https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110692
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu.org
Keywords| |missed-optimization
Component|middle-end |tree-optimization
Blocks| |53947
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
With the variable bound we have
a[i_11] 1 times vector_load costs 12 in body
_1 + 1 1 times scalar_to_vec costs 4 in prologue
_1 + 1 1 times vector_stmt costs 4 in body
_2 1 times vector_store costs 12 in body
t.c:4:28: note: operating on full vectors for epilogue loop.
t.c:4:28: note: cost model: epilogue peel iters set to vf/2 because loop
iterations are unknown .
a[i_11] 1 times scalar_load costs 12 in epilogue
_1 + 1 1 times scalar_stmt costs 4 in epilogue
_2 1 times scalar_store costs 12 in epilogue
<unknown> 1 times cond_branch_taken costs 16 in epilogue
t.c:4:28: note: Cost model analysis:
Vector inside of loop cost: 28
Vector prologue cost: 4
Vector epilogue cost: 44
Scalar iteration cost: 28
Scalar outside cost: 32
Vector outside cost: 48
prologue iterations: 0
epilogue iterations: 1
Calculated minimum iters for profitability: 1
t.c:4:28: note: Runtime profitability threshold = 2
t.c:4:28: note: Static estimate profitability threshold = 4
t.c:4:28: missed: not vectorized: estimated iteration count too small.
with the static bound:
a[i_10] 1 times vector_load costs 12 in body
_1 + 1 1 times scalar_to_vec costs 4 in prologue
_1 + 1 1 times vector_stmt costs 4 in body
_2 1 times vector_store costs 12 in body
t.c:4:28: note: operating on full vectors for epilogue loop.
a[i_10] 1 times scalar_load costs 12 in epilogue
_1 + 1 1 times scalar_stmt costs 4 in epilogue
_2 1 times scalar_store costs 12 in epilogue
t.c:4:28: note: Cost model analysis:
Vector inside of loop cost: 28
Vector prologue cost: 4
Vector epilogue cost: 28
Scalar iteration cost: 28
Scalar outside cost: 0
Vector outside cost: 32
prologue iterations: 0
epilogue iterations: 1
Calculated minimum iters for profitability: 2
t.c:4:28: note: Runtime profitability threshold = 2
t.c:4:28: note: Static estimate profitability threshold = 2
so it's the branch of the epilog of the epilog that ups the cost, not sure
whether in a reasonable way for this case. In the end I think if the
count is unknown a fully peeled epilog with 2 iterations is a reasonable
implementation.
We could decide that costing the epilogue vectorization isn't worthwhile
but note we are not implementing all 8 byte vector ops with the same
efficiency as the 16 byte ones.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
