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

Mehdi Amini via llvm-dev llvm-dev at lists.llvm.org
Sat Sep 24 15:16:35 PDT 2016

> On Sep 24, 2016, at 3:06 PM, Duncan P. N. Exon Smith via llvm-dev <llvm-dev at lists.llvm.org> 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.

I wonder if the constant class hierarchy should not be revisited in light of this?
For instance you identified that some are local to a module while others are “context-wide”.

I haven’t given too much thoughts about this, but I'm curious if you did?


>  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?
> <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>_______________________________________________
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