[llvm] [SandboxIR][Doc] Add a Doc file for Sandbox IR (PR #98691)

[via llvm-commits]

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+# Sandbox IR: A transactional layer over LLVM IR
+
+Sandbox IR is an IR layer on top of LLVM IR that allow you to save/restore its state.
+
+# API
+The Sandbox IR API is designed to make it feel like LLVM.
+So Sandbox IR replicates many common API classes and functions to mirror the LLVM API.
+The class hierarchy is quite similar (but in the `sandboxir` namespace), for example here is a small part of it:
+```
+namespace sandboxir {
+              Value
+              /  \
+            User BasicBlock ...
+           /   \
+  Instruction Constant
+        /
+     ...
+}
+```
+
+Sandbox IR is also using a similar type system with `isa<>, cast<>, dyn_cast<>`.
+
+# Design
+
+## Sandbox IR Value <-> LLVM IR Value Mapping
+LLVM IR and Sandbox IR are always in sync.
+
+- Forward mapping: Sandbox IR Value -> LLVM IR Value
+Each Sandbox IR Value contains an `llvm::Value *Val` member variable that points to the corresponding LLVM IR Value.
+
+- Reverse mapping: LLVM IR Value -> Sandbox IR Value
+This mapping is stored in `sandboxir::Context::LLVMValueToValue`.
+
+For example `sandboxir::User::getOperand(OpIdx)` for a `sandboxir::User * U` works as follows:
+- First we find the LLVM User: `llvm::User * LLVMU = U->Val`.
+- Next we get the LLVM Value operand: `llvm::Value *LLVMOp = LLVMU->getOperand(OpIdx)`
+- Finally we get the Sandbox IR operand that corresponds to `LLVMOp` by querying the map in the SandboxIR context: `retrun Ctx.getValue(LLVMOp)`.
+
+## Sandbox IR is Write-Through
+Sandbox IR is designed to rely on LLVM IR for its state.
+So any change made to Sandbox IR objects directly updates the corresponding LLVM IR.
+
+This has the following benefits:
+- It minimizes the replication of state, and
+- It makes sure that Sandbox IR and LLVM IR are always in sync, which helps avoid bugs and removes the need for a lowering step.
+- No need for serialization/de-serialization infrastructure as we can rely on LLVM IR for it.
+
+The implementation is straight-forward:
----------------
vporpo wrote:

Done

https://github.com/llvm/llvm-project/pull/98691