Professional slashbox alternative

No vertical rules and no vertical words. The column of inputs can be topped by the word "Input" and the output columns can be easily grouped. I suggest two possibilities.

\documentclass{article}
\usepackage{booktabs}

\begin{document}

\begin{tabular}{l*{6}{c}}
\toprule
& \multicolumn{6}{c}{Output} \\
\cmidrule(lr){2-7}
Input & int8\_t & int16\_t & int32\_t & uint8\_t & uint16\_t & uint32\_t \\
\midrule
int8\_t & a & b & c & d & e & f \\
int16\_t & g & h & i & j & k & l \\
int32\_t & m & n & o & p & q & r \\
uint8\_t & s & t & u & v & w & x \\
uint16\_t & y & z & a & b & c & d \\
uint32\_t & e & f & g & h & i & j \\
\bottomrule
\end{tabular}

\bigskip

\begin{tabular}{l*{6}{c}}
\toprule
& \multicolumn{6}{c}{Output} \\
\cmidrule(lr){2-7}
Input & int8\_t & int16\_t & int32\_t & uint8\_t & uint16\_t & uint32\_t \\
\cmidrule(lr){1-1}
\cmidrule(lr){2-7}
int8\_t & a & b & c & d & e & f \\
int16\_t & g & h & i & j & k & l \\
int32\_t & m & n & o & p & q & r \\
uint8\_t & s & t & u & v & w & x \\
uint16\_t & y & z & a & b & c & d \\
uint32\_t & e & f & g & h & i & j \\
\bottomrule
\end{tabular}

\end{document}

First of all, as I suggested in my comment, I would rather have the \midrule one row below:

I think grouping 'Output' with the headers makes more sense, logically, just as you did with 'Input'.

As an alternative to the slashbox package there is diagbox. The description in the documentation states:

diagbox is a modern alternative of slashbox.

I also like it better, is easier to customize and the diagonal line (now drawn with pict2e) is much nicer. Here's an example using it:

\begin{table}
  \centering
    \begin{tabular}{l|*{6}r}
    \toprule
    \diagbox{Input}{Output} & int8\_t & int16\_t & int32\_t & uint8\_t & uint16\_t & uint32\_t \\
    \midrule
    int8\_t & a & b & c & d & e & f \\
    int16\_t & g & h & i & j & k & l \\
    int32\_t & m & n & o & p & q & r \\
    uint8\_t & s & t & u & v & w & x \\
    uint16\_t & y & z & a & b & c & d \\
    uint32\_t & e & f & g & h & i & j \\
    \bottomrule
    \end{tabular}%
  \label{tab:addanotherlabel}%
\end{table}%

Overall a good result, but I was a bit dissatisfied with the placement of 'Output', so I tweaked the table. (Fortunately, diagbox is rather flexible.) The final result I came up with is a heavily modified (but IMHO visually the most appealing) version:

\begin{table}
  \centering
    \begin{tabular}{l|*{6}r}
    \toprule
    %\backslashbox{Input}{Output} & int8\_t & int16\_t & int32\_t & uint8\_t & uint16\_t & uint32\_t \\
    \diagbox[width=2.5cm, height=2.5cm]{\raisebox{5pt}{\hspace*{0.25cm}Input}}{\raisebox{-1.27cm}{\rotatebox{90}{Output}}} & \raisebox{-0.25cm}{\rotatebox{90}{int8\_t}} & \raisebox{-0.25cm}{\rotatebox{90}{int16\_t}} & \raisebox{-0.25cm}{\rotatebox{90}{int32\_t}} & \raisebox{-0.25cm}{\rotatebox{90}{uint8\_t}} & \raisebox{-0.25cm}{\rotatebox{90}{uint16\_t}} & \raisebox{-0.25cm}{\rotatebox{90}{uint32\_t}} \\
    \midrule\\
    \hspace*{0.25cm}int8\_t & a & b & c & d & e & f \\ \\
    \hspace*{0.25cm}int16\_t & g & h & i & j & k & l \\ \\
    \hspace*{0.25cm}int32\_t & m & n & o & p & q & r \\ \\
    \hspace*{0.25cm}uint8\_t & s & t & u & v & w & x \\ \\
    \hspace*{0.25cm}uint16\_t & y & z & a & b & c & d \\ \\
    \hspace*{0.25cm}uint32\_t & e & f & g & h & i & j \\ \\
    \bottomrule
    \end{tabular}%
  \label{tab:addlabel}%
\end{table}%

On a final note, I believe vertical lines are sometimes necessary and I don't think there is anything wrong with using one here. It is a guide for the eye to help separate logical groups from each other (e.g. headers from raw data) in the table. (For a discussion see here: Why not use vertical lines ('|') in a tabular?)