[llvm] [AMDGPU][LTO] Introduce AMDGPUCloneModuleLDS (PR #89683)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,201 @@
+//===-- AMDGPUCloneModuleLDSPass.cpp ------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+// The purpose of this pass is to ensure that the combined module contains
+// as many LDS global variables as there are kernels that (indirectly) access
+// them. As LDS variables behave like C++ static variables, it is important that
+// each partition contains a unique copy of the variable on a per kernel basis.
+// This representation also prepares the combined module to eliminate
+// cross-module dependencies of LDS variables.
+//
+// This pass operates as follows:
+// 1. Firstly, traverse the call graph from each kernel to determine the number
+//    of kernels calling each device function.
+// 2. For each LDS global variable GV, determine the function F that defines it.
+//    Collect it's caller functions. Clone F and GV, and finally insert a
+//    call/invoke instruction in each caller function.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPU.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/Analysis/CallGraph.h"
+#include "llvm/Passes/PassBuilder.h"
+#include "llvm/Support/ScopedPrinter.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "amdgpu-clone-module-lds"
+
+static cl::opt<unsigned int> MaxCountForClonedFunctions(
+    "clone-lds-functions-max-count", cl::init(16), cl::Hidden,
+    cl::desc("Specify a limit to the number of clones of a function"));
+
+/// Return the function that defines \p GV
+/// \param GV The global variable in question
+/// \return The function defining \p GV
+static Function *getFunctionDefiningGV(GlobalVariable &GV) {
+  SmallVector<User *> Worklist(GV.users());
+  while (!Worklist.empty()) {
+    User *U = Worklist.pop_back_val();
+    if (auto *Inst = dyn_cast<Instruction>(U))
+      return Inst->getFunction();
+    if (auto *Op = dyn_cast<Operator>(U))
+      append_range(Worklist, Op->users());
+  }
+  llvm_unreachable("GV must be used in a function.");
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arsenm wrote:

This can't be unreachable and you also need to handle initializer references (in particular llvm.used) 

https://github.com/llvm/llvm-project/pull/89683