How to submit an LLVM bug report

Introduction - Got bugs?

If you’re working with LLVM and run into a bug, we definitely want to know about it. This document describes what you can do to increase the odds of getting it fixed quickly.

🔒 If you believe that the bug is security related, please follow How to report a security issue?. 🔒

Basically you have to do two things at a minimum. First, decide whether the bug crashes the compiler or if the compiler is miscompiling the program (i.e., the compiler successfully produces an executable, but it doesn’t run right). Based on what type of bug it is, follow the instructions in the linked section to narrow down the bug so that the person who fixes it will be able to find the problem more easily.

Once you have a reduced test-case, go to the LLVM Bug Tracking System and fill out the form with the necessary details (note that you don’t need to pick a label, just use if you’re not sure). The bug description should contain the following information:

  • All information necessary to reproduce the problem.

  • The reduced test-case that triggers the bug.

  • The location where you obtained LLVM (if not from our Git repository).

Thanks for helping us make LLVM better!

Crashing Bugs

More often than not, bugs in the compiler cause it to crash—often due to an assertion failure of some sort. The most important piece of the puzzle is to figure out if it is crashing in the Clang front-end or if it is one of the LLVM libraries (e.g. the optimizer or code generator) that has problems.

To figure out which component is crashing (the front-end, middle-end optimizer, or backend code generator), run the clang command line as you were when the crash occurred, but with the following extra command line options:

  • -emit-llvm -Xclang -disable-llvm-passes: If clang still crashes when passed these options (which disable the optimizer and code generator), then the crash is in the front-end. Jump ahead to front-end bugs.

  • -emit-llvm: If clang crashes with this option (which disables the code generator), you found a middle-end optimizer bug. Jump ahead to middle-end bugs.

  • Otherwise, you have a backend code generator crash. Jump ahead to code generator bugs.

Front-end bugs

On a clang crash, the compiler will dump a preprocessed file and a script to replay the clang command. For example, you should see something like

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/foo-xxxxxx.c
clang: note: diagnostic msg: /tmp/foo-xxxxxx.sh

The creduce tool helps to reduce the preprocessed file down to the smallest amount of code that still replicates the problem. You’re encouraged to use creduce to reduce the code to make the developers’ lives easier. The clang/utils/creduce-clang-crash.py script can be used on the files that clang dumps to help with automating creating a test to check for the compiler crash.

cvise is an alternative to creduce.

Middle-end optimization bugs

If you find that a bug crashes in the optimizer, compile your test-case to a .bc file by passing “-emit-llvm -O1 -Xclang -disable-llvm-passes -c -o foo.bc”. The -O1 is important because -O0 adds the optnone function attribute to all functions and many passes don’t run on optnone functions. Then run:

opt -O3 foo.bc -disable-output

If this doesn’t crash, please follow the instructions for a front-end bug.

If this does crash, then you should be able to debug this with the following bugpoint command:

bugpoint foo.bc -O3

Run this, then file a bug with the instructions and reduced .bc files that bugpoint emits.

If bugpoint doesn’t reproduce the crash, llvm-reduce is an alternative way to reduce LLVM IR. Create a script that repros the crash and run:

llvm-reduce --test=path/to/script foo.bc

which should produce reduced IR that reproduces the crash. Be warned the llvm-reduce is still fairly immature and may crash.

If none of the above work, you can get the IR before a crash by running the opt command with the --print-before-all --print-module-scope flags to dump the IR before every pass. Be warned that this is very verbose.

Backend code generator bugs

If you find a bug that crashes clang in the code generator, compile your source file to a .bc file by passing “-emit-llvm -c -o foo.bc” to clang (in addition to the options you already pass). Once your have foo.bc, one of the following commands should fail:

  1. llc foo.bc

  2. llc foo.bc -relocation-model=pic

  3. llc foo.bc -relocation-model=static

If none of these crash, please follow the instructions for a front-end bug. If one of these do crash, you should be able to reduce this with one of the following bugpoint command lines (use the one corresponding to the command above that failed):

  1. bugpoint -run-llc foo.bc

  2. bugpoint -run-llc foo.bc --tool-args -relocation-model=pic

  3. bugpoint -run-llc foo.bc --tool-args -relocation-model=static

Please run this, then file a bug with the instructions and reduced .bc file that bugpoint emits. If something goes wrong with bugpoint, please submit the “foo.bc” file and the option that llc crashes with.

Miscompilations

If clang successfully produces an executable, but that executable doesn’t run right, this is either a bug in the code or a bug in the compiler. The first thing to check is to make sure it is not using undefined behavior (e.g. reading a variable before it is defined). In particular, check to see if the program is clean under various sanitizers (e.g. clang -fsanitize=undefined,address) and valgrind. Many “LLVM bugs” that we have chased down ended up being bugs in the program being compiled, not LLVM.

Once you determine that the program itself is not buggy, you should choose which code generator you wish to compile the program with (e.g. LLC or the JIT) and optionally a series of LLVM passes to run. For example:

bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments]

bugpoint will try to narrow down your list of passes to the one pass that causes an error, and simplify the bitcode file as much as it can to assist you. It will print a message letting you know how to reproduce the resulting error.

The OptBisect page shows an alternative method for finding incorrect optimization passes.

Incorrect code generation

Similarly to debugging incorrect compilation by mis-behaving passes, you can debug incorrect code generation by either LLC or the JIT, using bugpoint. The process bugpoint follows in this case is to try to narrow the code down to a function that is miscompiled by one or the other method, but since for correctness, the entire program must be run, bugpoint will compile the code it deems to not be affected with the C Backend, and then link in the shared object it generates.

To debug the JIT:

bugpoint -run-jit -output=[correct output file] [bitcode file]  \
         --tool-args -- [arguments to pass to lli]              \
         --args -- [program arguments]

Similarly, to debug the LLC, one would run:

bugpoint -run-llc -output=[correct output file] [bitcode file]  \
         --tool-args -- [arguments to pass to llc]              \
         --args -- [program arguments]

Special note: if you are debugging MultiSource or SPEC tests that already exist in the llvm/test hierarchy, there is an easier way to debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which will pass the program options specified in the Makefiles:

cd llvm/test/../../program
make bugpoint-jit

At the end of a successful bugpoint run, you will be presented with two bitcode files: a safe file which can be compiled with the C backend and the test file which either LLC or the JIT mis-codegenerates, and thus causes the error.

To reproduce the error that bugpoint found, it is sufficient to do the following:

  1. Regenerate the shared object from the safe bitcode file:

    llc -march=c safe.bc -o safe.c
gcc -shared safe.c -o safe.so

  2. If debugging LLC, compile test bitcode native and link with the shared object:

    llc test.bc -o test.s
gcc test.s safe.so -o test.llc
./test.llc [program options]

  3. If debugging the JIT, load the shared object and supply the test bitcode:

    lli -load=safe.so test.bc [program options]