Fix a case in which the vector cost model was ignored
Richard Sandiford
richard.sandiford@arm.com
Mon Mar 18 09:59:00 GMT 2019
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.sandiford@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))
+ {
+ /* 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))
+ {
+ /* Now that we know the minimum number of vector iterations,
+ find the minimum niters for which the scalar cost is larger:
+
+ SIC * niters > VIC * vniters + VOC - SOC
+
+ We know that the minimum niters is no more than
+ vniters * VF + NPEEL, but it might be (and often is) less
+ than that if a partial vector iteration is cheaper than the
+ equivalent scalar code. */
+ int threshold = (vec_inside_cost * min_vec_niters
+ + vec_outside_cost
+ - scalar_outside_cost);
+ if (threshold <= 0)
+ min_profitable_iters = 1;
+ else
+ min_profitable_iters = threshold / scalar_single_iter_cost + 1;
+ }
+ else
+ /* Convert the number of vector iterations into a number of
+ scalar iterations. */
+ min_profitable_iters = (min_vec_niters * assumed_vf
+ + peel_iters_prologue
+ + peel_iters_epilogue);
+ }
+ else
{
min_profitable_iters = ((vec_outside_cost - scalar_outside_cost)
* assumed_vf
@@ -3617,8 +3692,7 @@ vect_estimate_min_profitable_iters (loop
min_profitable_iters = 0;
else
{
- min_profitable_iters /= ((scalar_single_iter_cost * assumed_vf)
- - vec_inside_cost);
+ min_profitable_iters /= saving_per_viter;
if ((scalar_single_iter_cost * assumed_vf * min_profitable_iters)
<= (((int) vec_inside_cost * min_profitable_iters)
@@ -3627,24 +3701,6 @@ vect_estimate_min_profitable_iters (loop
min_profitable_iters++;
}
}
- /* vector version will never be profitable. */
- else
- {
- 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;
- }
if (dump_enabled_p ())
dump_printf (MSG_NOTE,
@@ -3668,10 +3724,34 @@ vect_estimate_min_profitable_iters (loop
Non-vectorized variant is SIC * niters and it must win over vector
variant on the expected loop trip count. The following condition must hold true:
- SIC * niters > VIC * ((niters-PL_ITERS-EP_ITERS)/VF) + VOC + SOC */
+ SIC * niters > VIC * ((niters - NPEEL) / VF) + VOC + SOC */
if (vec_outside_cost <= 0)
min_profitable_estimate = 0;
+ else if (LOOP_VINFO_FULLY_MASKED_P (loop_vinfo))
+ {
+ /* This is a repeat of the code above, but with + SOC rather
+ than - SOC. */
+ int outside_overhead = (vec_outside_cost
+ - scalar_single_iter_cost * peel_iters_prologue
+ - scalar_single_iter_cost * peel_iters_epilogue
+ + scalar_outside_cost);
+ int min_vec_niters = 1;
+ if (outside_overhead > 0)
+ min_vec_niters = outside_overhead / saving_per_viter + 1;
+
+ if (LOOP_VINFO_FULLY_MASKED_P (loop_vinfo))
+ {
+ int threshold = (vec_inside_cost * min_vec_niters
+ + vec_outside_cost
+ + scalar_outside_cost);
+ min_profitable_estimate = threshold / scalar_single_iter_cost + 1;
+ }
+ else
+ min_profitable_estimate = (min_vec_niters * assumed_vf
+ + peel_iters_prologue
+ + peel_iters_epilogue);
+ }
else
{
min_profitable_estimate = ((vec_outside_cost + scalar_outside_cost)
Index: gcc/testsuite/gcc.target/aarch64/sve/cost_model_1.c
===================================================================
--- /dev/null 2019-03-08 11:40:14.606883727 +0000
+++ gcc/testsuite/gcc.target/aarch64/sve/cost_model_1.c 2019-03-18 09:55:03.257194326 +0000
@@ -0,0 +1,12 @@
+/* { dg-options "-O2 -ftree-vectorize -fdump-tree-vect-details" } */
+
+void
+f (unsigned int *restrict x, unsigned int *restrict y,
+ unsigned char *restrict z, unsigned int n)
+{
+ for (unsigned int i = 0; i < n % 4; ++i)
+ x[i] = x[i] + y[i] + z[i];
+}
+
+/* { dg-final { scan-tree-dump "not vectorized: estimated iteration count too small" vect } } */
+/* { dg-final { scan-tree-dump "vectorized 0 loops" vect } } */
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