[llvm-dev] RFC: ConstantData should not have use-lists

[Duncan P. N. Exon Smith via llvm-dev]

r261464 added a type called ConstantData to the Value hierarchy.  This
is a parent type for constants with no operands, such as i32 0 and null.

Since then, I've removed most instances of iterating through the
use-lists of an instance of ConstantData.  I'd like to make this
illegal.  Since the users of ConstantData are spread across an
LLVMContext, most code that looks at the users is wrong.  Adding an
assertion should catch a lot of bugs (see r263853 and r263875) and
avoid some expensive walks through uninteresting code.

(The same is not true of Constant, generally.  A GlobalValue's use-list
will local to the GlobalValue's Module.  Any ConstantVector,
ConstantArray, or ConstantStruct that points at a GlobalValue will also
be local to the same Module.  In these cases, we also need RAUW
support.)

Besides catching bugs, removing use-lists from ConstantData will
guarantee that the compiler output *does not* depend on the use-list
order of something like i32 0.

Finally, this should dramatically reduce the overhead of serializing
use-list order in bitcode.  We will no longer track the arbitrary
order of references to things like i32 0 and null. 

What's left?
============

I just filed PR30513 to track remaining work.

 1. Avoid the remaining uses of ConstantData use-lists.  There are only
    a couple of cases left, highlighted in the WIP HACK patches attached
    below (0001 and 0002).
 
 2. Remove the use-lists!  Replace them with ref-counts to keep most of
    the use-list API functional (and minimize the size of the change).
    See the WIP patch below (0003).

 3. (Optional) Remove use-lists from other non-GlobalValue Constants
    that do not reference any GlobalValues.  This would require some
    sort of magic in, e.g., ConstantVector to conditionally have a
    use-list.  Call sites of API like Value::use_begin would have to
    check for Value::hasUseList.

 4. (Optional) Remove the ref-count from ConstantData (and, potentially,
    other use-list-free Constants).  This would eliminate ref-count
    traffic, but would also require checking at call sites before using
    any use-list-related API.

Feedback
========

- Does anyone disagree with this general direction?  Why?
- Any thoughts on #3?
- Any thoughts on #4?

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