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DEBUGGING.md

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Debugging the GraalVM LLVM Runtime

Using the Chrome Inspector

The Chrome Developer Tools can be used to debug programs running on the GraalVM LLVM runtime.

See the user documentation on debugging for details.

If running from the source directory using mx, note that the tools suite needs to be imported for debugging to work:

mx --dynamicimport /tools lli --inspect ...

Debugging on the LLVM Bitcode Level

To diagnose problems in the LLVM runtime itself, it it sometimes useful to debug programs running on GraalVM on the level of the LLVM bitcode directly, instead of showing the original high-level (e.g. C language) source code.

This debugging mode can be enabled using the --llvm.llDebug option:

lli --inspect --experimental-options --llvm.llDebug ...

To debug on the LLVM-IR level, you need to provide disassembled bitcode files next to the binary files that are loaded by GraalVM. These can be produced with the llvm-dis tool. Use the --log.llvm.LLDebug.level=FINER option to get diagnostic messages about missing disassembled bitcode files.

There is a helper script in mx to produce the disassembled bitcode files and put them in the correct place:

mx llvm-dis <path-to-file>

You can pass any file to mx llvm-dis that the GraalVM LLVM runtime can open, e.g. bitcode files, ELF files with embedded bitcode and so on. The script will extract the bitcode from the file, disassemble it, and put the result in a file next to it with the correct name for the --llvm.llDebug code to find it.

Tracing the Execution of LLVM Bitcode

GraalVM can produce an LLVM IR-level trace of its program execution. You ca enable this feature by passing the --log.llvm.TraceIR.level=FINER option to lli. This requires --llvm.llDebug to be enabled and the disassembled bitcode to be available.

$ mx lli --experimental-options --log.llvm.TraceIR.level=FINER --llvm.llDebug hello.bc
[llvm::TraceIR] FINER: >> Entering function @main at hello.ll:9:1 with arguments:[StackPointer 0x7fd46bfff010 (Bounds: 0x7fd466fff010 - 0x7fd46bfff010), 1, 0x7fd4ec261978, 0x7fd4ec261988]
[llvm::TraceIR] FINER: >> hello.ll:10:1 ->   %1 = alloca i32, align 4
[llvm::TraceIR] FINER: >> hello.ll:11:1 ->   store i32 0, i32* %1, align 4
[llvm::TraceIR] FINER: >> hello.ll:12:1 ->   %2 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([15 x i8], [15 x i8]* @.str, i32 0, i32 0)), !dbg !13
Hello, World!
[llvm::TraceIR] FINER: >> hello.ll:13:1 ->   ret i32 0, !dbg !14
[llvm::TraceIR] FINER: >> Leaving @main

Debugging the Runtime

For debugging the internals of the runtime, it is recommended to run the GraalVM LLVM runtime in JVM mode (i.e., from a JVM standalone). Then a regular Java debugger can be attached.

If running with mx, the -d option can be used to enable debugging:

$ mx -d lli hello
Listening for transport dt_socket at address: 8000

Or for running unit tests (see TESTS):

$ mx -d unittest SulongSuite
Listening for transport dt_socket at address: 8000

Alternatively, from a built GraalVM, debugging can be enabled using the standard Java debugging flags, for example:

lli --vm.Xdebug --vm.Xrunjdwp:transport=dt_socket,server=y,address=8000,suspend=y ...

Mixed Debugging

Sometimes it is useful to debug in both modes simultaneously. For that you can just pass both options, and attach both a Java IDE and the Chrome Inspector to the VM:

mx --dynamicimport /tools -d lli --inspect ...

That way, it is possible to for example step through a C program to the interesting point, and then enable a breakpoint in the Java debugger, so it will suspend on the next "step" command in the Chrome Inspector, or vice versa.

Detail Formatters

Editors such as Eclipse support the specification of custom code formatters. The LLVMNodeUtils class provides methods for printing the stack trace and AST of any LLVMNode. The follow steps configure this in Eclipse:

  1. Open Window -> Preferences
  2. Go to Java -> Debug -> Detail Formatters
  3. Add a new entry
    1. Set the qualified type name to com.oracle.truffle.llvm.runtime.nodes.api.LLVMNode
    2. Set the snippet to com.oracle.truffle.llvm.runtime.LLVMNodeUtils.stackTraceAndAST(this)

Debugging Utilities

The class LLVMDebugPointer contains functions that may be useful while debugging an application. For example, given the pointer char ** myPtr, the code snippet LLVMDebugPointer.of(myPtr).deref().readHex(18) can be used to return the bytes myPtr[0][0] to myPtr[0][17], such as:

00000000  4e 75 6c 6c 21 00 00 00 01 00 00 00 00 00 00 00  Null!...........
00000010  00 00                                            ..

The class contains the following functions:

  • readI8/.../readI64 to read the integer value at the pointer address
  • readHex(n) returns the hex output of the n bytes at the pointer address
  • readAsciiString returns the null-terminated string at the pointer address
  • deref debug the pointer at the pointer address
  • asHex format the pointer address as a hex value
  • increment(n) shift the pointer by n bytes