https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84512
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |ASSIGNED
Version|tree-ssa |8.0
Keywords| |missed-optimization
Last reconfirmed| |2018-02-27
Blocks| |53947
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot
gnu.org
Ever confirmed|0 |1
Summary|Missed optimization: should |[8 Regression] Missed
|be precalculated in |optimization: should be
|compile-time |precalculated in
| |compile-time
Target Milestone|--- |8.0
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. This is another case where we vectorize one loop but not the other
and DOM doesn't handle removing vector loads against scalar stores. Later
store-merging aggregates the stores but nothig performs CSE after it.
The vectorizer decides that vectorizing the reduction is profitable while
vectorizing the init is not:
t.c:4:3: note: Cost model analysis:
Vector inside of loop cost: 68
Vector prologue cost: 8
Vector epilogue cost: 128
Scalar iteration cost: 16
Scalar outside cost: 0
Vector outside cost: 136
prologue iterations: 0
epilogue iterations: 2
t.c:4:3: note: cost model: the vector iteration cost = 68 divided by the scalar
iteration cost = 16 is greater or equal to the vectorization factor = 4.
t.c:4:3: note: not vectorized: vectorization not profitable.
With -fno-vect-cost-model we vectorize both loops and optimize the function
like clang does.
The issue with the cost model here is that for the scalar iteration cost
we end up using builtin_vectorization_cost () while for the vector cost
we use add_stmt_cost. Only the latter makes a difference between the
different kind of operations.
I have a patch.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
