[Common] Remove volatile keyword in fused router kernel utils

[denera]

Description

Per recommendation from the compiler team, this helps avoid local memory loads/stores on SM100 and significantly improves performance of fused router kernels.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[greptile-apps]

Greptile Summary

Removed volatile keyword from 10 local variable declarations in CUDA device functions (warp_reduce_on_shmem, masked_warp_reduce_on_shmem, and naive_topk_and_mask) to optimize SM100 performance by avoiding unnecessary local memory spills.

Changes

• Removed volatile from double val variables used in warp reduction operations
• Removed volatile from int index and double cur_val variables used in topk selection
• Removed volatile from auto shuffled_val and auto shuffled_index variables used in warp shuffle operations

Analysis

The volatile keyword was originally used in these CUDA kernels, likely as a historical practice from older CUDA programming patterns. However, modern CUDA compilers and the CUDA memory model provide proper guarantees for warp-level operations through warp-synchronous programming primitives like __shfl_xor_sync() and __syncwarp(). The code correctly uses these synchronization primitives to ensure proper memory ordering within the warp.

The removal is safe because:

• All warp-level data sharing uses explicit shuffle intrinsics (__shfl_xor_sync) which provide the necessary synchronization
• Explicit warp synchronization barriers (__syncwarp()) are present where needed
• The variables are local to each thread and don't require volatile semantics for correctness
• Modern compiler optimizations benefit from removing unnecessary volatile qualifiers

Confidence Score: 5/5

• This PR is safe to merge with no risk - it's a targeted performance optimization based on compiler team recommendation
• The change removes outdated volatile keywords that were causing performance degradation on SM100 architecture. The code uses proper warp synchronization primitives (__shfl_xor_sync, __syncwarp) that provide correct memory ordering guarantees without needing volatile. The change is mechanical, well-scoped to a single header file, and based on official compiler team guidance.
• No files require special attention

Important Files Changed

Filename	Overview
transformer_engine/common/fused_router/utils.h	Removed volatile keyword from local variables in warp reduction and topk functions to prevent local memory spills on SM100 architecture

_{Last reviewed commit: 17c055a}

[greptile-apps]

_{1 file reviewed, no comments}

_{Edit Code Review Agent Settings | Greptile}

[ptrendx]

/te-ci pytorch