llvm dev - Jan 2021 - TableGen: behavior of list subscript

TableGen allows subscripting a list:

  string Foo = SomeList[1];

The documentation for this feature:

value[4...7,17,2...3,4] 
The final value is a new list that is a slice of the list value (note the
brackets). The new list contains elements 4, 5, 6, 7, 17, 2, 3, and 4. Elements
may be included multiple times and in any order.

It turns out that in some contexts a single subscript, as in the example above,
does indeed produce a list of element 1, but in most contexts it produces
element 1 itself (i.e., it's a typical subscripting operation). In all
contexts, multiple subscripts produce a list of the specified elements.

The most obvious case where it produces a list of the single element is if
SomeList were a defvar.

Ouch. I'm tempted to change the subscripting feature so that a single
subscript *always* produces the element itself, not a list of the element.
Multiple searches of all the TableGen files did not uncover any reliance on the
contexts where a single subscript produces a list.

Opinions, please.

> On Jan 21, 2021, at 7:34 AM, Paul C. Anagnostopoulos via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 
> TableGen allows subscripting a list:
> 
>  string Foo = SomeList[1];
> 
> The documentation for this feature:
> 
> value[4...7,17,2...3,4] 
> The final value is a new list that is a slice of the list value (note the
brackets). The new list contains elements 4, 5, 6, 7, 17, 2, 3, and 4. Elements
may be included multiple times and in any order.
> 
> It turns out that in some contexts a single subscript, as in the example
above, does indeed produce a list of element 1, but in most contexts it produces
element 1 itself (i.e., it's a typical subscripting operation). In all
contexts, multiple subscripts produce a list of the specified elements.
> 
> The most obvious case where it produces a list of the single element is if
SomeList were a defvar.
> 
> Ouch. I'm tempted to change the subscripting feature so that a single
subscript *always* produces the element itself, not a list of the element.
Multiple searches of all the TableGen files did not uncover any reliance on the
contexts where a single subscript produces a list.
> 
> Opinions, please.
This makes a lot of sense to me Paul!