Sure, the helper makes the code simpler. I'll test the new patch and push if
there is no other issue.
Thanks,
Hao
________________________________________
From: Richard Sandiford <richard.sandif...@arm.com>
Sent: Monday, July 31, 2023 17:11
To: Hao Liu OS
Cc: Richard Biener; GCC-patches@gcc.gnu.org
Subject: Re: [PATCH] AArch64: Do not increase the vect reduction latency by
multiplying count [PR110625]
Hao Liu OS <h...@os.amperecomputing.com> writes:
>> Which test case do you see this for? The two tests in the patch still
>> seem to report correct latencies for me if I make the change above.
>
> Not the newly added tests. It is still the existing case causing the
> previous ICE (i.e. assertion problem): gcc.target/aarch64/sve/cost_model_13.c.
>
> It's not the test case itself failed, but the dump message of vect says the
> "reduction latency" is 0:
>
> Before the change:
> cost_model_13.c:7:21: note: Original vector body cost = 6
> cost_model_13.c:7:21: note: Scalar issue estimate:
> cost_model_13.c:7:21: note: load operations = 1
> cost_model_13.c:7:21: note: store operations = 0
> cost_model_13.c:7:21: note: general operations = 1
> cost_model_13.c:7:21: note: reduction latency = 1
> cost_model_13.c:7:21: note: estimated min cycles per iteration = 1.000000
> cost_model_13.c:7:21: note: estimated cycles per vector iteration (for VF
> 8) = 8.000000
> cost_model_13.c:7:21: note: Vector issue estimate:
> cost_model_13.c:7:21: note: load operations = 1
> cost_model_13.c:7:21: note: store operations = 0
> cost_model_13.c:7:21: note: general operations = 1
> cost_model_13.c:7:21: note: reduction latency = 2
> cost_model_13.c:7:21: note: estimated min cycles per iteration = 2.000000
>
> After the change:
> cost_model_13.c:7:21: note: Original vector body cost = 6
> cost_model_13.c:7:21: note: Scalar issue estimate:
> cost_model_13.c:7:21: note: load operations = 1
> cost_model_13.c:7:21: note: store operations = 0
> cost_model_13.c:7:21: note: general operations = 1
> cost_model_13.c:7:21: note: reduction latency = 0 <--- seems not
> consistent with above result
> cost_model_13.c:7:21: note: estimated min cycles per iteration = 1.000000
> cost_model_13.c:7:21: note: estimated cycles per vector iteration (for VF
> 8) = 8.000000
> cost_model_13.c:7:21: note: Vector issue estimate:
> cost_model_13.c:7:21: note: load operations = 1
> cost_model_13.c:7:21: note: store operations = 0
> cost_model_13.c:7:21: note: general operations = 1
> cost_model_13.c:7:21: note: reduction latency = 0 <--- seems not
> consistent with above result
> cost_model_13.c:7:21: note: estimated min cycles per iteration = 1.000000
> <--- seems not consistent with above result
>
> BTW. this should be caused by the reduction stmt is not live, which indicates
> whether this stmts is part of a computation whose result is used outside the
> loop (tree-vectorized.h:1204):
> <bb 3>:
> # res_18 = PHI <res_15(7), 0(6)>
> # i_20 = PHI <i_16(7), 0(6)>
> _1 = (long unsigned int) i_20;
> _2 = _1 * 2;
> _3 = x_14(D) + _2;
> _4 = *_3;
> _5 = (unsigned short) _4;
> res.0_6 = (unsigned short) res_18;
> _7 = _5 + res.0_6; <-- This is not live, may be
> caused by the below type cast stmt.
> res_15 = (short int) _7;
> i_16 = i_20 + 1;
> if (n_11(D) > i_16)
> goto <bb 7>;
> else
> goto <bb 4>;
>
> <bb 7>:
> goto <bb 3>;
Ah, I see, thanks. My concern was: if requiring !STMT_VINFO_LIVE_P stmts
can cause "normal" reductions to have a latency of 0, could the same thing
happen for single-cycle reductions? But I suppose the answer is "no".
Introducing a cast like the above would cause reduc_chain_length > 1,
and so:
if (ncopies > 1
&& (STMT_VINFO_RELEVANT (stmt_info) <= vect_used_only_live)
&& reduc_chain_length == 1
&& loop_vinfo->suggested_unroll_factor == 1)
single_defuse_cycle = true;
wouldn't trigger. Which makes the single-cycle thing a bit hit-and-miss...
So yeah, I agree the patch is safe after all.
Please split the check out into a helper though, to avoid the awkward
formatting:
/* Return true if STMT_INFO is part of a reduction that has the form:
r = r op ...;
r = r op ...;
with the single accumulator being read and written multiple times. */
static bool
aarch64_force_single_cycle (vec_info *vinfo, stmt_vec_info stmt_info)
{
if (!STMT_VINFO_LIVE_P (stmt_info))
return false;
auto reduc_info = info_for_reduction (vinfo, stmt_info);
return STMT_VINFO_FORCE_SINGLE_CYCLE (reduc_info);
}
OK with that change, thanks.
Richard
