Remove unsafe code from Log2SoftwareFallback

[stephentoub]

#109900

[Copilot]

Pull Request Overview

This PR removes unsafe code from the Log2SoftwareFallback method by replacing a manual unsafe memory access with a simple array indexing operation. The change simplifies the code while maintaining the same functionality for computing logarithm base 2 using a de Bruijn sequence lookup table.

Key Changes

• Replaces Unsafe.AddByteOffset with direct array indexing
• Removes unnecessary MemoryMarshal.GetReference call
• Eliminates manual IntPtr casting

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[tannergooding]

This should be fine since all platforms have a baseline accelerated path.

We might, however, want to double check Mono to ensure that Arm and x64 isn't regressed for them.

The JIT could probably elide the bounds check itself here, but it would require it understanding that x >> 27 produces something where only the lower 5 bits can be set and so therefore must be in the range [0, 31]. Might be worth logging an issue for that as part of this.

[stephentoub]

The JIT could probably elide the bounds check itself here, but it would require it understanding that x >> 27 produces something where only the lower 5 bits can be set and so therefore must be in the range [0, 31]. Might be worth logging an issue for that as part of this.

Isn't that what #109900 does?