fix_simd_access_plan.md

Plan to Fix SIMD Access in VM Implementation

Current Issue

The VM's vector_add implementation is trying to directly access the private data field of the Stack struct to perform SIMD operations. This violates encapsulation and won't compile.

Solution Plan

1. Add SIMD Support Methods to Stack

Add two new methods to the Stack struct in stack.rs:

impl Stack {
    /// Returns a pointer to the last 8 elements for SIMD reading
    /// 
    /// # Safety
    /// Caller must ensure:
    /// - At least 8 elements are on the stack
    /// - Pointer is only used for reading
    #[inline(always)]
    pub fn as_simd_ptr(&self) -> Result<*const Value, VMError> {
        if self.depth() < 8 {
            return Err(VMError::StackUnderflow);
        }
        Ok(self.data.as_ptr().add(self.depth() - 8))
    }

    /// Returns a mutable pointer to the last 8 elements for SIMD writing
    /// 
    /// # Safety
    /// Caller must ensure:
    /// - At least 8 elements are on the stack
    /// - Pointer is only used for writing valid Values
    #[inline(always)] 
    pub fn as_simd_mut_ptr(&mut self) -> Result<*mut Value, VMError> {
        if self.depth() < 8 {
            return Err(VMError::StackUnderflow);
        }
        Ok(self.data.as_mut_ptr().add(self.depth() - 8))
    }
}

2. Update Vector Add Implementation

Modify the vector_add function in core.rs to use the new safe methods:

#[cfg(target_arch = "x86_64")]
#[inline(always)]
unsafe fn vector_add(&mut self) -> Result<(), VMError> {
    let values = _mm256_loadu_si256(self.stack.as_simd_ptr()? as *const __m256i);
    let result = _mm256_add_epi64(values, values);
    _mm256_storeu_si256(self.stack.as_simd_mut_ptr()? as *mut __m256i, result);
    Ok(())
}

Benefits

1. Maintains Encapsulation: The Stack's data remains private
2. Safe Interface: New methods include proper bounds checking
3. Performance: Preserves SIMD optimization capabilities
4. Clean API: Provides clear methods specifically for SIMD operations

Implementation Steps

1. Add new SIMD support methods to Stack
2. Update vector_add implementation
3. Add tests to verify SIMD operations work correctly
4. Document safety requirements for SIMD methods

Testing Plan

1. Test stack underflow conditions
2. Verify SIMD operations produce correct results
3. Ensure stack integrity is maintained after operations
4. Test with various stack sizes and values