[WIP] Add hashing changes for performance validation

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

please take only Hashing changes (don't remove other changes for now, just don't commit them) and send a PR to dotnet/runtime. Use the benchmark skill to validate perf impact.

[Chronological Review] 1. Initial request: User asked to create a plan to find and fix trivially-replaceable unsafe code in dotnet/runtime Libraries and CoreLib 2. Phase 1 - Exploration: Launched multiple parallel explore agents to search for unsafe patterns (pointers, fixed, Unsafe.*, MemoryMarshal.*, Marshal.*) 3. Phase 2 - Verification: Launched additional agents to verify specific candidates, check build contexts, determine which polyfill files are netstandard2.0-only 4. Phase 3 - First plan: Created plan with 4 items including Enum.cs (Unsafe.As replacement) and Hashing.cs (MemoryMarshal.Cast) 5. Phase 4 - Plan rejected: User rejected because replacing unsafe with Unsafe.As/MemoryMarshal is NOT acceptable. User wants truly safe code only (Span, indexing, etc.) 6. Phase 5 - Re-search: Launched new agents to find truly safe replacements 7. Phase 6 - Updated plan: Created corrected plan with 4 truly safe changes (FixedBufferExtensions, WebHeaderEncoding, StringParser, Hashing) 8. Phase 7 - Plan approved, autopilot mode: Implemented all 4 changes 9. Phase 8 - Build & test: All builds passed, 24,819 tests passed across 4 test suites 10. Phase 9 - Code review: Performed code review, all changes approved 11. Phase 10 - User returned, asked if still working. Marked task complete. 12. Phase 11 - User had modified GetHashCodeOrdinal's default case to a simpler for-loop shape, asked me to do the same for GetHashCodeOrdinalIgnoreCaseAscii 13. Phase 12 - Updated IgnoreCase method to match. Built & tested (22,068 tests pass). 14. Phase 13 - User asked to save all changes EXCEPT Hashing.cs to a .diff patch file, keeping only Hashing.cs modified 15. Phase 14 - Created non-hashing-unsafe-cleanup.diff, reverted the 3 non-Hashing files. Only Hashing.cs remains modified.

[Intent Mapping]

• Primary: Find and fix trivially unsafe code in Libraries/CoreLib with truly safe alternatives
• Constraint: No Unsafe., MemoryMarshal., Marshal.* in replacements — only Span, indexing, ToString(), etc.
• Constraint: Only trivial changes, no invasive refactoring
• Constraint: Ignore interop/pinvokes, tests, samples, tools, mono, nativeaot
• Constraint: Do not commit changes in git
• Later: Match the JIT-friendly for-loop shape user applied to GetHashCodeOrdinal in GetHashCodeOrdinalIgnoreCaseAscii
• Later: Save non-Hashing changes to a .diff patch, revert those files, keep only Hashing.cs

[Technical Inventory]

• dotnet/runtime codebase on Windows
• .NET 11.0 preview SDK (11.0.100-preview.1.26104.118)
• Build command: dotnet build <project>.csproj
• Test command: dotnet build /t:test <test-project>.csproj
• Branch: replace-fixed-buffers (at same commit as origin/main)
• Target frameworks verified: net11.0, netstandard2.0, net481

[Code Archaeology]

• FixedBufferExtensions.cs: removed unsafe, fixed, MemoryMarshal.GetReference, replaced with span.Slice(0, len).ToString() — REVERTED, saved to patch
• WebHeaderEncoding.cs: removed unsafe, fixed pointer loops, replaced with Span indexing — REVERTED, saved to patch
• StringParser.cs: removed unsafe, fixed(char* bufferPtr = _buffer), replaced with _buffer.AsSpan() indexing in 4 methods — REVERTED, saved to patch
• Hashing.cs: removed unsafe, fixed, MemoryMarshal.GetReference, (uint*) pointer casts. Replaced with span indexing + bit packing ((uint)s[i] | ((uint)s[i+1] << 16)). User then modified GetHashCodeOrdinal default case to use for (; i < s.Length - 3; i += 4) shape. I matched that shape in GetHashCodeOrdinalIgnoreCaseAscii. — STILL MODIFIED

[Progress Assessment]

• Hashing.cs: DONE, still modified in working tree
• Other 3 files: DONE but reverted, saved to non-hashing-unsafe-cleanup.diff
• All builds passed, all tests passed (24,819 total)

[Context Validation]

• The .diff patch file exists at C:\prj\runtime-main2\non-hashing-unsafe-cleanup.diff
• Only Hashing.cs remains modified (84 insertions, 100 deletions)
• Plan file exists at session-state folder

[Recent Commands Analysis]

• Last commands: git diff to create patch of 3 non-Hashing files → saved to .diff, then git checkout -- to revert those 3 files, then git diff --stat confirming only Hashing.cs remains modified
• task_complete called with summary

1. Conversation Overview: - Primary Objectives: User requested: "Please walk over the codebase and try to find places with unsafe code that you can trivially make safe." Specifically targeting: 1) unmanaged pointers/dereferencing/fixed statements, 2) Unsafe, MemoryMarshal, Marshal APIs. Rules: trivial changes only, ignore interop/pinvokes, focus on optimization-related unsafe code replaceable with Span, only Libraries and CoreLib (no tests/samples/tools/mono/nativeaot), no git commits. - Session Context: Explored do...

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