Several people on this list have reported issues with the linker regarding a
named StructType instance with the same name in two different modules
being resolved into two StructTypes with different names due to StructType::
setName(…) collision behavior. Looking at BitcodeReader::ParseTypeTableBody(…),
I don't see use of LLVMContextImpl::NamedStructTypes or of Module::getTypeByName(…).
Nor do I see this use anywhere else in BitcodeReader's implementation (.cpp file).
Doesn't the context's NamedStructTypes (checking for a previously created StructType
with the same name), have to be used before setName(…) is called so that a new structure
is not erroneously created?
My apologies if this is noise.
Garrison
The BitcodeReader has nothing to do with the issue in question; the
relevant code is in lib/Linker.
-Eli
Thanks for the clarification Eli.
In looking at a stack trace from:
Linker::LinkInFiles(…) to:
BitcodeReader::ParseTypeTableBody()
it seemed that not using NamedStructTypes would cause an issue.
Thanks again
Garrison
Hi Garrison,
Do the other two threads answer the question here?
-Chris
Thanks for the response Chris,
Several people on this list have reported issues with the linker regarding a
named StructType instance with the same name in two different modules
being resolved into two StructTypes with different names due to StructType::
setName(…) collision behavior. Looking at BitcodeReader::ParseTypeTableBody(…),
I don't see use of LLVMContextImpl::NamedStructTypes or of Module::getTypeByName(…).
Nor do I see this use anywhere else in BitcodeReader's implementation (.cpp file).
Doesn't the context's NamedStructTypes (checking for a previously created StructType
with the same name), have to be used before setName(…) is called so that a new structure
is not erroneously created?
Hi Garrison,
Do the other two threads answer the question here?
-Chris
Yes I believe the other threads concerning the same issue answer why the system, because
of the lack of type linkage, is lenient in "unioning" types. What I'm still uncomfortable with is, I
don't understand under what conditions the system should union two opaque types with the
same name and def. The above method BitcodeReader::ParseTypeTableBody, never seems to do
this, and therefore my reading of Linker::LinkInFiles won't do it either for say two bitcode files
accessing/realizing the same named StructType. On the other hand when hand coding two
modules using in memory IR, I have a choice in either checking with NamedStructTypes to to force
two equivalently named struct types to resolve to the same struct type with the same name, or
ignoring the issue, and just have each module create two different struct types (via StructType
name collision behavior), with different names but with the same definition. Does the name
matter given that the definitions are the same?
Regardless unless you feel there is some pedagogic value to the list, you can ignore my lack of
understanding here because I'm now pretty sure I'm just creating noise that will be resolved
for me by writing tests; not just by looking at code where I'm am probably missing something
anyway. If there is a more concrete question I can ask, when looking at my tests, I'll ask again
then.
Thanks again
Garrison
Hi Garrison,
Do the other two threads answer the question here?
-Chris
Yes I believe the other threads concerning the same issue answer why the system, because
of the lack of type linkage, is lenient in "unioning" types. What I'm still uncomfortable with is, I
don't understand under what conditions the system should union two opaque types with the
same name and def. The above method BitcodeReader::ParseTypeTableBody, never seems to do
this, and therefore my reading of Linker::LinkInFiles won't do it either for say two bitcode files
accessing/realizing the same named StructType.
The idea is that the linker does this when there is linkage that implies that two types should be unioned. For example:
%A = type opaque
@G = external global %A*
...and...
%A = type { i32 }
@G = global %A* null
should merge the two %a's, because that is implied by linking @G and there is no structural conflict.
On the other hand when hand coding two
modules using in memory IR, I have a choice in either checking with NamedStructTypes to to force
two equivalently named struct types to resolve to the same struct type with the same name, or
ignoring the issue, and just have each module create two different struct types (via StructType
name collision behavior), with different names but with the same definition. Does the name
matter given that the definitions are the same?
For linking or IR generation? In IR generation, names matter for purposes of type identity and equality. The linker cannot assume that linked symbols have the same type (for many reasons), so type merging is "nice to have" for it.
-Chris
Hi Chris,
Ok, it looks like I need to get my head out of the JIT world where linking between different
modules is done by hand--well it use to be via ExecutionEngine::updateGlobalMapping(...),
and therefore a developer controlled event.
Thanks for the explanation
Garrison