[llvm-dev] [IR] Modelling of GlobalIFunc

Tue Sep 14 11:13:33 PDT 2021

> On 2021-09-14, at 13:26, Fangrui Song <maskray at google.com> wrote:
> 
> On 2021-09-11, Itay Bookstein via llvm-dev wrote:
>> That really depends on what the correct behavior is for
>> getBaseObject(). As far as I can tell, this method was originally
>> added to pierce through GlobalAliases (including various
>> ConstantExpr-s) and get to the "subject" *GlobalObject* (which is its
>> return type).
>> 
>> When looking at getBaseObject() through these
>> GlobalAlias-colored-glasses, I think that the following invariant
>> should follow:
>> * getBaseObject() should be idempotent (at least for the cases that it
>> does not return a nullptr): GV->getBaseObject()->getBaseObject() ==
>> GV->getBaseObject().
>> Note that a GlobalAlias may have a different type than its base object
>> [4], so my previous arguments about a same-type-invariant are
>> incorrect.
>> 
>> Now, let us consider the case of a GlobalAlias where the aliasee is a
>> GlobalIFunc. This happens in glibc for example.
>> What should GA->getBaseObject() return in this case? I think there are
>> three options here:
>> 1. nullptr; this sort-of-misses the original point of getBaseObject(),
>> since there *is* an aliasee, you don't even need to pierce through
>> ConstantExpr-s to get it.
>> 2. The GlobalIFunc itself; this is only possible with either:
>> 2. a. Making GlobalIFunc inherit from GlobalObject [1].
>> 2. b. Changing the return type of getBaseObject() to be GlobalValue
>> (which also seems to be missing the original point of
>> getBaseObject()).
>> 3. The GlobalIFunc's resolver; this is similar to the behavior of
>> getRootObject() in my PR [1].
>> 
>> The current situation on top of master is that GA->getBaseObject()
>> would return nullptr, but GI->getBaseObject() would return the
>> resolver, which makes it non-idempotent.
>> To resolve that, I think that either 2.a. or 3. make sense here.
> 
> I'd just avoid llvm::GlobalIndirectSymbol::getBaseObject for GlobalIFunc.
> getBaseObject can be moved to GlobalAlias:: to avoid GlobalIndirectSymbol confusion.
> It seems that most users only call getBaseObject with a GlobalAlias, so the required changes are
> few.
> 
> For GlobalIFunc, just use getResolver().

MaskRay, are you suggesting removing GlobalValue::getBaseObject, forcing an initial cast to GlobalAlias? I agree that sounds like a simple approach.

If there’s a need to keep GlobalValue::getBaseObject, another option would be to return a utility that stands in for ifunc-or-object (wrapping `GlobalValue*` but requiring it not be a `GlobalAlias*`). But sinking it to GlobalAlias is simpler if it’ll work.

Itay/others, WDYT?

>> Some users [2] consider the result of GlobalAlias::getBaseObject() to
>> be the aliasee [2], which option 3 doesn't uphold.
>> Other users [3] consider the result of getBaseObject() to be some
>> "root" / "key" / "leader" for a group of GlobalValues that need to be
>> handled together, which option 2 doesn't uphold. This is the reason
>> for the getBaseObject/getRootObject split in my PR, which demonstrates
>> 2.a.
>> 
>> There's also the replication of some of this type hierarchy and
>> semantics in {GlobalValue,Alias,Function,GlobalVar}Summary::getBaseObject(),
>> which I haven't yet considered. Note:
>> * IFunc is missing from that list
>> * GlobalValueSummary::getBaseObject() returns a GlobalValueSummary,
>> not a GlobalObjectSummary (which doesn't exist).
> 
> With the changes above, there is probably not much in GlobalIndirectSymbol which can be shared with
> GlobalAlias and GlobalIFunc, but that seems ok to me.
> 
>> Thoughts?
>> 
>> [1] https://reviews.llvm.org/D108872
>> [2] https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4037
>> https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/Analysis/ModuleSummaryAnalysis.cpp#L640
>> https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp#L1675
>> https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/LTO/LTO.cpp#L736
>> [3] https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/Transforms/Utils/SplitModule.cpp#L130
>> https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/Transforms/Utils/SplitModule.cpp#L229
>> https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/Object/ModuleSymbolTable.cpp#L207
>> [4] https://github.com/llvm/llvm-project/blob/6aacc69338787bfa1ad814928459e3cb94522298/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp#L1664-L1668
>> 
>> 
>> On Wed, 8 Sept 2021 at 20:03, Duncan P. N. Exon Smith
>> <dexonsmith at apple.com> wrote:
>>> 
>>> For the specific problem hit below, it feels like another available approach would be to change GlobalIndirectSymbol to behave correctly for both GlobalAlias and GlobalIFunc, without changing the class hierarchy, by reducing its scope and deferring more to its derived classes (e.g., change getBaseObject() to do the right thing).
>>> 
>>> Can you comment further on the tradeoffs vs. the refactoring you’re proposing? (I see your argument that globalifunc shares some properties globalobject, but it’s not obvious to me that it’s more similar to globalobject than it is to globalalias, or that in aggregate code dealing with these classes will be cleaner after moving globalifunc over to the globalobject bucket.)
>>> 
>>> > On 2020-09-07, at 13:06, Itay Bookstein via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>> >
>>> > I was working on https://reviews.llvm.org/D81911 to try and fix
>>> > https://bugs.llvm.org/show_bug.cgi?id=46340 . Through the review
>>> > process we happened upon a design limitation or perhaps a potential
>>> > mis-modelling of GlobalIFunc in the IR object hierarchy, which leads
>>> > to some problems in LTO flows.
>>> >
>>> > To summarize, as it currently stands (and in the hopes of faithfully
>>> > representing the conclusions of that discussion):
>>> > * Calling getBaseObject() on a GlobalAlias whose aliasee is a
>>> > GlobalIFunc currently returns null.
>>> > * Calling getBaseObject() on a GlobalIFunc returns its resolver function.
>>> > * This causes computeAliasSummary in ModuleSummaryAnalysis to crash on
>>> > a null dereference for an alias-to-ifunc situation.
>>> > * A GlobalIFunc and its resolver are *not* interchangeable: at the
>>> > interface level, they have different signatures (conceptually, the
>>> > IFunc has the same signature of the function pointer that the resolver
>>> > potentially returns, not of the resolver itself).
>>> > * It makes sense for the IFunc to be its own base object (which is
>>> > GlobalObject-like-behavior), but type-hierarchy-wise it can't. This is
>>> > because GlobalIFunc derives from GlobalIndirectSymbol which derives
>>> > directly from GlobalValue, and therefore it is not a GlobalObject.
>>> >
>>> > Would it make sense for GlobalIFunc to derive from GlobalObject
>>> > instead of GlobalIndirectSymbol? If so, GlobalIndirectSymbol would
>>> > lose its purpose somewhat, and could be merged into GlobalAlias. It
>>> > would, however, require updating a considerable amount of code.
>>> >
>>> > ~Itay
>>> > _______________________________________________
>>> > LLVM Developers mailing list
>>> > llvm-dev at lists.llvm.org
>>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> 
>> 
>> 
>> -- 
>> 
>> 
>> 
>> 
>> Itay Bookstein
>> 
>> Software Engineer
>> 
>> 
>> NEXTSILICON
>> 
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