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

[Zhen Lei]

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@xxxxxxxxxx>
---
 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];
+
+	cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
+	cmd[1] = 0;
 
 	switch (ent->opcode) {
 	case CMDQ_OP_TLBI_EL2_ALL:
@@ -332,6 +334,9 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
 		return -ENOENT;
 	}
 
+	out_cmd[0] = cmd[0];
+	out_cmd[1] = cmd[1];
+
 	return 0;
 }
 
-- 
2.26.0.106.g9fadedd