Bernhard Reutner-Fischer <rep.dot....@gmail.com> writes:
> On 18 March 2019 10:58:53 CET, Richard Sandiford <richard.sandif...@arm.com>
> wrote:
>>This patch fixes a case in which we vectorised something with a
>>fully-predicated loop even after the cost model had rejected it.
>>E.g. the loop in the testcase has the costs:
>>
>> Vector inside of loop cost: 27
>> Vector prologue cost: 0
>> Vector epilogue cost: 0
>> Scalar iteration cost: 7
>> Scalar outside cost: 6
>> Vector outside cost: 0
>> prologue iterations: 0
>> epilogue iterations: 0
>>
>>and we can see that the loop executes at most three times, but we
>>decided to vectorise it anyway.
>>
>>(The costs here are equal for three iterations, but the same thing
>>happens even when the vector code is strictly more expensive.)
>>
>>The problem is the handling of "/VF" in:
>>
>> /* Calculate number of iterations required to make the vector version
>> profitable, relative to the loop bodies only. The following condition
>> must hold true:
>> SIC * niters + SOC > VIC * ((niters-PL_ITERS-EP_ITERS)/VF) + VOC
>> where
>> SIC = scalar iteration cost, VIC = vector iteration cost,
>> VOC = vector outside cost, VF = vectorization factor,
>> PL_ITERS = prologue iterations, EP_ITERS= epilogue iterations
>> SOC = scalar outside cost for run time cost model check. */
>>
>>We treat the "/VF" as truncating, but for fully-predicated loops, it's
>>closer to a ceil division, since fractional iterations are handled by a
>>full iteration with some predicate bits set to false.
>>
>>The easiest fix seemed to be to calculate the minimum number of vector
>>iterations first, then use that to calculate the minimum number of
>>scalar
>>iterations.
>>
>>Calculating the minimum number of vector iterations might make sense
>>for
>>unpredicated loops too, since calculating the scalar niters directly
>>doesn't take into account the fact that the VIC multiple has to be an
>>integer. But the handling of PL_ITERS and EP_ITERS for unpredicated
>>loops is a bit hand-wavy anyway, so maybe vagueness here cancels out
>>vagueness there?
>>
>>Either way, changing this for unpredicated loops would be much too
>>invasive for stage 4, so the patch keeps it specific to
>>fully-predicated
>>loops (i.e. SVE) for now. There's no functional change for other
>>targets.
>>
>>Tested on aarch64-linux-gnu with and without SVE, and on
>>x86_64-linux-gnu.
>>This is a regression introduced by the original cost model patches for
>>fully-predicated loops, so OK for GCC 9?
>>
>>Thanks,
>>Richard
>>
>>
>>2019-03-18 Richard Sandiford <richard.sandif...@arm.com>
>>
>>gcc/
>> * tree-vect-loop.c (vect_estimate_min_profitable_iters): Fix the
>> calculation of the minimum number of scalar iterations for
>> fully-predicated loops.
>>
>>gcc/testsuite/
>> * gcc.target/aarch64/sve/cost_model_1.c: New test.
>>
>>Index: gcc/tree-vect-loop.c
>>===================================================================
>>--- gcc/tree-vect-loop.c 2019-03-08 18:15:33.668751875 +0000
>>+++ gcc/tree-vect-loop.c 2019-03-18 09:55:03.257194326 +0000
>>@@ -3600,14 +3600,89 @@ vect_estimate_min_profitable_iters (loop
>> /* Calculate number of iterations required to make the vector version
>> profitable, relative to the loop bodies only. The following condition
>> must hold true:
>>- SIC * niters + SOC > VIC * ((niters-PL_ITERS-EP_ITERS)/VF) + VOC
>>+ SIC * niters + SOC > VIC * ((niters - NPEEL) / VF) + VOC
>> where
>> SIC = scalar iteration cost, VIC = vector iteration cost,
>> VOC = vector outside cost, VF = vectorization factor,
>>- PL_ITERS = prologue iterations, EP_ITERS= epilogue iterations
>>+ NPEEL = prologue iterations + epilogue iterations,
>> SOC = scalar outside cost for run time cost model check. */
>>
>>- if ((scalar_single_iter_cost * assumed_vf) > (int) vec_inside_cost)
>>+ int saving_per_viter = (scalar_single_iter_cost * assumed_vf
>>+ - vec_inside_cost);
>>+ if (saving_per_viter <= 0)
>>+ {
>>+ if (LOOP_VINFO_LOOP (loop_vinfo)->force_vectorize)
>>+ warning_at (vect_location.get_location_t (), OPT_Wopenmp_simd,
>>+ "vectorization did not happen for a simd loop");
>>+
>>+ if (dump_enabled_p ())
>>+ dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
>>+ "cost model: the vector iteration cost = %d "
>>+ "divided by the scalar iteration cost = %d "
>>+ "is greater or equal to the vectorization factor = %d"
>>+ ".\n",
>>+ vec_inside_cost, scalar_single_iter_cost, assumed_vf);
>>+ *ret_min_profitable_niters = -1;
>>+ *ret_min_profitable_estimate = -1;
>>+ return;
>>+ }
>>+
>>+ /* ??? The "if" arm is written to handle all cases; see below for
>>what
>>+ we would do for !LOOP_VINFO_FULLY_MASKED_P. */
>>+ if (LOOP_VINFO_FULLY_MASKED_P (loop_vinfo))
>>+ {
>
> The condition above seems to contain...
>
>
>>+ /* Rewriting the condition above in terms of the number of
>>+ vector iterations (vniters) rather than the number of
>>+ scalar iterations (niters) gives:
>>+
>>+ SIC * (vniters * VF + NPEEL) + SOC > VIC * vniters + VOC
>>+
>>+ <==> vniters * (SIC * VF - VIC) > VOC - SIC * NPEEL - SOC
>>+
>>+ For integer N, X and Y when X > 0:
>>+
>>+ N * X > Y <==> N >= (Y /[floor] X) + 1. */
>>+ int outside_overhead = (vec_outside_cost
>>+ - scalar_single_iter_cost * peel_iters_prologue
>>+ - scalar_single_iter_cost * peel_iters_epilogue
>>+ - scalar_outside_cost);
>>+ /* We're only interested in cases that require at least one
>>+ vector iteration. */
>>+ int min_vec_niters = 1;
>>+ if (outside_overhead > 0)
>>+ min_vec_niters = outside_overhead / saving_per_viter + 1;
>>+
>>+ if (dump_enabled_p ())
>>+ dump_printf (MSG_NOTE, " Minimum number of vector iterations: %d\n",
>>+ min_vec_niters);
>>+
>>+ if (LOOP_VINFO_FULLY_MASKED_P (loop_vinfo))
>>+ {
>
> ... this identical condition, AFAICS?
> So this second conditions else arm should be dead, shouldn't it?
Yeah, that's what:
/* ??? The "if" arm is written to handle all cases; see below for what
we would do for !LOOP_VINFO_FULLY_MASKED_P. */
was trying to say. Like I mentioned in the covering note, in principle
the approach of calculating the minimum number of vector iterations
should work for all cases, and we might want to consider doing that
for stage 1. I wanted to show what the !LOOP_VINFO_FULLY_MASKED_P
code would look like if we did that.
I'd wondered about putting the inner else in an #if 0 or comment instead,
but this way makes it easier to experiment with.
Thanks,
Richard
