CODEWITHSUNDEEP
llvm
user2022455
user2022455

Reputation:

llvm line number of an instruction

I want to get the line number of an instruction (and also of a variable declaration - alloca and global). The instruction is saved in an array of instructions. I have the function:

Constant* metadata::getLineNumber(Instruction* I){
    if (MDNode *N = I->getMetadata("dbg")) { // this if is never executed
        DILocation Loc(N);
        unsigned Line = Loc.getLineNumber();
        return ConstantInt::get(Type::getInt32Ty(I->getContext()), Line);
    }   // else {
      //  return NULL; }
}

and in my main() I have :

errs()<<"\nLine number is "<<*metadata::getLineNumber(allocas[p]);

the result is NULL since I->getMetadata("dbg") is false.

Is there a possibility to enable dbg flags in LLVM without rebuilding the LLVM framework, like using a flag when compiling the target program or when running my pass (I used -debug) ?

Compiling a program with “-O3 -g” should give full debug information, but I still have the same result. I am aware of http://llvm.org/docs/SourceLevelDebugging.html , from where I can see that is quite easy to take the source line number from a metadata field.

PS: for Allocas, it seems that I have to use findDbgDeclare method from DbgInfoPrinter.cpp.

Thank you in advance !

Upvotes: 5

Views: 4185

Answers (1)

Eli Bendersky
Eli Bendersky

Reputation: 273416

LLVM provides debugging information if you specify the -g flag to Clang. You don't need to rebuild LLVM to enable/disable it - any LLVM will do (including a pre-built one from binaries or binary packages).

The problem may be that you're trying to have debug information in highly optimized code (-O3). This is not necessarily possible, since LLVM simply optimizes some code away in such cases and there's not much meaning to debug information. LLVM tries to preserve debug info during optimizations, but it's not an easy task.

Start by generating unoptimized code with debug info (-O0 -g) and write your code/passes to work with that. Then graduate to optimized code, and try to examine what specifically gets lost. If you think that LLVM is being stupid, don't hesitate to open a bug.

Some random tips:

  • Generate IR from clang (-emit-llvm) and see the debug metadata nodes in it. Then you can run through opt with optimizations and see what remains.
  • The -debug option to llc and other LLVM tools is quite unrelated to debug info in the source.

Upvotes: 6

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