On 01/25/2018 03:52 PM, Peter Robinson wrote:
On Thu, Jan 25, 2018 at 2:46 PM, Florian Weimer <fweimer@xxxxxxxxxx> wrote:
GCC offers a generic tuning option for Arm these days, but we select
-mtune=cortex-8a instead.
Is this still a good choice?
I suspect the generic tuning is likely a better choice, is there any
details about it anywhere? Basically Cortex-A8 is pretty much the
lowest common denominator for ARMv7
The generic tuning has this:
/* Generic Cortex tuning. Use more specific tunings if appropriate. */
const struct tune_params arm_cortex_tune =
{
&generic_extra_costs,
&generic_addr_mode_costs, /* Addressing mode costs. */
NULL, /* Sched adj cost. */
arm_default_branch_cost,
&arm_default_vec_cost,
1, /* Constant limit. */
5, /* Max cond insns. */
8, /* Memset max inline. */
2, /* Issue rate. */
ARM_PREFETCH_NOT_BENEFICIAL,
tune_params::PREF_CONST_POOL_FALSE,
tune_params::PREF_LDRD_FALSE,
tune_params::LOG_OP_NON_SHORT_CIRCUIT_TRUE, /* Thumb. */
tune_params::LOG_OP_NON_SHORT_CIRCUIT_TRUE, /* ARM. */
tune_params::DISPARAGE_FLAGS_NEITHER,
tune_params::PREF_NEON_64_FALSE,
tune_params::PREF_NEON_STRINGOPS_FALSE,
tune_params::FUSE_NOTHING,
tune_params::SCHED_AUTOPREF_OFF
};
The Cortex-A8 tuning is:
const struct tune_params arm_cortex_a8_tune =
{
&cortexa8_extra_costs,
&generic_addr_mode_costs, /* Addressing mode costs. */
NULL, /* Sched adj cost. */
arm_default_branch_cost,
&arm_default_vec_cost,
1, /* Constant limit. */
5, /* Max cond insns. */
8, /* Memset max inline. */
2, /* Issue rate. */
ARM_PREFETCH_NOT_BENEFICIAL,
tune_params::PREF_CONST_POOL_FALSE,
tune_params::PREF_LDRD_FALSE,
tune_params::LOG_OP_NON_SHORT_CIRCUIT_TRUE, /* Thumb. */
tune_params::LOG_OP_NON_SHORT_CIRCUIT_TRUE, /* ARM. */
tune_params::DISPARAGE_FLAGS_NEITHER,
tune_params::PREF_NEON_64_FALSE,
tune_params::PREF_NEON_STRINGOPS_TRUE,
tune_params::FUSE_NOTHING,
tune_params::SCHED_AUTOPREF_OFF
};
The real difference is in generic_extra_costs vs generic_extra_costs,
and too large to include here. One of the differences seems to be that
on Cortex-A8, floating point multiply & divide is considered relatively
more expensive, if I read the sources correctly. But this all a bit
black magic.
Thanks,
Florian
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