TableGen: Streamline the semantics of NAME
The new rules are straightforward. The main rules to keep in mind
- NAME is an implicit template argument of class and multiclass, and will be substituted by the name of the instantiating def/defm.
- The name of a def/defm in a multiclass must contain a reference to NAME. If such a reference is not present, it is automatically prepended.
And for some additional subtleties, consider these:
- defm with no name generates a unique name but has no special behavior otherwise.
- def with no name generates an anonymous record, whose name is unique but undefined. In particular, the name won't contain a reference to NAME.
Keeping rules 1&2 in mind should allow a predictable behavior of
name resolution that is simple to follow.
The old "rules" were rather surprising: sometimes (but not always),
NAME would correspond to the name of the toplevel defm. They were
also plain bonkers when you pushed them to their limits, as the old
version of the TableGen test case shows.
Having NAME correspond to the name of the toplevel defm introduces
"spooky action at a distance" and breaks composability:
refactoring the upper layers of a hierarchy of nested multiclass
instantiations can cause unexpected breakage by changing the value
of NAME at a lower level of the hierarchy. The new rules don't
suffer from this problem.
Some existing .td files have to be adjusted because they ended up
depending on the details of the old implementation.
Reviewers: tra, simon_tatham, craig.topper, MartinO, arsenm, javed.absar
Subscribers: wdng, llvm-commits
Differential Revision: https://reviews.llvm.org/D47430