Debugging an llvm pass with gdb

Question

Is it possible to debug an llvm pass using gdb? I couldn't find any docs on the llvm site.

Richard Pennington · Accepted Answer · 2010-02-09 13:20:00Z

10

Yes. Build LLVM in non-release mode (the default). It takes a bit longer than a release build, but you can use gdb to debug the resulting object file.

One note of caution: I had to upgrade my Linux box to 3GB of memory to make LLVM debug mode link times reasonable.

answered Feb 9, 2010 at 13:20

Richard Pennington

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2 Comments

Arjun Singri Over a year ago

But can I debug an llvm pass? LLVM passes are usually run using opt so I don't know how to use gdb to run it. Can you tell me the command?

Richard Pennington Over a year ago

You can run e.g. "gdb /usr/local/bin/opt" and type "run <your command line>".

Donal Fellows · Accepted Answer · 2015-12-24 14:08:09Z

7

First make sure LLVM is compiled with debug options enabled, which is basically the default setting. If you didn't compile LLVM with non-default options then your current build should be fine.

All LLVM passes are run using LLVM's opt (optimizer) tool. Passes are compiled into shared object files, i.e., LLVMHello.so file in build/lib and then loaded by the opt tool. To debug or step through the pass we have to halt LLVM before it starts executing the .so file because there is no way to put a break point in a shared object file. Instead, we can put a break in the code before it invokes the pass.

We're going to put a breakpoint in llvm/lib/IR/Pass.cpp

Here's how to do it:

Navigate to build/bin and open terminal and type gdb opt. If you compiled llvm with the debug symbols added then gdb will take some time to load debugging symbols, otherwise gdb will say loading debugging symbols ... (no debugging symbols found).
Now we need to set a break point at the void Pass::preparePassManager(PMStack &) method in Pass.cpp. This is probably the first (or one of the first) methods involved in loading the pass. You can do this by by typing break llvm::Pass::preparePassManager in terminal.
Running the pass. I have a bitcode file called trial.bc and the same LLVMHello.so pass so I run it with
```
run -load ~/llvm/build/lib/LLVMHello.so  -hello < ~/llvmexamples/trial.bc > /dev/null
```
gdb will now stop at Pass::preparePassManager and from here on we can use step and next to trace the execution.

edited Dec 24, 2015 at 14:08

Donal Fellows

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answered Aug 10, 2015 at 18:42

Muhammad Safi

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2 Comments

algoProg Over a year ago

Hi, I think I did not add debug symbols while building LLVM. Can I still use gdb for debugging? Will that have any limitations on debugging?

Muhammad Safi Over a year ago

There shouldn't be any problems.

alex · Accepted Answer · 2015-01-13 06:43:10Z

1

Following Richard Penningtons advice + adding backticks works for me:

gdb /usr/local/bin/opt

then type

run `opt -load=/pathTo/LLVMHello.so  -hello < /pathTo/your.bc > /dev/null`

Note: I would have commented, but couldn't (missing rep.)

answered Jan 13, 2015 at 6:43

alex

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