct 2020 14:16:42 +0200
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On 10/1/20 8:37 PM, Leif Lindholm wrote:
> There may be architectures on which there are benefits to
>    eor r0, r0(, r0)
> but ARM was never one of them. Change to more readable
>    mov r0, #0
> instead.
> 
> Signed-off-by: Leif Lindholm <leif@nuviainc.com>
> ---
>   MdePkg/Library/BaseLib/Arm/SetJumpLongJump.S   | 2 +-
>   MdePkg/Library/BaseLib/Arm/SetJumpLongJump.asm | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.S b/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.S
> index 54b11ad2197c..407df5f41ac5 100644
> --- a/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.S
> +++ b/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.S
> @@ -36,7 +36,7 @@ ASM_PFX(SetJump):
>     pop   {r0, lr}
>     mov   r3, r13
>     stmia r0, {r3-r12,r14}
> -  eor   r0, r0, r0
> +  mov   r0, #0

Actually, 'movs r0, #0' produces a smaller opcode here when building in 
Thumb2 mode :-)


>     bx    lr
>   
>   #/**
> diff --git a/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.asm b/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.asm
> index 6d47033975f2..3a45f045460a 100644
> --- a/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.asm
> +++ b/MdePkg/Library/BaseLib/Arm/SetJumpLongJump.asm
> @@ -36,7 +36,7 @@ SetJump
>     POP  {R0, LR}
>     MOV  R3, R13
>     STM  R0, {R3-R12,R14}
> -  EOR  R0, R0
> +  MOV  RO, #0
>     BX   LR
>   
>   ;/**
>