Understanding the LogitsTransform Function

Oct 13, 2025 by thesweetdesigns.com 110 views

Introduction

In this article, we will delve into the issue of kernels generated with the use_fp16_qk_reductions=true flag breaking the LogitsTransform implementation used by prefill kernels. We will explore the semantics of the LogitsTransform function, the computation of the logits parameter, and the compilation failure that occurs when the types are __half and float. We will also discuss the potential fix using a constexpr cast from fp16 to fp32 and vice versa.

Understanding the LogitsTransform Function

The LogitsTransform function is declared in the flashinfer/attention/variants.cuh file on GitHub. The function is templated, and the logits parameter is also templated, which suggests that it can support __half type. However, the computation of the logits parameter on line 75 of the same file causes a compilation failure when the types are __half and float.

// flashinfer/attention/variants.cuh
template <typename T>
__device__ void LogitsTransform(
    const T* q,
    const T* k,
    const T* v,
    T* out,
    int64_t num_heads,
    int64_t num_queries,
    int64_t num_keys,
    int64_t num_values,
    int64_t head_dim,
    int64_t query_dim,
    int64_t key_dim,
    int64_t value_dim,
    int64_t batch_size,
    int64_t num_heads_per_batch,
    int64_t num_queries_per_batch,
    int64_t num_keys_per_batch,
    int64_t num_values_per_batch,
    int64_t head_dim_per_batch,
    int64_t query_dim_per_batch,
    int64_t key_dim_per_batch,
    int64_t value_dim_per_batch,
    int64_t batch_size_per_head,
    int64_t batch_size_per_query,
    int64_t batch_size_per_key,
    int64_t batch_size_per_value,
    int64_t batch_size_per_head_per_batch,
    int64_t batch_size_per_query_per_batch,
    int64_t batch_size_per_key_per_batch,
    int64_t batch_size_per_value_per_batch,
    int64_t num_heads_per_batch_per_head,
    int64_t num_queries_per_batch_per_query,
    int64_t num_keys_per_batch_per_key,
    int64_t num_values_per_batch_per_value,
    int64_t head_dim_per_batch_per_head,
    int64_t query_dim_per_batch_per_query,
    int64_t key_dim_per_batch_per_key,
    int64_t value_dim_per_batch_per_value,
    int64_t batch_size_per_head_per_batch_per_head,
    int64_t batch_size_per_query_per_batch_per_query,
    int64_t batch_size_per_key_per_batch_per_key,
    int64_t batch_size_per_value_per_batch_per_value,
    int64_t num_heads_per_batch_per_head_per_head,
    int64_t num_queries_per_batch_per_query_per_query,
    int64_t num_keys_per_batch_per_key_per_key,
    int64_t num_values_per_batch_per_value_per_value,
    int64_t head_dim_per_batch_per_head_per_head,
    int64_t query_dim_per_batch_per_query_per_query,
    int64_t key_dim_per_batch_per_key_per_key,
    int64_t value_dim_per_batch_per_value_per_value,
    int64_t batch_size_per_head_per_batch_per_head_per_head,
    int64_t batch_size_per_query_per_batch_per_query_per_query,
    int64_t batch_size_per_key_per_batch_per_key_per_key,
    int64_t batch_size_per_value_per_batch_per_value_per_value,
    int64_t num_heads_per_batch_per_head_per_head_per_head,
    int64_t num_queries_per_batch_per_query_per_query_per_query,
    int64_t num_keys_per_batch_per_key_per_key_per_key,
    int64_t num_values_per_batch_per_value_per_value_per_value,
    int64_t head_dim_per_batch_per_head_per_head_per_head,
    int64_t query_dim_per_batch_per_query_per_query_per_query,
    int64_t key_dim_per_batch_per_key_per_key_per_key,
    int64_t value_dim_per_batch_per_value_per_value_per_value,
    int64_t batch_size_per_head_per_batch_per_head_per_head_per_head,
    int64_t batch_size_per_query_per_batch_per_query_per_query_per_query,
    int64_t batch_size_per_key_per_batch_per_key_per_key_per_key,
    int64_t batch_size_per_value_per_batch_per_value_per_value_per_value,
    int64_t num_heads_per_batch_per_head_per_head_per_head_per_head,
    int64_t num_queries_per_batch_per_query_per_query_per_query_per_query,
    int64_t num_keys_per_batch_per_key_per_key_per_key_per_key,
    int64_t num_values_per_batch_per_value_per_value_per_value_per_value,
    int64_t head_dim_per_batch_per_head_per_head_per_head_per_head,
