[llvm-dev] Can't find symbol from llvm backend

[张雨姗(ZHANG Yushan)-11310380 via llvm-dev]

Hi,


Did you check your variable is inserted successfully? As far as I know, to insert functions you need to instrument the bitcode as the following:


        Constant *c = module->getOrInsertFunction("cmpr",
                FunctionType::getVoidTy(*context),
                Type::getInt64Ty(*context),
                Type::getInt64Ty(*context), NULL);

        f = &cast<Function>(*c);


The upper code inserts a new function "cmpr" (declaration) to the function table, but later you need to add definitions.



Or if you want to insert a variable (e.g. int), the following is code from StackOverflow I found work for my case: 





  GlobalVariable* gvar_int = new GlobalVariable(
            /*Type=*/i64_type,
            /*isConstant=*/false,
            /*Linkage=*/GlobalValue::CommonLinkage,
            /*Initializer=*/0, // has initializer, specified below
            /*Name=*/name);

 
And you could retrieve this variable with:
    module->getGlobalVariable("var_name")





If you have further questions, I'm happy to discuss.




Regards,
Yushan


------------------ Original ------------------
From:  "n3v3rm03 via llvm-dev"<llvm-dev at lists.llvm.org>;
Date:  Sat, Jan 19, 2019 02:52 PM
To:  "paul.robinson"<paul.robinson at sony.com>; 
Cc:  "llvm-dev"<llvm-dev at lists.llvm.org>; 
Subject:  Re: [llvm-dev] Can't find symbol from llvm backend

 
No, I don't. But I’m sure it is not optimized because I can see this variable in the final binary file emitted. 



Do you mean if I don’t refer to this variable in the front end, then I can’t see it in the backend? (although it’s in the final binary file)


Thank you!
2019年1月19日(土) 2:00 <paul.robinson at sony.com>:




   
Do you have a use of the new variable as well?  If not, it might be optimized out between the point where you create it and your MachineFunction pass.
 
--paulr
 
 
    
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of n3v3rm03 via llvm-dev
 Sent: Friday, January 18, 2019 5:57 AM
 To: llvm-dev at lists.llvm.org
 Subject: [llvm-dev] Can't find symbol from llvm backend
 
 



 
 
  
Hi, I am new to llvm. 
  
I create a global variable called "test_llvm_var" in the front end function pass like this:
 
  
 
 
 
            new GlobalVariable(*m, tmp->getType(), true, llvm::GlobalValue::ExternalLinkage, tmp, "test_llvm_var");
  
 
 
  
where tmp is a function.
 
  
 
 
  
Then I tried to access this global variable in the back end MachineFunctionPass like this:
 
  
 
 
 
            MCContext& MCC = MF.getContext();
             MCSymbol* MCS = MCC.lookupSymbol("test_llvm_var");
             if (MCS == NULL) {
                 errs() << ">>> MCSymbol is null\n";
             }  
             else {
                 MCS->dump();
             }  
  
 
  
 (MF is a MachineFunction).
 
  
 
 
  
However I always get MCS==NULL, there is no symbol called test_llvm_var in the backend.
 
  
 
 
  
Am I doing something wrong? What I would like to do is to modify the global variable's value from the backend pass.
 
  
 
 
  
Any help would be appreciated.
 
  
Thank you!
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