[wasm][anyref] Implement sub-typing of anyref, anyfunc, and nullref
For the reference types anyref, anyfunc, and nullref, there exist sub-typing rules. The spec says A reference type reftype1 matches a reference type reftype2 if and only if: * Either both reftype1 and reftype2 are the same. * Or reftype1 is nullref. * Or reftype2 is anyref. This CL introduces the type nullref for ref-null, and implements the sub-typing rules in the function-body-decoder. Note that because of the sub-typing check validation performance may regresses. In that case we can optimize the sub-typing check. R=titzer@chromium.org, clemensh@chromium.org Bug: v8:7581 Change-Id: I55bab72a109f3374da3770d141b0fc8067aad8b2 Reviewed-on: https://chromium-review.googlesource.com/c/1430061 Commit-Queue: Andreas Haas <ahaas@chromium.org> Reviewed-by: Clemens Hammacher <clemensh@chromium.org> Reviewed-by: Ben Titzer <titzer@chromium.org> Cr-Commit-Position: refs/heads/master@{#59043}
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