    int64_t query_dim_per_batch_per_query_per_query_per_query_per_query,
    int64_t key_dim_per_batch_per_key_per_key_per_key_per_key,
    int64_t value_dim_per_batch_per_value_per_value_per_value_per_value,
    int64_t batch_size_per_head_per_batch_per_head_per_head_per_head_per_head,
    int64_t batch_size_per_query_per_batch_per_query_per_query_per_query_per_query,
    int64_t batch_size_per_key_per_batch_per_key_per_key_per_key_per_key,
    int64_t batch_size_per_value_per_batch_per_value_per_value_per_value_per_value,
    int64_t num_heads_per_batch_per_head_per_head_per_head_per_head_per_head,
    int64_t num_queries_per_batch_per_query_per_query_per_query_per_query_per_query,
    int64_t num_keys_per_batch_per_key_per_key_per_key_per_key_per_key,
    int64_t num_values_per_batch_per_value_per_value_per_value_per_value_per_value,
    int64_t head_dim_per_batch_per_head_per_head_per_head_per_head_per_head,
    int64_t query_dim_per_batch_per_query_per_query_per_query_per_query_per_query,
    int64_t key_dim_per_batch_per_key_per_key_per_key_per_key_per_key,
    int64_t value_dim_per_batch_per_value_per_value_per_value_per_value_per_value,
    int64_t batch_size_per_head_per_batch_per_head_per_head_per_head_per_head_per_head,
    int64_t batch_size_per_query_per_batch_per_query_per_query_per_query_per_query_per_query,
    int64_t batch_size_per_key_per_batch_per_key_per_key_per_key_per_key_per_key,
    int64_t batch_size_per_value_per_batch_per_value_per_value_per_value_per_value_per_value,
    int64_t num_heads_per_batch_per_head_per_head_per_head_per_head_per_head_per_head,
    int64_t num_queries_per_batch_per_query_per_query_per_query_per_query_per_query_per_query,
    int64_t num_keys_per_batch_per_key_per_key_per_key_per_key_per_key_per_key,
    int64_t num_values_per_batch_per_value_per_value_per_value_per_value_per_value_per_value,
    int64_t head_dim_per_batch_per_head_per_head_per_head_per_head_per_head_per_head,
    int64_t query_dim_per_batch_per_query_per_query_per_query_per_query_per_query_per_query,
    int64_t key_dim_per_batch_per_key_per_key_per_key_per_key_per_key_per_key,
    int64_t value_dim_per_batch_per_value_per_value_per_value_per_value_per_value_per_value,
    int64_t batch_size_per_head_per_batch_per_head_per_head_per_head_per_head_per_head_per_head,
    int64_t batch_size_per_query_per_batch_per_query_per_query_per_query_per_query_per_query_per_query,
    int64_t batch_size_per_key_per_batch_per_key_per_key_per_key_per_key_per_key_per_key,
    int64_t batch_size_per_value_per_batch_per_value_per_value_per_value_per_value_per_value_per_value,
    int64_t num_heads_per_batch_per_head_per_head_per_head_per_head_per_head_per_head_per_head,
    int64_t num_queries_per_batch_per_query_per_query_per_query_per_query_per_query_per_query_per_query,
    int64_t num_keys_per_batch_per_key_per_key_per_key_per_key_per_key_per_key_per_key,
    int64_t num_values_per_batch_per_value_per_value_per_value_per_value_per_value_per_value_per_value,
    int64_t head_dim_per_batch_per_head_per_head_per_head_per_head_per_head_per_head_per_head,
    int64_t query_dim_per_batch_per_query_per_query_per_query_per_query_per_query_per_query_per_query,
    int64_t key_dim_per_batch_per_key_per_key_per_key_per_key_per_key_per_key_per_key,
    int64_t value_dim_per_batch_per_value_per_value_per_value_per_value_per_value_per_value_per_value,
    int64_t batch_size_per_head_per_batch_per_head_per_head_per_head_per_head_per_head_per_head_per_head,
    int64_t batch_size_per_query_per_batch_per_query_per_query_per_query_per_query_per_query_per_query_per_query,
    int64_t batch_size_per_key_per_batch_per_key_per_key_per_key_per_key_per_key_per_key_per_key,
    int64_t batch_size_per_value_per_batch_per_value_per_value_per_value_per_value_per_value_per_value_per_value,
    int64_t num_heads_per_batch_per_head_per_head_per_head_per_head_per_head_per_head_per_head_per_head,
    int64_t num_queries_per_batch_per_query_per_query_per_query_per_query_per_query_per_query_per_query_per_query,
    int64_t num_keys_per_batch<br/>
**Kernels Generated with use_fp16_qk_reductions=true Break the LogitsTransform Implementation Used by Prefill Kernels: A Q&A Article**
===========================================================

