[llvm] Add DebugSSAUpdater class to track debug value liveness (PR #135349)

[Jeremy Morse via llvm-commits]

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+//===- DebugSSAUpdater.h - Debug SSA Update Tool ----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file declares the DebugSSAUpdater class, which is used to evaluate the
+// live values of debug variables in IR. This uses SSA construction, treating
+// debug value records as definitions, to determine at each point in the program
+// which definition(s) are live at a given point. This is useful for analysis of
+// the state of debug variables, such as measuring the change in values of a
+// variable over time, or calculating coverage stats.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_DEBUGSSAUPDATER_H
+#define LLVM_TRANSFORMS_UTILS_DEBUGSSAUPDATER_H
+
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/DebugInfoMetadata.h"
+#include "llvm/IR/DebugProgramInstruction.h"
+#include "llvm/IR/Instruction.h"
+
+namespace llvm {
+
+////////////////////////////////////////
+// SSAUpdater specialization classes
+
+class DbgSSAPhi;
+template <typename T> class SmallVectorImpl;
+template <typename T> class SSAUpdaterTraits;
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jmorse wrote:

How come the fwd-decl is needed instead of just including `SmallVector.h`?

https://github.com/llvm/llvm-project/pull/135349