3 ms·
> Who is going to be making heavy use of x87 or MMX (both obsolete) along with AVX-512 Edge case and still not likely to cause contention: x87 fadd has 3-cycle
by zbjornson 6y ago
> Who is going to be making heavy use of x87 or MMX (both obsolete) along with AVX-512
Edge case and still not likely to cause contention: x87 fadd has 3-cycle latency while AVX has 4-c lat through ICL. If you need to prepare some constant with a chain of high-precision additions to broadcast into an AVX reg, it's faster. (Especially if the alternative is compensated addition.)
- BeeOnRope 6y agoYes, that is interesting. fadd also uses a different port (p5) as compared to SSE/AVX FP stuff which uses p01, so it seems likely that there is separate dedicated hardware for the x87 stuff, probably in the slice between the main vector pipes and this can also handle the 80 bit stuff.