Re: [PATCH v2 2/2] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd()

[Will Deacon <will@kernel.org>]

On Wed, Aug 18, 2021 at 04:04:52PM +0800, Zhen Lei wrote:
> Although the parameter 'cmd' is always passed by a local array variable,
> and only this function modifies it, the compiler does not know this. The
> compiler almost always reads the value of cmd[i] from memory rather than
> directly using the value cached in the register. This generates many
> useless instruction operations and affects the performance to some extent.
> 
> To guide the compiler for proper optimization, 'cmd' is defined as a local
> array variable, marked as register, and copied to the output parameter at
> a time when the function is returned.
> 
> The optimization effect can be viewed by running the "size arm-smmu-v3.o"
> command.
> 
> Before:
>    text    data     bss     dec     hex
>   26954    1348      56   28358    6ec6
> 
> After:
>    text    data     bss     dec     hex
>   26762    1348      56   28166    6e06
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 01e95b56ffa07d1..7cec0c967f71d86 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -234,10 +234,12 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent)
>  }
>  
>  /* High-level queue accessors */
> -static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> +static int arm_smmu_cmdq_build_cmd(u64 *out_cmd, struct arm_smmu_cmdq_ent *ent)
>  {
> -	memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT);
> -	cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode);
> +	register u64 cmd[CMDQ_ENT_DWORDS];

'register' is pretty badly specified outside of a handful of limited uses in
conjunction with inline asm, so I really don't think we should be using it
here.

> +	cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);

This generates a compiler warning about taking the address of a 'register'
variable.

Will