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.leiz...@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 _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu