[llvm] [RISCV] Prefer VLS over VLA if costs are equal (PR #100564)

[Craig Topper via llvm-commits]

topperc wrote:

Is the cost affected by Zvl*b? I see in the test case, SCALABLE is LMUL=2 vectorization. For FIXED, we use LMUL=2 registers, but VL=8. If the runtime VLEN is greater than 128 the scalable code will be able to process more elements per iteration.

If the microarchitecture doesn't dynamically reduce LMUL based on VL, then the FIXED code will waste resources with VLEN > 128 since all 8 elements will fit in half of an LMUL=2 register.

https://github.com/llvm/llvm-project/pull/100564