https://sigarch.org/simd-instructions-considered-harmful
\item Setup and corner-cases alone are extremely complex.\\
Hardware is easy, but software is hell.
- \item O($N^{6}$) ISA opcode proliferation!\\
+ \item O($N^{6}$) ISA opcode proliferation (1000s of instructions)\\
opcode, elwidth, veclen, src1-src2-dest hi/lo
\end{itemize}
}
\begin{itemize}
\item Effectively a variant of SIMD / SIMT (arbitrary length)\vspace{4pt}
+ \item Fascinatingly, despite being a SIMD-variant, RVV only has
+ O(N) opcode proliferation! (extremely well designed)
\item Extremely powerful (extensible to 256 registers)\vspace{4pt}
\item Supports polymorphism, several datatypes (inc. FP16)\vspace{4pt}
\item Requires a separate Register File (32 w/ext to 256)\vspace{4pt}
\end{itemize}
However...
\begin{itemize}
- \item 98 percent opcode duplication with rest of RV (CLIP)
+ \item 98 percent opcode duplication with rest of RV
\item Extending RVV requires customisation not just of h/w:\\
gcc, binutils also need customisation (and maintenance)
- \item Fascinatingly, despite being a SIMD-variant, RVV only has
- O(N) opcode proliferation! (extremely well designed)
\end{itemize}
}
\frametitle{Register key-value CSR table decoding pseudocode}
\begin{semiverbatim}
-struct vectorised fp\_vec[32], int\_vec[32]; // 64 in future
+struct vectorised fp\_vec[32], int\_vec[32];
for (i = 0; i < 16; i++) // 16 CSRs?
tb = int\_vec if CSRvec[i].type == 0 else fp\_vec
idx = CSRvec[i].regkey // INT/FP src/dst reg in opcode
tb[idx].elwidth = CSRvec[i].elwidth
tb[idx].regidx = CSRvec[i].regidx // indirection
+ tb[idx].regidx += CSRvec[i].bank << 5 // 0 (1=rsvd)
tb[idx].isvector = CSRvec[i].isvector
tb[idx].packed = CSRvec[i].packed // SIMD or not
- tb[idx].bank = CSRvec[i].bank // 0 (1=rsvd)
tb[idx].enabled = true
\end{semiverbatim}
\frametitle{Predication key-value CSR table decoding pseudocode}
\begin{semiverbatim}
-struct pred fp\_pred[32], int\_pred[32]; // 64 in future
+struct pred fp\_pred[32], int\_pred[32];
for (i = 0; i < 16; i++) // 16 CSRs?
tb = int\_pred if CSRpred[i].type == 0 else fp\_pred
idx = CSRpred[i].regkey
tb[idx].zero = CSRpred[i].zero // zeroing
tb[idx].inv = CSRpred[i].inv // inverted
tb[idx].predidx = CSRpred[i].predidx // actual reg
- tb[idx].bank = CSRpred[i].bank // 0 for now
+ tb[idx].predidx += CSRvec[i].bank << 5 // 0 (1=rsvd)
tb[idx].enabled = true
\end{semiverbatim}