[polly] [Polly] Allocate packed arrays of matrix multiplication on heap (PR #226163)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur requested changes to this pull request.

Use of the stack is generally preferable. Heap allocation is additional overhead. See e.g. #153374 #152639. Users may run run performance-sensitive applications with `ulimit -s unlimited`.

The usual approach:
1. Use stack if the array size is statically known to be below a threshold
2. Allow that threshold to be configured or switched off (Fortran: `-fhead-array`, `-fstack-array`)

Whether to use heap or stack arrays in JSON import is a configuration paramter, the default being the stack. The claim from the summary is incorrect.

https://github.com/llvm/llvm-project/pull/226163