[HLSL] define the matrix element accessor semantics

[farzonl]

fixes #677

Moved the Vector Swizzle stuff that repeats with Matrix and moved it to a generic swizzle section.

The new Matrix Swizzle looks like this:

[farzonl]

What is the propper way in the grammar to specify that ._m and ._ can happen multiple times like in vector swizzle?

[llvm-beanz]

You'll want something like:

Suggested change

	postfix-expression \terminal{.\_<integer-literal><integer-literal>}\br
	postfix-expression \terminal{.\_m<integer-literal><integer-literal>}\br
	postfix-expression matrix-zero-indexed-accessor\br
	postfix-expression matrix-one-indexed-accessor\br
	\define{matrix-zero-indexed-accessor}\br
	\opt{matrix-zero-indexed-accessor} \terminal{_m} integer-literal\br
	\define{matrix-one-indexed-accessor}\br
	\opt{matrix-one-indexed-accessor} \terminal{_} integer-literal\br

[llvm-beanz]

Oh, we also should not use integer-literal here, we should define separate index grammar elements because the valid characters are 0 1 2 3 and 1 2 3 4 not the full space of integer-literal which is more like [0-9]+.

[farzonl]

need to escape the _ via \_ otherwise my latex compiler breaks.

As an asside I'm also seeing this warning everywhere

• Underfull \hbox (badness 10000) in paragraph at lines 172--181
• Underfull \hbox (badness 10000) in paragraph at lines 129--131

Doesn't seem to impact spec generation so I'm going to ignore it for now.

[llvm-beanz]

I think I set you on the wrong path for the grammar. The grammar should probably be something more like:

\begin{grammar}
  \define {matrix-accessor-sequence}\br
  matrix-zero-indexed-accessor-sequence\br
  matrix-one-indexed-accessor-sequence\br

  \define{matrix-zero-indexed-accessor-sequence}\br
  matrix-zero-indexed-accessor\br
  matrix-zero-indexed-accessor-sequence matrix-zero-indexed-accessor\br

  \define{matrix-zero-indexed-accessor}\br
  \terminal{\_m} zero-index-value\br

  \define{zero-index-value}\br
  \terminal{0}\br \terminal{1}\br \terminal{2}\br \terminal{3}\br

  \define{matrix-one-indexed-accessor-sequence}\br
  matrix-one-indexed-accessor\br
  matrix-one-indexed-accessor-sequence matrix-one-indexed-accessor\br

  \define{matrix-one-indexed-accessor}\br
  \terminal{\_m} one-index-value\br

  \define{one-index-value}\br
  \terminal{1}\br \terminal{2}\br \terminal{3}\br \terminal{4}\br

\end{grammar}

Two things to note:

(1) you're not defining postfix-expression, that's defined above, you should be adding a formulation something like postix-expression \terminal{.} matrix-accessor-sequence.

(2) We should describe subscripts on matrices separately because they behave quite differently, but you actually don't need to describe the grammar since it works recursively from the already defined [] grammars in Expr.Post.

[farzonl]

Should we define zero-index-value and one-index-value in Lex.Literal.Int and reuse concepts like nonzero-digit

I had a thought to define it as

\define{one-index-value}\br
  nonzero-dim-constraint-digit}\br

\[
\begin{array}{rcl}
\textit{nonzero-digit} &::=& 1 \mid 2 \mid 3 \mid 4 \mid 5 \mid 6 \mid 7 \mid 8 \mid 9 \\[4pt]
\textit{nonzero-dim-constraint-digit} &::=& \textit{nonzero-digit} \quad \text{where value} \le 4
\end{array}
\]

[llvm-beanz]

I wouldn't put them in the Lex chapter. They're only valid grammar constructs in this narrow space, and I think it's a lot simpler and clearer to read to just list the four possible values for each context.

[farzonl]

@llvm-beanz One unique thing I was thinking about indicating is that a matrix swizzle can not be more than 4 elements. That seems to be implied later when we indicate that a swizzle expression is a vector. However, with long vectors coming in SM 6.9 does this 4 element limitation on matrix swizzles go away? https://hlsl.godbolt.org/z/r9o5PKPWr because I am still seeing it in godbolt: https://hlsl.godbolt.org/z/fdnWqq7cK.

[llvm-beanz]

We also don't allow vector swizzles to be more than 4 elements. I think we should keep the 4 element restriction.

[llvm-beanz]

A few suggestions, but otherwise LGTM.