Quantization Support

Quantization Support in LLMIR

Quantization is a critical optimization technique for large language models, reducing memory footprint and computation requirements by using lower-precision representations of weights and activations. LLMIR provides comprehensive support for quantization through specialized representations and transformations.

Quantization in LLMIR ¶

LLMIR supports various quantization strategies tailored for LLM inference:

Custom Quantized Types ¶

LLMIR defines specialized types for representing quantized tensors:

// INT8 asymmetric quantized tensor
!llm.quantized_tensor<4x1024xi8, scale=f32, zp=i8, group_size=128>

// INT4 symmetric grouped quantized tensor
!llm.quantized_tensor<4x1024xi4, scale=f32, symmetric=true, group_size=128>

// Mixed precision quantized tensor
!llm.mixed_quantized_tensor<4x1024x!llm.mixed<i8,i4>, scale=f32>

Quantization Operations ¶

// Quantize a tensor from FP16 to INT8
%quantized = llm.quantize(%input) {
  scale = dense<0.01> : tensor<256xf32>,
  zero_point = dense<-2> : tensor<256xi8>,
  bits = 8 : i32,
  symmetric = false
} : (tensor<1x256xf16>) -> tensor<1x256xi8>

// Dequantize from INT8 back to FP16
%dequantized = llm.dequantize(%quantized) {
  scale = dense<0.01> : tensor<256xf32>,
  zero_point = dense<-2> : tensor<256xi8>
} : (tensor<1x256xi8>) -> tensor<1x256xf16>

// Quantized matrix multiplication
%result = llm.quantized_matmul(%input, %weight, %scales, %zero_points) {
  bits = 8 : i32,
  group_size = 128 : i32
} : (tensor<?x?xf16>, tensor<?x?xi8>, tensor<?xf32>, tensor<?xi8>) -> tensor<?x?xf16>

Quantization Methods ¶

LLMIR will support multiple quantization strategies:

Post-Training Quantization (PTQ) ¶

• Symmetric Quantization: Uses a symmetric range around zero
• Asymmetric Quantization: Uses zero-point offsets for asymmetric ranges
• Per-Channel/Per-Tensor: Supports different scaling granularities
• Group-wise Quantization: Applies quantization parameters to groups of weights

Quantization-Aware Inference (QAI) ¶

• Weight-only Quantization: Keeps activations in higher precision
• Activation Quantization: Quantizes intermediate activations
• Mixed-precision Inference: Different precision for different parts of the model

Optimization Passes ¶

LLMIR will include several quantization-related optimization passes:

1. QuantizationCalibrationPass: Analyze model to determine optimal quantization parameters
2. WeightQuantizationPass: Convert model weights to quantized formats
3. ActivationQuantizationPass: Add quantization/dequantization for activations
4. QuantizedOperationFusionPass: Fuse quantized operations for efficient execution
5. HardwareSpecificQuantizationPass: Customize quantization for specific hardware features

Integration with Hardware Backends ¶

LLMIR’s quantization system is designed to integrate with various hardware backends:

• NVIDIA Tensor Cores: Support for INT8/INT4 computation
• Intel AMX: Optimizations for x86 Advanced Matrix Extensions
• ARM Matrix multiplier: Support for efficient ARM-based inference
• Custom Accelerators: Extensible for specialized ML hardware

Runtime Support ¶

The LLMIR runtime will provide efficient implementations for quantized operations:

// Quantized Matrix Multiplication (Planned API)
void quantizedMatMul(
    const void* input,           // Input activations (typically FP16)
    const int8_t* weights,       // Quantized weights (INT8/INT4)
    const float* scales,         // Quantization scales
    const int8_t* zeroPoints,    // Zero points (for asymmetric)
    void* output,                // Output buffer
    int M, int N, int K,         // Matrix dimensions
    int groupSize,               // Group size for scales
    int bits                     // Bit width (8/4)
);

Future Directions ¶

As part of LLMIR’s advanced features (Phase 3), quantization support will be enhanced with:

• Sparse-Quantized Representations: Combining sparsity and quantization
• Dynamic Quantization: Adaptive precision based on content
• Calibration Tools: Utilities for determining optimal quantization parameters
• Automated Mixed Precision: Intelligent selection of precision for different model parts

This feature is planned for Phase 3 of the LLMIR project development.