https://gcc.gnu.org/g:4238e3470d3fa9b9697f9cf6ad26d4ef76fdf248
commit r16-1300-g4238e3470d3fa9b9697f9cf6ad26d4ef76fdf248
Author: Tamar Christina <tamar.christ...@arm.com>
Date: Mon Jun 9 07:03:27 2025 +0100
middle-end: Add new parameter to scale scalar loop costing in vectorizer
This patch adds a new param vect-scalar-cost-multiplier to scale the scalar
costing during vectorization. If the cost is set high enough and when using
the dynamic cost model it has the effect of effectively disabling the
costing vs scalar and assumes all vectorization to be profitable.
This is similar to using the unlimited cost model, but unlike unlimited it
does not fully disable the vector cost model. That means that we still
perform comparisons between vector modes. And it means it also still does
costing for alias analysis.
As an example, the following:
void
foo (char *restrict a, int *restrict b, int *restrict c,
int *restrict d, int stride)
{
if (stride <= 1)
return;
for (int i = 0; i < 3; i++)
{
int res = c[i];
int t = b[i * stride];
if (a[i] != 0)
res = t * d[i];
c[i] = res;
}
}
compiled with -O3 -march=armv8-a+sve -fvect-cost-model=dynamic fails to
vectorize as it assumes scalar would be faster, and with
-fvect-cost-model=unlimited it picks a vector type that's so big that the
large
sequence generated is working on mostly inactive lanes:
...
and p3.b, p3/z, p4.b, p4.b
whilelo p0.s, wzr, w7
ld1w z23.s, p3/z, [x3, #3, mul vl]
ld1w z28.s, p0/z, [x5, z31.s, sxtw 2]
add x0, x5, x0
punpklo p6.h, p6.b
ld1w z27.s, p4/z, [x0, z31.s, sxtw 2]
and p6.b, p6/z, p0.b, p0.b
punpklo p4.h, p7.b
ld1w z24.s, p6/z, [x3, #2, mul vl]
and p4.b, p4/z, p2.b, p2.b
uqdecw w6
ld1w z26.s, p4/z, [x3]
whilelo p1.s, wzr, w6
mul z27.s, p5/m, z27.s, z23.s
ld1w z29.s, p1/z, [x4, z31.s, sxtw 2]
punpkhi p7.h, p7.b
mul z24.s, p5/m, z24.s, z28.s
and p7.b, p7/z, p1.b, p1.b
mul z26.s, p5/m, z26.s, z30.s
ld1w z25.s, p7/z, [x3, #1, mul vl]
st1w z27.s, p3, [x2, #3, mul vl]
mul z25.s, p5/m, z25.s, z29.s
st1w z24.s, p6, [x2, #2, mul vl]
st1w z25.s, p7, [x2, #1, mul vl]
st1w z26.s, p4, [x2]
...
With -fvect-cost-model=dynamic --param vect-scalar-cost-multiplier=200
you get more reasonable code:
foo:
cmp w4, 1
ble .L1
ptrue p7.s, vl3
index z0.s, #0, w4
ld1b z29.s, p7/z, [x0]
ld1w z30.s, p7/z, [x1, z0.s, sxtw 2]
ptrue p6.b, all
cmpne p7.b, p7/z, z29.b, #0
ld1w z31.s, p7/z, [x3]
mul z31.s, p6/m, z31.s, z30.s
st1w z31.s, p7, [x2]
.L1:
ret
This model has been useful internally for performance exploration and
cost-model
validation. It allows us to force realistic vectorization overriding the
cost
model to be able to tell whether it's correct wrt to profitability.
gcc/ChangeLog:
* params.opt (vect-scalar-cost-multiplier): New.
* tree-vect-loop.cc (vect_estimate_min_profitable_iters): Use it.
* doc/invoke.texi (vect-scalar-cost-multiplier): Document it.
gcc/testsuite/ChangeLog:
* gcc.target/aarch64/sve/cost_model_16.c: New test.
Diff:
---
gcc/doc/invoke.texi | 4 ++++
gcc/params.opt | 4 ++++
.../gcc.target/aarch64/sve/cost_model_16.c | 21 +++++++++++++++++++++
gcc/tree-vect-loop.cc | 3 ++-
4 files changed, 31 insertions(+), 1 deletion(-)
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 189a52b9b959..17929b3cf150 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -17325,6 +17325,10 @@ this parameter. The default value of this parameter
is 50.
@item vect-induction-float
Enable loop vectorization of floating point inductions.
+@item vect-scalar-cost-multiplier
+Apply the given multiplier % to scalar loop costing during vectorization.
+Increasing the cost multiplier will make vector loops more profitable.
+
@item vrp-block-limit
Maximum number of basic blocks before VRP switches to a lower memory algorithm.
diff --git a/gcc/params.opt b/gcc/params.opt
index 1f0abeccc4b9..a67f900a63f7 100644
--- a/gcc/params.opt
+++ b/gcc/params.opt
@@ -1253,6 +1253,10 @@ The maximum factor which the loop vectorizer applies to
the cost of statements i
Common Joined UInteger Var(param_vect_induction_float) Init(1) IntegerRange(0,
1) Param Optimization
Enable loop vectorization of floating point inductions.
+-param=vect-scalar-cost-multiplier=
+Common Joined UInteger Var(param_vect_scalar_cost_multiplier) Init(100)
IntegerRange(0, 10000) Param Optimization
+The scaling multiplier as a percentage to apply to all scalar loop costing
when performing vectorization profitability analysis. The default value is 100.
+
-param=vrp-block-limit=
Common Joined UInteger Var(param_vrp_block_limit) Init(150000) Optimization
Param
Maximum number of basic blocks before VRP switches to a fast model with less
memory requirements.
diff --git a/gcc/testsuite/gcc.target/aarch64/sve/cost_model_16.c
b/gcc/testsuite/gcc.target/aarch64/sve/cost_model_16.c
new file mode 100644
index 000000000000..bfe49ef15f3a
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/sve/cost_model_16.c
@@ -0,0 +1,21 @@
+/* { dg-do compile } */
+/* { dg-options "-Ofast -march=armv8-a+sve --param
vect-scalar-cost-multiplier=1000 -fdump-tree-vect-details" } */
+
+void
+foo (char *restrict a, int *restrict b, int *restrict c,
+ int *restrict d, int stride)
+{
+ if (stride <= 1)
+ return;
+
+ for (int i = 0; i < 3; i++)
+ {
+ int res = c[i];
+ int t = b[i * stride];
+ if (a[i] != 0)
+ res = t * d[i];
+ c[i] = res;
+ }
+}
+
+/* { dg-final { scan-tree-dump "vectorized 1 loops in function" "vect" } } */
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 8c5761d3c55d..9ac4d7e5f7a0 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -4646,7 +4646,8 @@ vect_estimate_min_profitable_iters (loop_vec_info
loop_vinfo,
TODO: Consider assigning different costs to different scalar
statements. */
- scalar_single_iter_cost = loop_vinfo->scalar_costs->total_cost ();
+ scalar_single_iter_cost = (loop_vinfo->scalar_costs->total_cost ()
+ * param_vect_scalar_cost_multiplier) / 100;
/* Add additional cost for the peeled instructions in prologue and epilogue
loop. (For fully-masked loops there will be no peeling.)
