1//===- SplitModule.cpp - Split a module into partitions -------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file defines the function llvm::SplitModule, which splits a module
11// into multiple linkable partitions. It can be used to implement parallel code
12// generation for link-time optimization.
13//
14//===----------------------------------------------------------------------===//
15
16#include "llvm/Transforms/Utils/SplitModule.h"
17#include "llvm/ADT/Hashing.h"
18#include "llvm/IR/Function.h"
19#include "llvm/IR/GlobalAlias.h"
20#include "llvm/IR/GlobalObject.h"
21#include "llvm/IR/GlobalValue.h"
22#include "llvm/IR/Module.h"
23#include "llvm/Support/MD5.h"
24#include "llvm/Support/raw_ostream.h"
25#include "llvm/Transforms/Utils/Cloning.h"
26
27using namespace llvm;
28
29static void externalize(GlobalValue *GV) {
30  if (GV->hasLocalLinkage()) {
31    GV->setLinkage(GlobalValue::ExternalLinkage);
32    GV->setVisibility(GlobalValue::HiddenVisibility);
33  }
34
35  // Unnamed entities must be named consistently between modules. setName will
36  // give a distinct name to each such entity.
37  if (!GV->hasName())
38    GV->setName("__llvmsplit_unnamed");
39}
40
41// Returns whether GV should be in partition (0-based) I of N.
42static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
43  if (auto GA = dyn_cast<GlobalAlias>(GV))
44    if (const GlobalObject *Base = GA->getBaseObject())
45      GV = Base;
46
47  StringRef Name;
48  if (const Comdat *C = GV->getComdat())
49    Name = C->getName();
50  else
51    Name = GV->getName();
52
53  // Partition by MD5 hash. We only need a few bits for evenness as the number
54  // of partitions will generally be in the 1-2 figure range; the low 16 bits
55  // are enough.
56  MD5 H;
57  MD5::MD5Result R;
58  H.update(Name);
59  H.final(R);
60  return (R[0] | (R[1] << 8)) % N == I;
61}
62
63void llvm::SplitModule(
64    std::unique_ptr<Module> M, unsigned N,
65    std::function<void(std::unique_ptr<Module> MPart)> ModuleCallback) {
66  for (Function &F : *M)
67    externalize(&F);
68  for (GlobalVariable &GV : M->globals())
69    externalize(&GV);
70  for (GlobalAlias &GA : M->aliases())
71    externalize(&GA);
72
73  // FIXME: We should be able to reuse M as the last partition instead of
74  // cloning it.
75  for (unsigned I = 0; I != N; ++I) {
76    ValueToValueMapTy VMap;
77    std::unique_ptr<Module> MPart(
78        CloneModule(M.get(), VMap, [=](const GlobalValue *GV) {
79          return isInPartition(GV, I, N);
80        }));
81    if (I != 0)
82      MPart->setModuleInlineAsm("");
83    ModuleCallback(std::move(MPart));
84  }
85}
86