• Seth Brenith's avatar
    [torque] Add a way to specify that a class field is optional · c2d419a3
    Seth Brenith authored
    Currently, some ScopeInfo fields are defined as indexed fields with a
    length of either one or zero, because the field might be present or it
    might not. Based on comments in https://crrev.com/c/v8/v8/+/2601880 ,
    this strategy is not sustainable and we need a better way to represent
    optional fields so that we don't have to pass zero when accessing their
    only element. This change is a proposal to fix that problem.
    
    Syntax:
    
    I'm proposing using a question mark because TypeScript does, and Torque
    syntax looks somewhat like TypeScript. I don't feel strongly about this
    though, and I'm open to other suggestions.
      field_name?[condition_expression]: FieldType;
    
    Internal Torque compiler representation:
    
    Internally, I've updated the Torque compiler to still treat these fields
    as indexed, but with an extra flag saying they're optional. When getting
    a LocationReference for a field access expression on an optional field,
    Torque produces a Slice like it would for any other indexed field and
    subsequently calls AtIndex(0) to get a Reference.
    
    AtIndex can crash the process if the index is out of bounds (which is
    good), so some other parts of the Torque compiler need minor adjustments
    so that it doesn't take references to optional fields unless it actually
    needs them.
    
    Initialization:
    
    This proposal doesn't include any changes to initialization logic, so an
    optional field can still be initialized using '...' and an iterator.
    Perhaps we could introduce an Optional<T> struct for prettier
    initialization in a future change.
    
    Bug: v8:7793
    Change-Id: I37649495f4c259e685261f53e4cf2859da66a31f
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2706306
    Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
    Reviewed-by: 's avatarNico Hartmann <nicohartmann@chromium.org>
    Reviewed-by: 's avatarLeszek Swirski <leszeks@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#73018}
    c2d419a3
types.cc 45 KB