llvm dev - Feb 2019 - Create the GlobalVariable which have extern in one header file

Hi
Good day. I am facing issue with creating a GlobalVariable. I already have an
extern for that global in a header file to use it in the following way

extern const void* DATA_TABLE[];

And with a LLVM PASS, I am trying to create this array with the same name and
with initializer. So, I have following:

GlobalVariable *gvar_data = new GlobalVariable(
    M, blockItems->getType(), true, GlobalValue::CommonLinkage,
    blockItems, "DATA_TABLE");
gvar_data->setAlignment(16);
gvar_data->setSection("data_section");
gvar_data->addAttribute(llvm::Attribute::OptimizeNone);

I am not sure if I am using the correct Linkage or not.
The pass has failed to complete it. Here is the runtime fault. Any guess what I
am doing incorrect?

'common' global must have a zero initializer!
[10 x i8*]* @DATA_TABLE.1
LLVM ERROR: Broken module found, compilation aborted:::::::::!
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On Thu, 21 Feb 2019 at 18:42, Mustakimur Khandaker via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> GlobalVariable *gvar_data = new GlobalVariable(
>     M, blockItems->getType(), true, GlobalValue::CommonLinkage,
>     blockItems, "DATA_TABLE");
> gvar_data->setAlignment(16);
> gvar_data->setSection("data_section");
> gvar_data->addAttribute(llvm::Attribute::OptimizeNone);
>
> I am not sure if I am using the correct Linkage or not.
> The pass has failed to complete it. Here is the runtime fault. Any guess
what I am doing incorrect?
As you've guessed, it's the linkage.

"common" is the name for the C situation where multiple source files
simply declare "int a;" with nothing else specified. The names get
merged, and the variable gets zero-initialized by default. A "real"
definition (like you're trying to provide) would take precedence over
this and specify the actual data.

You almost certainly want GlobalValue::ExternalLinkage for a
bog-standard global variable.

Cheers.

Tim.

I have changed it to ExternalLinkage. Now, in LLVM IR, it looks like that:

@DATA_TABLE = external global [0 x i8*], section "data_section", align
16 #0
@DATA_TABLE.1 = global [10 x i8*] [i8* inttoptr (i64 53415718 to i8*), i8*
bitcast (void (%class.Hello*)* @_ZN5Hello5ptofnEv to i8*),...], section
"data_section", align 16 #0

In my uses:
%arrayidx = getelementptr inbounds [0 x i8*], [0 x i8*]* @DATA_TABLE, i64 0, i64
%3

Why it is creating another global variable instead of linking? Is that because
of array size mismatch?

Here is my code:

GlobalVariable *old = M.getGlobalVariable("DATA_TABLE");
old->setAlignment(16);
old->setSection("data_section");
old->addAttribute(llvm::Attribute::OptimizeNone);
old->setDSOLocal(false);

GlobalVariable *gNew = new GlobalVariable(
M, blockItems->getType(), false, GlobalValue::ExternalLinkage,
blockItems, "DATA_TABLE");
gNew->setAlignment(16);
gNew->setSection("data_section");
gNew->addAttribute(llvm::Attribute::OptimizeNone);

On Feb 22 2019, at 12:02 am, Tim Northover <t.p.northover at gmail.com>
wrote:
On Thu, 21 Feb 2019 at 18:42, Mustakimur Khandaker via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
GlobalVariable *gvar_data = new GlobalVariable(
M, blockItems->getType(), true, GlobalValue::CommonLinkage,
blockItems, "DATA_TABLE");
gvar_data->setAlignment(16);
gvar_data->setSection("data_section");
gvar_data->addAttribute(llvm::Attribute::OptimizeNone);

I am not sure if I am using the correct Linkage or not.
The pass has failed to complete it. Here is the runtime fault. Any guess what I
am doing incorrect?

As you've guessed, it's the linkage.

"common" is the name for the C situation where multiple source files
simply declare "int a;" with nothing else specified. The names get
merged, and the variable gets zero-initialized by default. A "real"
definition (like you're trying to provide) would take precedence over
this and specify the actual data.

You almost certainly want GlobalValue::ExternalLinkage for a
bog-standard global variable.

Cheers.

Tim.
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