llvm dev - Sep 2020 - Creating a global variable for a struct array

Hello,

I would like to create a global variable for the following struct array, h1

dhash* h1 = new dhash[10];

typedef struct dhash{
        char* filenm;
        dlist* llist;
}dhash;

typedef struct dlist{
    int soffst;
    int eoffst;
    uint8_t* dptr;
}dlist;

I also need to allocate space for:
1) the field llist in struct dhash which is a pointer to another struct dlist
and
2) the field dptr in struct dlist

Is there an example that I can refer to for doing this ?

I tried to create a GlobalVariable using ConstantStruct::get(StructType*,
ArrayRef<Constant *>). I'm not sure how to get the second argument of
type ArrayRef<Constant *> from the above variable h1.

Thanks,
Chaitra
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On Tue, 22 Sep 2020 at 23:52, Niddodi, Chaitra via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> dhash* h1 = new dhash[10];
Note that this "new" expression needs a runtime call to actually
allocate the memory on the heap. In LLVM terms this is A
GlobalVariable with type PointerType::get(StructTy, ...), initialized
to null, together with a global init function (runs when the program
starts) that makes the call to new and stores it to that global. My
guess is that's not what you intended
> I also need to allocate space for:
> 1) the field llist in struct dhash which is a pointer to another struct
dlist and
> 2) the field dptr in struct dlist
>
> Is there an example that I can refer to for doing this ?
I think you should start by writing exactly what you want in C++, and
then (when you're happy with the assembly output) using Clang's "-S
-emit-llvm" options to see what LLVM IR that produces.

That will clear up the question of how things should be allocated,
which isn't quite obvious from the questions you're asking, and make
translating the IR into LLVM API calls more straightforward.
> I tried to create a GlobalVariable using ConstantStruct::get(StructType*,
ArrayRef<Constant *>). I'm not sure how to get the second argument of
type ArrayRef<Constant *> from the above variable h1.
Hopefully it'll be clearer when you have the real IR you want (in
textual form) in front of you.

Cheers.

Tim.

Let me clarify my question.

I have a struct array h1 as follows:
dhash h1[10];

I want to get a Constant* to variable h1. It looks like I can use
ConstantStruct::get(StructType*, ArrayRef<Constant *>) to do this.

My question is how to get the second argument of type ArrayRef<Constant *>
from the above variable h1.

Thanks,
Chaitra
________________________________
From: Tim Northover <t.p.northover at gmail.com>
Sent: Wednesday, September 23, 2020 03:04
To: Niddodi, Chaitra
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Creating a global variable for a struct array

On Tue, 22 Sep 2020 at 23:52, Niddodi, Chaitra via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> dhash* h1 = new dhash[10];
Note that this "new" expression needs a runtime call to actually
allocate the memory on the heap. In LLVM terms this is A
GlobalVariable with type PointerType::get(StructTy, ...), initialized
to null, together with a global init function (runs when the program
starts) that makes the call to new and stores it to that global. My
guess is that's not what you intended
> I also need to allocate space for:
> 1) the field llist in struct dhash which is a pointer to another struct
dlist and
> 2) the field dptr in struct dlist
>
> Is there an example that I can refer to for doing this ?
I think you should start by writing exactly what you want in C++, and
then (when you're happy with the assembly output) using Clang's "-S
-emit-llvm" options to see what LLVM IR that produces.

That will clear up the question of how things should be allocated,
which isn't quite obvious from the questions you're asking, and make
translating the IR into LLVM API calls more straightforward.
> I tried to create a GlobalVariable using ConstantStruct::get(StructType*,
ArrayRef<Constant *>). I'm not sure how to get the second argument of
type ArrayRef<Constant *> from the above variable h1.
Hopefully it'll be clearer when you have the real IR you want (in
textual form) in front of you.

Cheers.

Tim.
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