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

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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On Fri, 22 Feb 2019 at 04:15, Mustakimur Khandaker <mrk15e at my.fsu.edu>
wrote:
> Why it is creating another global variable instead of linking?
You're explicitly calling "new" which creates a global variable.
If
there's an existing global you want to reuse you should call
Module::getOrInsertGlobal instead. But...
> Is that because of array size mismatch?
It's definitely not helping, and if you can't come up with a way to
get the type when creating the original variable you might have to
create two variables after all.

In that case you'd create your new variable with a known array length,
use ConstantExpr::getBitCast to cast it to [0 x i8*]* and then call
replaceAllUsesWith to make the existing code use your new variable
with initializer.

Cheers.

Tim.