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