686589 - Aliasing region for vector metadata

Status: RESOLVED WONTFIX

(Tamarin Graveyard :: Baseline JIT (CodegenLIR), defect)

Description

[Lars T Hansen]

Suppose a loop like this:

  var v: Vector.<int> = ...
  var n: int = 0
  for ( var i:int=0 ; i < v.length ; i++ )
    n += v[i]

We want to set up an aliasing region that covers vector metadata: the length field and the "data" pointer.  Operations (stores, calls) that do not destroy vector metadata should be marked as such, allowing the metadata to be cached across those operations.  In the loop above the length should be loaded once and used both for the loop check and the access bounds check.  (A reasonable optimizer would then remove the latter.)  The vector's data pointer should be loaded once and kept for the entire loop.

Operations that definitely do not alter vector metadata are traps in general, vector "read" helpers, but probably many JIT helpers.  TBD.