[MLAS] Fix rotary avx2 kernel invalid access (#26389)
This fixes an issue that _mm256_maskload_ps intrinsic used in
remainder-handling logic introduced in
https://github.com/microsoft/onnxruntime/pull/23694.
The core of the problem is that _mm256_maskload_ps (and its store
equivalent) can read beyond the masked elements.
Even if mask correctly specifies that you only want to load, for
example, 3 floats, the intrinsic may still read the full 32 bytes (8
floats) from the provided memory address.
The invalid access occurs when one of buffers (input, sin_data, or
cos_data) ends near the boundary of a memory page, and the part of the
32-byte read that you don't care about (i.e., the masked-off part) falls
onto an unmapped page. This will cause a segmentation fault (invalid
access).
The Solution: Use a Scalar Remainder Loop
The simplest, safest, and most robust solution is to replace the masked
AVX remainder logic with a simple scalar loop. This is the exact
strategy already used by your RopeKernel_Avx2_fp16_Impl functions, which
are safe from this bug.
The performance impact of this change will be negligible, as this loop
only processes the final 1-15 elements.
---------
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
