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

[Chandler Carruth via llvm-dev]

On Sat, Sep 24, 2016 at 4:39 PM Duncan P. N. Exon Smith via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> > On 2016-Sep-24, at 15:16, Mehdi Amini <mehdi.amini at apple.com> wrote:
> >
> >>
> >> 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?
>
> You mean something like PureConstantVector (which cannot transitively
> reference GlobalValue) vs ConstantVectorWithGlobalRef (which can/must
> transitively reference GlobalValue), right?  (And also for ConstantStruct,
> ConstantArray, and ConstantExpr, etc.)
>
> I hadn't considered that, and it seems worth thinking about.  I'm unsure
> whether using isa<>() would really be cleaner than using Value::hasUseList;
> and it would certainly be intrusive.  Do you see any concrete benefits?
>
> One possible long-term thing (after #4)... we could add ConstantDataUser
> (vs. User), which can only reference a Constant-with-no-GlobalValue, and
> has operands the size of a pointer.  Obviously nice to save on
> operand-size, but I'm not convinced it would save sufficient memory to be
> worthwhile: IIRC, Instruction accounts for most instances of User.
>

FWIW, I still would find this interesting, at least for conceptual
improvements in the IR.

I'd really like it if we could separate the ideas of true constants (that
are inherently foldable and don't need use lists) and things that
transitively reference globals.

Recently I've been thinking that the split which might make sense would be
to have Constants which have no use-lists and must be manifest (no
references to globals, and GlobalExprs which have use lists and can
reference Globals (as wall as Constants).

This might in turn allow us to consider better models for things like
expanding constantexprs that don't fit any relocations on the platform.


Personally, I'd probably stop after your #2 unless/until we do something to
re-think the class hierarchy here.

-Chandler
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