Add LLVM for code generation proposal

Signed-off-by: Alan Jowett <alanjo@microsoft.com>

docs/eBpfLlvmCodeGenerationProposal.md

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+# Clang/LLVM as code generator for eBPF-for-Windows
+
+## Overview
+
+Clang/LLVM has a much richer tool chain for generating optimal ISA specific
+instruction sequences. Proposal is to switch to LLVM to emit optimal native
+instruction sequences.
+
+## Background
+
+Clang/LLVM has a language neutral internal representation (IR). Various
+front-ends produce IR that is then handed off to back-ends to produce optimal
+and secure ISA specific machine code. LLVM code generation is platform aware
+and can take advantage of advanced processor features like vectored / SIMD
+instructions as well as having support for speculative load hardening and
+other security features.
+
+## Proposal
+
+1. Write a Clang/LLVM front-end that translates eBPF byte code to LLVM IR.
+2. Detect CPU features at runtime and configure LLVM.
+3. Use LLVM back-end to generate optimal native instruction sequence.
+4. Optionally provide LLVM IR versions of map helper functions (to permit
+inlining of map lookups).
+
+## Expected benefits
+
+1. LLVM already supports x86, ARM and ARM64, so eBPF-for-Windows will be able
+to JIT correctly on all these platforms.
+2. LLVM can optimize the generated machine code as it has the entire control
+flow graph as IR.
+3. LLVM mitigations for branch prediction side-channel attacks will be applied
+to the generated code.
+
+## Future options
+
+The conversion of source code -> LLVM IR -> eBPF -> LLVM IR -> machine code is
+lossy in that some context about how to best optimize the resulting machine code
+is lost at each step. A possible future change would be to have eBPF for Windows
+accept LLVM IR directly, then internally convert LLVM IR -> eBPF for the verifier
+and LLVM IR -> machine code for execution.