Mill Computing, Inc. Forums The Mill Architecture Scratchpad design decision Reply To: Scratchpad design decision

Ivan Godard
Keymaster
Post count: 629

As a slight variation on the multiple values in one go theme, would a “touch” like operation that specifies multiple belt positions (maybe just in the last N positions) that are needed soon again help in any way?

That’s what “rescue” does.

Also, never having worked with an architecture that exposed a scratchpad to me, I am wondering how its typically used. If it would be for constants that are needed a few times during execution of code, I imagine normal memory addressing and the cache system would work just fine. Is there a typical cutoff point where the scratchpad starts to benefit and at what sizes?

The compiler doesn’t load anything twice from memory/cache that it knows is unchanged; memory bandwidth is limited and cache is a power hog. Generally literal constants and some LEAs are simply redone if needed a second time, rather than being run off to scratch. We should probably revisit this for quad FP literals, assuming that a spill/fill is cheaper than a 16-byte literal, but we don’t have much real experience with quad yet. Some programs have memory arrays of constants that they load with explicit addressing; LLVM is a little erratic in how it treats those loads (to constexpr or not), but we just take what LLVM gives us.

  • This reply was modified 3 years, 5 months ago by  Ivan Godard.