Philip Reames 2e7c7d20d5 [RISCV][TTI] Adjust cost for extract/insert element when VLEN is known (#108595) ...

If we know an exact VLEN, then the index is effectively modulo the
number of elements in a single vector register. Our lowering performs
this subvector optimization.

A bit of context. This change may look a bit strange on it's own given
we are currently *not* scaling insert/extract cost by LMUL. This costing
decision needs to change, but is very intertwined with SLP
profitability, and is thus a bit hard to adjust. I'm hoping that
https://github.com/llvm/llvm-project/pull/108419 will let me start to
untangle this. This change is basically a case of finding a subset I can
tackle before other dependencies are in place which does no real harm in
the meantime.

2024-09-17 08:43:40 -07:00

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