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

[via llvm-commits]

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

This is under User Guides > Additional Topics > Sandbox IR.

>From 85c16beb3eadb7e23c1795d07b4638eb2884c957 Mon Sep 17 00:00:00 2001
From: Vasileios Porpodas <vporpodas at google.com>
Date: Fri, 12 Jul 2024 13:19:14 -0700
Subject: [PATCH] [SandboxIR][Doc] Add a Doc file for Sandbox IR

This is under User Guides > Additional Topics > Sandbox IR.
---
 llvm/docs/SandboxIR.md   | 53 ++++++++++++++++++++++++++++++++++++++++
 llvm/docs/UserGuides.rst |  5 ++++
 2 files changed, 58 insertions(+)
 create mode 100644 llvm/docs/SandboxIR.md

diff --git a/llvm/docs/SandboxIR.md b/llvm/docs/SandboxIR.md
new file mode 100644
index 0000000000000..0665cc0437a30
--- /dev/null
+++ b/llvm/docs/SandboxIR.md
@@ -0,0 +1,53 @@
+# 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:
+Sandbox IR API functions that modify the IR state call the corresponding LLVM IR function that modifies the LLVM IR's state.
+For example, for `sandboxir::User::setOperand(OpIdx, sandboxir::Value *Op)`:
+- We get the corresponding LLVM User: `llvm::User *LLVMU = cast<llvm::User>(Val)`
+- Next we get the corresponding LLVM Operand: `llvm::Value *LLVMOp = Op->Val`
+- Finally we modify `LLVMU`'s operand: `LLVMU->setOperand(OpIdx, LLVMOp)`.
diff --git a/llvm/docs/UserGuides.rst b/llvm/docs/UserGuides.rst
index bf7cdda89a009..ff956234fee25 100644
--- a/llvm/docs/UserGuides.rst
+++ b/llvm/docs/UserGuides.rst
@@ -67,6 +67,7 @@ intermediate LLVM representation.
    RISCV/RISCVVectorExtension
    SourceLevelDebugging
    SPIRVUsage
+   SandboxIR
    StackSafetyAnalysis
    SupportLibrary
    TableGen/index
@@ -192,6 +193,7 @@ Optimizations
    This document specifies guidelines for contributions for InstCombine and
    related passes.
 
+
 Code Generation
 ---------------
 
@@ -288,3 +290,6 @@ Additional Topics
 
 :doc:`RISCV/RISCVVectorExtension`
    This document describes how the RISC-V Vector extension can be expressed in LLVM IR and how code is generated for it in the backend.
+
+:doc:`Sandbox IR <SandboxIR>`
+   Describes the design and usage of Sandbox IR, a transactional layer over LLVM IR.