[PATCH v3 1/1] 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. Every
time the 'cmd' variable is updated, a memory write operation is generated.
This generates many useless instruction operations.

To guide the compiler for proper optimization, 'cmd' is defined as a local
array variable, 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
  28246    1332      56   29634    73c2

After:
   text    data     bss     dec     hex
  28134    1332      56   29522    7352

For example:
	cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
case CMDQ_OP_TLBI_EL2_VA:
	cmd[0] |= FIELD_PREP(CMDQ_TLBI_0_NUM, ent->tlbi.num);
	cmd[0] |= FIELD_PREP(CMDQ_TLBI_0_SCALE, ent->tlbi.scale);
	cmd[0] |= FIELD_PREP(CMDQ_TLBI_0_ASID, ent->tlbi.asid);
	cmd[1] |= FIELD_PREP(CMDQ_TLBI_1_LEAF, ent->tlbi.leaf);
	cmd[1] |= FIELD_PREP(CMDQ_TLBI_1_TTL, ent->tlbi.ttl);
	cmd[1] |= FIELD_PREP(CMDQ_TLBI_1_TG, ent->tlbi.tg);
	cmd[1] |= ent->tlbi.addr & CMDQ_TLBI_1_VA_MASK;

Before:
  Each "cmd[0] |=" or "cmd[1] |=" operation generates a "str" instruction,
sum = 8.

     ldrb	w4, [x1, #8]		//w4 = ent->tlbi.num
     ubfiz	x4, x4, #12, #5
     mov	w0, #0x0
     orr	x4, x4, x3
     str	x4, [x2]
     autiasp
     ldrb	w3, [x1, #9]		//w3 = ent->tlbi.scale
     ubfiz	x3, x3, #20, #5
     orr	x3, x3, x4
     str	x3, [x2]
     ldrh	w4, [x1, #10]		//w4 = ent->tlbi.asid
     orr	x3, x3, x4, lsl #48
     str	x3, [x2]
     ldrb	w3, [x1, #14]		//w3 = ent->tlbi.leaf
     str	x3, [x2, #8]
     ldrb	w4, [x1, #15]		//w4 = ent->tlbi.ttl
     ubfiz	x4, x4, #8, #2
     orr	x4, x4, x3
     str	x4, [x2, #8]
     ldrb	w3, [x1, #16]		//ent->tlbi.tg
     ubfiz	x3, x3, #10, #2
     orr	x3, x3, x4
     str	x3, [x2, #8]
     ldr	x1, [x1, #24]		//ent->tlbi.addr
     and	x1, x1, #0xfffffffffffff000
     orr	x1, x1, x3
     str	x1, [x2, #8]
     ret

After:
  All "cmd[0] |=" and "cmd[1] |=" operations generate a "stp" instruction,
sum = 1.

3e8:
     mov	w0, #0x0
     autiasp
     stp	x2, x1, [x3]
     ret
     bti	j
3fc:
     ldrb	w5, [x1, #8]		//w5 = ent->tlbi.num
     mov	x2, #0x22		//x2 = ent->opcode = CMDQ_0_OP
     ldrb	w6, [x1, #9]		//w6 = ent->tlbi.scale
     ubfiz	x5, x5, #12, #5
     ldrb	w0, [x1, #16]		//w0 = ent->tlbi.tg
     orr	x5, x5, x2
     ldrb	w7, [x1, #15]		//w7 = ent->tlbi.ttl
     ldr	x4, [x1, #24]		//x4 = ent->tlbi.addr
     ubfiz	x0, x0, #10, #2
     ldrh	w2, [x1, #10]		//w2 = ent->tlbi.asid
     ubfiz	x6, x6, #20, #5
     ldrb	w8, [x1, #14]		//w8 = ent->tlbi.leaf
     and	x4, x4, #0xfffffffffffff000
     ubfiz	x1, x7, #8, #2
     orr	x1, x0, x1
     orr	x2, x6, x2, lsl #48
     orr	x0, x4, x8
     orr	x2, x2, x5
     orr	x1, x1, x0
     b		3e8

Signed-off-by: Zhen Lei <thunder.leizhen at 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 f5848b351b19359..e55dfc14cac6005 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);
+	int i;
+	u64 cmd[CMDQ_ENT_DWORDS] = {0};
+
+	cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
 
 	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;
 	}
 
+	for (i = 0; i < CMDQ_ENT_DWORDS; i++)
+		out_cmd[i] = cmd[i];
+
 	return 0;
 }
 
-- 
2.25.1