https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116562
Bug ID: 116562
Summary: wrong cost of gather load preventing loop from
vectored
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: kugan at gcc dot gnu.org
Target Milestone: ---
typedef int real_t;
extern __attribute__((aligned(64))) real_t a[32000],b[32000],c[32000],d[32000];
void s4117()
{
for (int i = 0; i < 32000; i++) {
a[i] = b[i] + c[i/2] * d[i];
}
}
is not vectored for AdvSIMD due to wrong cost calculation.
Compiler option used: cc1plus -Ofast -fdump-tree-vect-all -mcpu=neoverse-v2
--param=aarch64-autovec-preference=1
tt.c:6:21: note: Cost model analysis:
Vector inside of loop cost: 64
Vector prologue cost: 0
Vector epilogue cost: 0
Scalar iteration cost: 15
Scalar outside cost: 0
Vector outside cost: 0
prologue iterations: 0
epilogue iterations: 0
tt.c:6:21: missed: cost model: the vector iteration cost = 64 divided by the
scalar iteration cost = 15 is greater or equal to the vectorization factor = 4.
tt.c:6:21: missed: not vectorized: vectorization not profitable.
tt.c:6:21: missed: not vectorized: vector version will never be profitable.
tt.c:6:21: missed: Loop costings may not be worthwhile.
tt.c:6:21: note: ***** Analysis failed with vector mode V4SI
We cost this c[i/2] as having the cost of 4 loads and one construct. I think we
should special case these sort of gather loads which as lower cost in practice?
11233 if (costing_p)
11234 {
11235 /* For emulated gathers N offset vector element
11236 offset add is consumed by the load). */
11237 inside_cost = record_stmt_cost (cost_vec,
const_nunits,
11238 vec_to_scalar,
stmt_info,
11239 0, vect_body);
11240 /* N scalar loads plus gathering them into a
11241 vector. */
11242 inside_cost
11243 = record_stmt_cost (cost_vec, const_nunits,
scalar_load,
11244 stmt_info, 0, vect_body);
11245 inside_cost
11246 = record_stmt_cost (cost_vec, 1, vec_construct,
11247 stmt_info, 0, vect_body);
11248 continue;
11249 }
