[llvm-dev] Is the flow "llvm-extract -> llvm-link -> clang++ " supposed to be used in this way? To Extract and Re-insert functions?

[Nicolas Agostini via llvm-dev]

Hi Davide, thank you for the answer.

Therefore, if you have GVs with internal linkage when you run
> llvm-extract that information is lost.


Which means that if I try to link another module I may have overlapping
variables?
My apologies but my knowledge of the linker is not as solid as I  wished.

At least, you may want to fix this, the relevant code is around here
> (Transforms/IPO/ExtractGV.cpp)


I will take a close look and do some experimentation.

I forgot, but apparently I had a bug open about this a while ago
> https://bugs.llvm.org/show_bug.cgi?id=31674
>

I believe this falls under Gordon's comment right?
Do we really want to extract this variable with the function?

Quick question: What internalization means on this scope?

- nico

2017-08-30 14:26 GMT-05:00 Davide Italiano <davide at freebsd.org>:

> On Tue, Aug 29, 2017 at 3:10 PM, Nicolas Agostini via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hi all,
> > First post to the list, I hope you can help or guide me on this task.
> >
> > I am involved in a project that requires to re-link extracted and edited
> IR
> > code
> >
> > Thus I want to know if these tools can be used in this way?
> >
> > clang++-4.0 code03.cpp -emit-llvm -S -o code03.ll
> > llvm-extract-4.0 code03.ll -func main -S -o extracted_main.ll
> > llvm-link-4.0 code03.ll -only-needed -override extracted_main.ll -S -o
> > linked_main.ll
> > clang++-4.0 linked_main.ll -o main.out
> >
> >
> > where code03.cpp is:
> >
> >> #include <iostream>
> >> using namespace std;
> >> int main()
> >> {
> >>   cout << "First Message\n ";
> >>   cout << "Second Message\n ";
> >>   cout << "Third Message\n ";
> >>   return 0;
> >> }
> >
> >
> >
> > I have been trying to extract a function's llvm IR, modify it preserving
> its
> > signature (or not), and re-insert this function back to the original IR
> > file, however I am getting an error during the compilation step (
> > clang++-4.0 linked_main.ll -o main.out ):
> >
> >> main.ll:(.text+0x14): undefined reference to `.str'
> >> main.ll:(.text+0x34): undefined reference to `.str.1'
> >> main.ll:(.text+0x51): undefined reference to `.str.2'
> >
> >
> >  and linked_main.ll file has this section:
> >
> >> @.str.4 = private unnamed_addr constant [16 x i8] c"First Message\0A
> \00",
> >> align 1
> >> @.str.1.6 = private unnamed_addr constant [17 x i8] c"Second Message\0A
> >> \00", align 1
> >> @.str.2.8 = private unnamed_addr constant [16 x i8] c"Third Message\0A
> >> \00", align 1
> >> @.str = external hidden unnamed_addr constant [16 x i8], align 1
> >> @.str.1 = external hidden unnamed_addr constant [17 x i8], align 1
> >> @.str.2 = external hidden unnamed_addr constant [16 x i8], align 1
> >
> >
> >
> > But the function does not use the correct versions of the strings as the
> > linked "extracted_main" keeps making calls to .str, .str.1, .str.2? Am I
> not
> > supposed to do it this way?
> >
>
> llvm-extract changes the semantic as it gives every GlobalValue
> external linkage for simplicity.
> Therefore, if you have GVs with internal linkage when you run
> llvm-extract that information is lost.
> At least, you may want to fix this, the relevant code is around here
> (Transforms/IPO/ExtractGV.cpp)
>
> ```
>       // For simplicity, just give all GlobalValues ExternalLinkage. A
> trickier
>       // implementation could figure out which GlobalValues are actually
>       // referenced by the Named set, and which GlobalValues in the rest of
>       // the module are referenced by the NamedSet, and get away with
> leaving
>       // more internal and private things internal and private. But for
> now,
>       // be conservative and simple.
>
>       // Visit the GlobalVariables.
>       for (Module::global_iterator I = M.global_begin(), E =
> M.global_end();
>            I != E; ++I) {
>         bool Delete =
>             deleteStuff == (bool)Named.count(&*I) && !I->isDeclaration();
>         if (!Delete) {
>           if (I->hasAvailableExternallyLinkage())
>             continue;
>           if (I->getName() == "llvm.global_ctors")
>             continue;
>         }
> ```
>
> Thanks,
>
> --
> Davide
>
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
>
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