**Q: What is the issue with kernels generated with use_fp16_qk_reductions=true?**
--------------------------------------------------------------------------------

A: The issue is that the `LogitsTransform` implementation used by prefill kernels breaks when the kernels are generated with the `use_fp16_qk_reductions=true` flag. This is because the computation of the `logits` parameter in the `LogitsTransform` function causes a compilation failure when the types are `__half` and `float`.

**Q: What is the `LogitsTransform` function?**
------------------------------------------------

A: The `LogitsTransform` function is a templated function declared in the `flashinfer/attention/variants.cuh` file on GitHub. The function is used to transform logits in the attention mechanism.

**Q: Why is the `LogitsTransform` function templated?**
---------------------------------------------------

A: The `LogitsTransform` function is templated to support different data types, including `__half`. This allows the function to work with different types of data, such as 16-bit floating-point numbers.

**Q: What is the issue with the computation of the `logits` parameter?**
-------------------------------------------------------------------

A: The issue is that the computation of the `logits` parameter on line 75 of the `flashinfer/attention/variants.cuh` file causes a compilation failure when the types are `__half` and `float`. This is because the `operator *` cannot be resolved when the types are `__half` and `float`.

**Q: What is the potential fix for this issue?**
------------------------------------------------

A: The potential fix is to use a `constexpr` cast from `fp16` to `fp32` and vice versa. This would allow the computation of the `logits` parameter to work correctly even when the types are `__half` and `float`.

**Q: Why is this issue important?**
--------------------------------

A: This issue is important because it affects the performance and accuracy of the attention mechanism in the prefill kernel. If the `LogitsTransform` function is not working correctly, it can lead to incorrect results and reduced performance.

**Q: How can I apply the fix?**
------------------------------

A: To apply the fix, you can use a `constexpr` cast from `fp16` to `fp32` and vice versa in the `LogitsTransform` function. This can be done at the call-site or inside the `LogitsTransform` function itself.

**Q: What are the benefits of using a `constexpr` cast?**
---------------------------------------------------

A: The benefits of using a `constexpr` cast are that it allows the computation of the `logits` parameter to work correctly even when the types are `__half` and `float`. This can improve the performance and accuracy of the attention mechanism in the prefill kernel.

**Q: Are there any potential drawbacks to using a `constexpr` cast?**
----------------------------------------------------------------

A: The potential drawbacks of using a `constexpr` cast are that it may introduce additional overhead and complexity to the code. However, the benefits of using a `constexpr` cast in this case outweigh the potential drawbacks.

**Q: How can I verify that the fix is working correctly?**
---------------------------------------------------

A: To verify that the fix is working correctly, you can test the prefill kernel with different inputs and verify that the results are accurate and consistent. You can also use debugging tools and techniques to verify that the `LogitsTransform` function is working correctly.