Messages in this thread | | | Subject | Re: [PATCH] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() | From | Robin Murphy <> | Date | Tue, 17 Aug 2021 14:23:53 +0100 |
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On 2021-08-17 12:34, 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.
Which compiler? GCC 4.9 does not make the same codegen decisions that GCC 10 does; Clang is different again. There are also various config options which affect a compiler's inlining/optimisation choices either directly or indirectly.
If it's something that newer compilers can get right anyway, then micro-optimising just for older ones might warrant a bit more justification.
> 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 > 27602 1348 56 29006 714e > > After: > text data bss dec hex > 27402 1348 56 28806 7086 > > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com> > --- > drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 18 +++++++++++++++--- > 1 file changed, 15 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 d76bbbde558b776..50a9db5bac466c7 100644 > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > @@ -233,11 +233,19 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent) > return 0; > } > > +#define arm_smmu_cmdq_copy_cmd(dst, src) \ > + do { \ > + dst[0] = src[0]; \ > + dst[1] = src[1]; \ > + } while (0) > + > /* 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: > @@ -309,6 +317,7 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent) > case PRI_RESP_SUCC: > break; > default: > + arm_smmu_cmdq_copy_cmd(out_cmd, cmd);
Why bother writing back a partial command when we're telling the caller it's invalid anyway?
> return -EINVAL; > } > cmd[1] |= FIELD_PREP(CMDQ_PRI_1_RESP, ent->pri.resp); > @@ -329,9 +338,12 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent) > cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB); > break; > default: > + arm_smmu_cmdq_copy_cmd(out_cmd, cmd);
Ditto.
> return -ENOENT; > } > > + arm_smmu_cmdq_copy_cmd(out_cmd, cmd);
...and then it would be simpler to open-code the assignment here.
I guess if you're really concerned with avoiding temporary commands being written back to the stack and reloaded, it might be worth experimenting with wrapping them in a struct which can be passed around by value - AAPCS64 allows passing a 16-byte composite type purely in registers.
Robin.
> + > return 0; > } > >
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