The LoopInterchange cost-model consists of several decision rules. They
are called one by one, and if some rule can determine the profitability,
then the subsequent rules aren't called. In the current implementation,
the rule for `CacheCostAnalysis` is called first, and if it fails to
determine the profitability, then the rule for vectorization is called.
However, there are cases where interchanging loops for vectorization
makes the code faster even if such exchanges are detrimental to the
cache. For example, exchanging the inner two loops in the following
example looks about x3 faster in my local (compiled with `-O3
-mcpu=neoverse-v2 -mllvm -cache-line-size=64`), even though it's
rejected by the rule based on cache cost. (NOTE: LoopInterchange cannot
exchange these loops due to legality checks. This should also be
improved.)
```c
__attribute__((aligned(64))) float aa[256][256],bb[256][256],cc[256][256],
dd[256][256],ee[256][256],ff[256][256];
// Alternative of TSVC s231 with more array accesses than the original.
void s231_alternative() {
for (int nl = 0; nl < 100*(100000/256); nl++) {
for (int i = 0; i < 256; ++i) {
for (int j = 1; j < 256; j++) {
aa[j][i] = aa[j-1][i] + bb[j][i] + cc[i][j]
+ dd[i][j] + ee[i][j] + ff[i][j];
}
}
}
}
```
This patch introduces a new option to prioritize the vectorization rule
over the cache cost rule.
Related issue: #131130
---------
Co-authored-by: Florian Hahn <flo@fhahn.com>
