llvm dev - Aug 2020 - Accessing constant contents

Hi,

I have a global variable in LLVM IR:

@mem8 = private unnamed_addr constant [9 x i8] c"Hello12\0A\00", align
1

I have loaded the module, so now have:
llvm::Value *value1;
...
value1->print(llvm::outs());
That will output:
@mem8 = private unnamed_addr constant [9 x i8] c"Hello12\0A\00", align
1

What method should I use to access the contents of the constant.
I.e. I want the "Hello12" bytes contained in @mem8 to be put in a
std::vector<int8_t>

I can access the Type hierarchy, but cannot find how to access the
value contents.

Kind Regards

James

Hi James,


I haven't tried it but if my memory is not failing me you should be able to

ask for the initializer of the llvm::GlobalVariable, which is a constant.

That should actually be a llvm::ConstantDataSequential in this case 
which has

methods like isString, getString, getCString,...


Oh, and your value1 is the llvm::GlobalVariable I am talking about, so 
something along the lines of:


auto *GV = cast<GlobalVariable>(value1);

auto *CDS = cast<ConstantDataSequential>(GV->getInitializer());

CDS->getString();


Hope this helps,

   Johannes


On 8/1/20 3:25 PM, James Courtier-Dutton via llvm-dev
wrote:
> Hi,
>
> I have a global variable in LLVM IR:
>
> @mem8 = private unnamed_addr constant [9 x i8] c"Hello12\0A\00",
align 1
>
> I have loaded the module, so now have:
> llvm::Value *value1;
> ...
> value1->print(llvm::outs());
> That will output:
> @mem8 = private unnamed_addr constant [9 x i8] c"Hello12\0A\00",
align 1
>
> What method should I use to access the contents of the constant.
> I.e. I want the "Hello12" bytes contained in @mem8 to be put in a
> std::vector<int8_t>
>
> I can access the Type hierarchy, but cannot find how to access the
> value contents.
>
> Kind Regards
>
> James
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

The "hello" string is a part of the GlobalVariable's initializer. 
You can use GlobalVariable::getInitializer to get a Constant with the
initializer value, and then, depending on the actual type of it you can get the
contents of it.

--
Krzysztof Parzyszek  kparzysz at quicinc.com   AI tools development

-----Original Message-----
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of James
Courtier-Dutton via llvm-dev
Sent: Saturday, August 1, 2020 3:25 PM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] Accessing constant contents

Hi,

I have a global variable in LLVM IR:

@mem8 = private unnamed_addr constant [9 x i8] c"Hello12\0A\00", align
1

I have loaded the module, so now have:
llvm::Value *value1;
...
value1->print(llvm::outs());
That will output:
@mem8 = private unnamed_addr constant [9 x i8] c"Hello12\0A\00", align
1

What method should I use to access the contents of the constant.
I.e. I want the "Hello12" bytes contained in @mem8 to be put in a
std::vector<int8_t>

I can access the Type hierarchy, but cannot find how to access the value
contents.

Kind Regards

James
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llvm-dev at lists.llvm.org
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