C++ Print Debugging Utilities

Status

Implemented

Proposed by: Adam Gibson (20-11-2024) Discussed with: Paul Dubs

Context

We need a way to provide debugging utilities for C++ code in the nd4j library. These utilities help in identifying issues such as out-of-bounds crashes and unexpected behavior in mathematical operations. The goal is to have a set of tools that can be easily enabled or disabled via configuration flags.

Decision

We implement three distinct debugging utilities that can be controlled through configuration flags:

1. Print Indices - For tracking loop execution and array access
2. Print Math - For debugging mathematical operations
3. Preprocessor Output - For debugging macro behavior

Each utility serves a specific debugging purpose and can be independently enabled or disabled through both Maven and CMake configuration.

Configuration

Maven Configuration (pom.xml)

<!-- Print Indices Configuration -->
<libnd4j.printindices>OFF</libnd4j.printindices>

<!-- Print Math Configuration -->
<libnd4j.printmath>OFF</libnd4j.printmath>

<!-- Preprocessor Configuration -->
<libnd4j.preprocess>OFF</libnd4j.preprocess>

Build System Integration

The utilities are integrated into the build system through a shell script that configures CMake. The script:

1. Echoes configuration status:

echo PRINT_INDICES       = "$PRINT_INDICES"
echo PRINT_MATH          = "$PRINT_MATH"
echo PREPROCESS          = "$PREPROCESS"

2. Passes configuration to CMake:

eval "$CMAKE_COMMAND" \
    -DPRINT_MATH="$PRINT_MATH" \
    -DPRINT_INDICES="$PRINT_INDICES" \
    # ... other CMake configurations

3. Special handling for preprocessing:

if [ "$PREPROCESS" == "ON" ]; then
    # Re-run CMake with preprocessing enabled
    eval "$CMAKE_COMMAND" \
        -DPRINT_MATH="$PRINT_MATH" \
        -DPRINT_INDICES="$PRINT_INDICES" \
        -DSD_PREPROCESS="$PREPROCESS" \
        # ... other configurations
    echo "Running preprocessing step..."
    exit 0
fi

Implementation Details

1. Print Indices Utility

Purpose

Tracks loop iterations and array access patterns to help identify out-of-bounds issues and iteration-related bugs.

Build Configuration

1. Set through Maven property libnd4j.printindices
2. Passed to CMake as PRINT_INDICES
3. Defines PRINT_INDICES macro in C++ code

Implementation

#if defined(PRINT_INDICES)
    printf("i: %lld xEws %lld ReduceBoolFunction<X, Z>::execScalar\n", i, xEws);
#endif

Example Usage

else {
    for (auto i = start; i < stop; i++) {
#if defined(PRINT_INDICES)
        printf("i: %lld xEws %lld ReduceBoolFunction<X, Z>::execScalar\n", i, xEws);
#endif
        intermediate[thread_id] = OpType::update(
            intermediate[thread_id], 
            OpType::op(x[i * xEws], extraParams), 
            extraParams
        );
    }
}

2. Print Math Utility

Purpose

Provides detailed tracking of mathematical operations, including input values, outputs, and execution flow.

Build Configuration

1. Set through Maven property libnd4j.printmath
2. Passed to CMake as PRINT_MATH
3. Defines SD_GCC_FUNCTRACE macro in C++ code when enabled

Implementation

template <>
SD_INLINE SD_HOST void sd_print_math2<uint16_t>(char* func_name, uint16_t input1, uint16_t input2, uint16_t output) {
#if defined(SD_GCC_FUNCTRACE)
    PRINT_IF_NECESSARY(func_name);
#endif
    printf("%s: input1 = %d, input2 = %d, output = %d\n", func_name, input1, input2, output);
    fflush(stdout);
}

3. Preprocessor Output Utility

Purpose

Generates preprocessed source files to help debug macro-related issues and understand macro expansion.

Build Configuration

1. Set through Maven property libnd4j.preprocess
2. Passed to CMake as SD_PREPROCESS
3. Triggers special build flow in shell script when enabled

Build Process

When preprocessing is enabled:

1. Maven sets libnd4j.preprocess=ON
2. Shell script detects PREPROCESS=ON
3. CMake is run with -DSD_PREPROCESS=ON
4. Build exits after preprocessing step
5. Preprocessed files are generated in the build directory

Implementation

if("${SD_PREPROCESS}" STREQUAL "ON")
    message("Preprocessing enabled ${CMAKE_BINARY_DIR}")
    include_directories(${CMAKE_BINARY_DIR}/../../include)
    list(REMOVE_DUPLICATES ALL_SOURCES)
    # Generate preprocessed files

Consequences

Print Indices

Advantages

• Easily identifies loop boundary issues
• Helps debug array access patterns
• Minimal code overhead

Drawbacks

• Can generate large amounts of output
• Performance impact when enabled

Print Math

Advantages

• Detailed visibility into mathematical operations
• Type-safe debugging
• Stack trace integration

Drawbacks

• Increased binary size
• Runtime overhead when enabled
• Additional complexity in template code

Preprocessor Output

Advantages

• Clear visibility into macro expansion
• Helps debug complex macro interactions
• Useful for template debugging
• Early exit prevents unnecessary compilation steps

Drawbacks

• Increases build complexity
• Additional disk space requirements
• Requires separate build run for preprocessing
• Cannot be combined with normal build flow

References

• platformmath.h
• templatemath.h
• CMakeLists.txt configuration
• pom.xml configuration
• build-dl4j-natives.sh script
• ReduceBoolFunction implementation