llvm dev - Feb 2016 - How to resolve debug info forward types

Before metadata was separated from values, I could create a debug info forward
declaration and eventually resolve it using LLVMReplaceAllUsesWith in core.h.
Now, I can't figure out how to resolve it.  I can find no function that
seems
to do this.  My one wild guess that giving the forward decl and the resolving
decl the same UniqueId might do it is not working.

I am currently using 3.6.1, but I see nothing the voluminous diff of DIBuilder
from 3.6.1 to 3.7.1 that looks like it has anything to do with this.
-- 
Rodney Bates
rodney.m.bates at acm.org

There are replaceable MD nodes - you could check how Clang does this when
encountering a declaration, then a definition:

struct foo;
void f(foo *) {
}
struct foo {
};
void g(foo) {
}

Should exercise "Build a declaration", "build a definition",
"replace
declaration with definition"

On Wed, Feb 24, 2016 at 2:09 PM, Rodney M. Bates via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Before metadata was separated from values, I could create a debug info
> forward
> declaration and eventually resolve it using LLVMReplaceAllUsesWith in
> core.h.
> Now, I can't figure out how to resolve it.  I can find no function that
> seems
> to do this.  My one wild guess that giving the forward decl and the
> resolving
> decl the same UniqueId might do it is not working.
>
> I am currently using 3.6.1, but I see nothing the voluminous diff of
> DIBuilder
> from 3.6.1 to 3.7.1 that looks like it has anything to do with this.
> --
> Rodney Bates
> rodney.m.bates at acm.org
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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It's now a multi-stage process.

First, there is a difference between a permanent forward declaration (i.e.
the actual definition is in a different file) and a temporary forward
declaration (i.e. it will appear later in the same file).

To create a permanent forward declaration:

    DICompositeType     *result = m_DIBuilder->createForwardDecl(
            dwarf::DW_TAG_structure_type,
            name,
            m_DICompileUnit,
            file,
            loc.m_Line);

To create a temporary forward declaration:

    DICompositeType     *result m_DIBuilder->createReplaceableCompositeType(
            dwarf::DW_TAG_structure_type,
            name,
            m_DICompileUnit,
            file,
            loc.m_Line);

And then, once the final value of the temporary is known (and you GOTTA do
this step otherwise you get an assert):

    DICompositeType     *result = m_DIBuilder->createStructType(
            m_DICompileUnit,
            name,
            file,
            loc.m_Line,
            sizeOf(actual) * 8, alignOf(actual) * 8,
            0, nullptr, elts);

    it = m_TmpStructDI.find(name);
    if (it != m_TmpStructDI.end()) {
        MDNode          *node = /* the result from
createReplaceableCompositeType earlier */
        llvm::TempMDNode fwd_decl(node);

        m_DIBuilder->replaceTemporary(std::move(fwd_decl), result);

You will need to keep a map of outstanding temporary forward declarations
to do this.

On Wed, Feb 24, 2016 at 5:09 PM, Rodney M. Bates via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Before metadata was separated from values, I could create a debug info
> forward
> declaration and eventually resolve it using LLVMReplaceAllUsesWith in
> core.h.
> Now, I can't figure out how to resolve it.  I can find no function that
> seems
> to do this.  My one wild guess that giving the forward decl and the
> resolving
> decl the same UniqueId might do it is not working.
>
> I am currently using 3.6.1, but I see nothing the voluminous diff of
> DIBuilder
> from 3.6.1 to 3.7.1 that looks like it has anything to do with this.
> --
> Rodney Bates
> rodney.m.bates at acm.org
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Thanks, this got me going.  I had to do things a bit differently for 3.6.1.
But the body of DIBuilder::replaceTemporary of 3.7.1 led me to what I need
in MDNodeFwdDecl::replaceAllUsesWith, in Metadata.h of 3.6.1.


On 02/24/2016 04:15 PM, David Jones wrote:
> It's now a multi-stage process.
>
> First, there is a difference between a permanent forward declaration (i.e.
the actual definition is in a different file) and a temporary forward
declaration (i.e. it will appear later in the same file).
>
> To create a permanent forward declaration:
>
>      DICompositeType     *result = m_DIBuilder->createForwardDecl(
>              dwarf::DW_TAG_structure_type,
>              name,
>              m_DICompileUnit,
>              file,
>              loc.m_Line);
>
> To create a temporary forward declaration:
>
>      DICompositeType     *result =
m_DIBuilder->createReplaceableCompositeType(
>              dwarf::DW_TAG_structure_type,
>              name,
>              m_DICompileUnit,
>              file,
>              loc.m_Line);
>
> And then, once the final value of the temporary is known (and you GOTTA do
this step otherwise you get an assert):
>
>      DICompositeType     *result = m_DIBuilder->createStructType(
>              m_DICompileUnit,
>              name,
>              file,
>              loc.m_Line,
>              sizeOf(actual) * 8, alignOf(actual) * 8,
>              0, nullptr, elts);
>
>      it = m_TmpStructDI.find(name);
>      if (it != m_TmpStructDI.end()) {
>          MDNode          *node = /* the result from
createReplaceableCompositeType earlier */
>          llvm::TempMDNode fwd_decl(node);
>
>          m_DIBuilder->replaceTemporary(std::move(fwd_decl), result);
>
> You will need to keep a map of outstanding temporary forward declarations
to do this.
>
> On Wed, Feb 24, 2016 at 5:09 PM, Rodney M. Bates via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     Before metadata was separated from values, I could create a debug info
forward
>     declaration and eventually resolve it using LLVMReplaceAllUsesWith in
core.h.
>     Now, I can't figure out how to resolve it.  I can find no function
that seems
>     to do this.  My one wild guess that giving the forward decl and the
resolving
>     decl the same UniqueId might do it is not working.
>
>     I am currently using 3.6.1, but I see nothing the voluminous diff of
DIBuilder
>     from 3.6.1 to 3.7.1 that looks like it has anything to do with this.
>     --
>     Rodney Bates
>     rodney.m.bates at acm.org <mailto:rodney.m.bates at acm.org>
>     _______________________________________________
>     LLVM Developers mailing list
>     llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>     http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-- 
Rodney Bates
rodney.m.bates at acm.org