It seems i cannot use upper half of ymm registers

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Summary

The issue at hand is the inability to use the upper half of YMM registers when writing code in Visual Studio 2022 using MASM and AVX2. The code provided attempts to fill vectors with set values, but only the bottom halves of the registers are being filled, while the upper halves remain unchanged.

Root Cause

The root cause of this issue is due to the following reasons:

  • Incorrect usage of AVX2 instructions: The code is using VPBROADCASTW and VPMOVSBW instructions, which only operate on the lower 128 bits of the YMM registers.
  • Insufficient register initialization: The code does not properly initialize the upper halves of the YMM registers before using them.

Why This Happens in Real Systems

This issue occurs in real systems because:

  • AVX2 instructions are designed to operate on XMM and YMM registers, but some instructions only access the lower 128 bits of the YMM registers.
  • Register initialization is crucial when working with AVX2 instructions to ensure that all parts of the register are properly set.

Real-World Impact

The impact of this issue is:

  • Incorrect results: The code will produce incorrect results because the upper halves of the YMM registers are not being properly utilized.
  • Performance issues: The code may experience performance issues due to the inefficient use of AVX2 instructions.

Example or Code

; === BUILD KERNEL VECTORS: [k0, k1, k2] ===
mov eax, 0Fh
vmovq xmm0, rax
vpbroadcastd ymm3, xmm0
vpbroadcastd ymm5, xmm0
mov eax, 61h
vmovd xmm0, eax
vpbroadcastd ymm4, xmm0

In this example, VPBROADCASTD is used instead of VPBROADCASTW to fill the entire YMM register.

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Using the correct AVX2 instructions: Choosing instructions that operate on the entire YMM register, such as VPBROADCASTD.
  • Properly initializing registers: Ensuring that all parts of the YMM registers are properly set before using them.
  • Verifying register contents: Using debugging tools to verify the contents of the YMM registers and ensure they are being used correctly.

Why Juniors Miss It

Junior engineers may miss this issue because:

  • Lack of experience with AVX2: Inexperience with AVX2 instructions and YMM registers can lead to incorrect usage.
  • Insufficient understanding of register initialization: Not fully understanding the importance of register initialization can result in incorrect results.
  • Inadequate debugging: Failing to properly debug and verify the contents of the YMM registers can lead to issues going unnoticed.

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