Messages in this thread |  | | Subject | Re: [PATCH v2 2/2] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() | From | "Leizhen (ThunderTown)" <> | Date | Wed, 20 Oct 2021 15:29:44 +0800 |
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On 2021/10/4 20:07, Will Deacon wrote: > 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.
OK, I think I was overly aggressive in the beginning, and I just tried to remove the register modifier and get the same optimization.
> >> + cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode); > > This generates a compiler warning about taking the address of a 'register' > variable. > > Will > . >
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