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

[Vedant Kumar via llvm-dev]

> On Sep 24, 2016, at 3:06 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 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.

Could you explain why #3 requires checking for hasListList() or e.g
isa<ConstantVectorWithGlobalRef>() before calling use_begin()?


> 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?
> 
> <0001-WIP-HACK-SimplifyLibCalls-Disable-optimizeSinCosPi-o.patch>
> <0002-WIP-HACK-LICM-Ignore-stores-to-UndefValue-and-Consta.patch>
> <0003-WIP-IR-Remove-use-lists-from-ConstantData.patch>

We should probably improve the use-list-order2.ll test instead of deleting it.
I've attached a patch to PR24755 which might help with this ("Improve use-list
order testing for function operands"). I think the test should fail if its uses
of ConstantData are replaced by e.g personality functions.

vedant