[LLVMdev] mapping types from a bitcode module

Michael Muller mmuller at enduden.com
Mon Feb 27 11:27:26 PST 2012

Hi Clemens - thanks for your response.

Clemens Hammacher wrote:
> Hi Michael,
> since noone of the experts answered, let me share our experiences. We 
> recently had exactly the same problem, I posted on this list on January 
> 31st.
> I didn't follow Duncans advice to "just use the linker", since for 
> several reasons we wanted to have unique struct types even in the 
> separate modules.
> > 1) duplicate the LinkModule internal code and copy the module we load from
> > bitcode to a new module with the correct types mapped.
> Sounds quite inefficient. If you have to duplicate the internal code 
> anyway, you can also iterate over the original module and mutateType().
> > 2) duplicate BitcodeReader and create a version that reuses existing
> > StructTypes.
> see below.
> > 3) destructively convert all of the types in the imported module to our
> > existing types.
> That's what we actually implemented, following the idea I described in 
> the mentioned post. We don't identify identical struct types by their 
> name, since even in the new type system, names don't actually mean 
> anything. You could just strip them off.
> Instead, we use the pointer value of the types to identify them, since 
> originally, all our modules reside in the same LLVMContext. Since that 
> doesn't seem to be the case in your situation, you propably would have 
> to use the name, or attach other metadata to uniquely identify your structs.

I was actually fearful of this approach, it looked to me like the linker was
at least partially copying data structures to the destination module.  I see
that there is a mutateType() method in Value, though it comes with a very
stern warning :-)

But given your success with it, and given that it seems to involve the least
amount of copy-pasting the existing code, I think I'll give it a try.

> > Also, when loading named StructTypes, would it be possible for LLVM to reuse
> > an existing type with the same name assuming the existing type is isomorphic?
> > This seems like it would be a win all around.
> Just out of interest, I also implemented that, because I thought it 
> could improve the overall performance. But I couldn't measure any 
> performance impact on the simple tests in the test-suite.
> The main problem is that the named struct could reference other types 
> defined later in the type table of the module, so you can only check 
> whether named structs are identical after the whole type table has been 
> parsed and the types are already created. So during parsing I am 
> remembering which StructTypes had to be renamed, and after that - but 
> before parsing the instructions - I check which of them are isomorphic 
> to the corresponding existing struct, and directly manipulate the type 
> list used when parsing the rest of the module.

Ah, I see.  That would definitely complicate things.

> A funny insight when implementing that is that I also had to change the 
> behaviour of the linker, since it again created copies of all types used 
> in the source module. So after linking, there again were different 
> instances of the same struct type, but only one of them had a meaningful 
> name, since the new copy that the linker creates steals the name of the 
> original type ;)
> For both implementations I can provide source code if you wish.

Thanks, I think we should be ok given your explanation.  If I get stuck,
I might take you up on it.  Although if we're both doing this, it may be
worthwhile for us to try to come up with something general enough to include
in LLVM.

> Cheers,
> Clemens

michaelMuller = mmuller at enduden.com | http://www.mindhog.net/~mmuller
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