Messages in this thread | | | Subject | Re: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() | From | "Leizhen (ThunderTown)" <> | Date | Wed, 8 Dec 2021 21:40:10 +0800 |
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On 2021/12/8 0:17, John Garry wrote: > >> + >> return 0; >> } > > Did you notice any performance change with this change?
Hi John: Thanks for the tip. I wrote a test case today, and I found that the performance did not go up but down. It's so weird. So I decided not to change it, because it's also poorly readable. So I plan to make only the following modifications: @@ -237,7 +237,7 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent) static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent) { memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT); - cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode); + cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
switch (ent->opcode) { case CMDQ_OP_TLBI_EL2_ALL:
This prevents the compiler from generating the following two inefficient instructions: 394: f9400002 ldr x2, [x0] //x2 = cmd[0] 398: aa020062 orr x2, x3, x2 //x3 = FIELD_PREP(CMDQ_0_OP, ent->opcode)
Maybe it's not worth changing because I've only seen a 0.x nanosecond reduction in performance. But one thing is, it only comes with benefits, no side effects.
> > Thanks, > John
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