[PATCH] arm64: fpsimd: avoid restoring fpcr if the contents haven't changed

[Ard Biesheuvel]

Hey Will,

On 26 June 2014 19:34, Will Deacon <will.deacon at arm.com> wrote:
> Writing to the FPCR is commonly implemented as a self-synchronising
> operation in the CPU, so avoid writing to the register when the saved
> value matches that in the hardware already.
>
> Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Signed-off-by: Will Deacon <will.deacon at arm.com>
> ---
>  arch/arm64/include/asm/fpsimdmacros.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/arch/arm64/include/asm/fpsimdmacros.h b/arch/arm64/include/asm/fpsimdmacros.h
> index 768414d55e64..137c0f45b0a8 100644
> --- a/arch/arm64/include/asm/fpsimdmacros.h
> +++ b/arch/arm64/include/asm/fpsimdmacros.h
> @@ -40,6 +40,7 @@
>         str     w\tmpnr, [\state, #16 * 2 + 4]
>  .endm
>
> +/* Clobbers \state */
>  .macro fpsimd_restore state, tmpnr
>         ldp     q0, q1, [\state, #16 * 0]
>         ldp     q2, q3, [\state, #16 * 2]
> @@ -60,7 +61,15 @@
>         ldr     w\tmpnr, [\state, #16 * 2]
>         msr     fpsr, x\tmpnr
>         ldr     w\tmpnr, [\state, #16 * 2 + 4]
> +       /*
> +        * Writes to fpcr may be self-synchronising, so avoid restoring
> +        * the register if it hasn't changed.
> +        */
> +       mrs     \state, fpcr
> +       cmp     x\tmpnr, \state
> +       b.eq    9999f
>         msr     fpcr, x\tmpnr
> +9999:
>  .endm
>

I would assume the same would apply to the partial restore that was
merged during the last window?

-- 
Ard.