On 6/16/2026 8:52 PM, Zhongyao Chen wrote:
This applies a default 2:1 scalar/vector unit ratio to integer vector
body costs, which helps the cost model better match the actual hardware
throughput.
Take an int64 vadd at VLEN=128 for example: 4 scalar units and 2 vector
units can both process 4 elements per cycle. Without this ratio, both
vector and scalar stmts just get a cost of 1, which ends up making
vectorization look too cheap.
Skip applying this ratio for:
- reduction stmts
- gather/scatter stmts
Since this is just an RFC patch, a few things need comments:
1. The ratio is hardcoded to 2 for now.
- Should this come from the CPU model, tune info, or a param
instead? I'm leaving it as is for future work.
- How should we handle non-integer ratios like 3/2 ?
I was thinking about using issue rate from the tune structure as a
reasonable proxy for how wide the scalar part of the design. Then you
compare that to the VL of the code you want to vectorize. If the scalar
bandwidth is >= the number of vector elements you're handling per vector
op, then the scalar loop is probably preferred. It's not exact, but it
doesn't rely on magic numbers.
Jeff
