1 //===- InlineAlways.cpp - Code to inline always_inline functions ----------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements a custom inliner that handles only functions that
10 // are marked as "always inline".
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/IPO/AlwaysInliner.h"
15 #include "llvm/ADT/SetVector.h"
16 #include "llvm/Analysis/AssumptionCache.h"
17 #include "llvm/Analysis/InlineCost.h"
18 #include "llvm/Analysis/TargetLibraryInfo.h"
19 #include "llvm/IR/CallSite.h"
20 #include "llvm/IR/CallingConv.h"
21 #include "llvm/IR/DataLayout.h"
22 #include "llvm/IR/Instructions.h"
23 #include "llvm/IR/Module.h"
24 #include "llvm/IR/Type.h"
25 #include "llvm/Transforms/IPO.h"
26 #include "llvm/Transforms/IPO/Inliner.h"
27 #include "llvm/Transforms/Utils/Cloning.h"
28 #include "llvm/Transforms/Utils/ModuleUtils.h"
29 
30 using namespace llvm;
31 
32 #define DEBUG_TYPE "inline"
33 
34 PreservedAnalyses AlwaysInlinerPass::run(Module &M, ModuleAnalysisManager &) {
35   InlineFunctionInfo IFI;
36   SmallSetVector<CallSite, 16> Calls;
37   bool Changed = false;
38   SmallVector<Function *, 16> InlinedFunctions;
39   for (Function &F : M)
40     if (!F.isDeclaration() && F.hasFnAttribute(Attribute::AlwaysInline) &&
41         isInlineViable(F)) {
42       Calls.clear();
43 
44       for (User *U : F.users())
45         if (auto CS = CallSite(U))
46           if (CS.getCalledFunction() == &F)
47             Calls.insert(CS);
48 
49       for (CallSite CS : Calls)
50         // FIXME: We really shouldn't be able to fail to inline at this point!
51         // We should do something to log or check the inline failures here.
52         Changed |=
53             InlineFunction(CS, IFI, /*CalleeAAR=*/nullptr, InsertLifetime);
54 
55       // Remember to try and delete this function afterward. This both avoids
56       // re-walking the rest of the module and avoids dealing with any iterator
57       // invalidation issues while deleting functions.
58       InlinedFunctions.push_back(&F);
59     }
60 
61   // Remove any live functions.
62   erase_if(InlinedFunctions, [&](Function *F) {
63     F->removeDeadConstantUsers();
64     return !F->isDefTriviallyDead();
65   });
66 
67   // Delete the non-comdat ones from the module and also from our vector.
68   auto NonComdatBegin = partition(
69       InlinedFunctions, [&](Function *F) { return F->hasComdat(); });
70   for (Function *F : make_range(NonComdatBegin, InlinedFunctions.end()))
71     M.getFunctionList().erase(F);
72   InlinedFunctions.erase(NonComdatBegin, InlinedFunctions.end());
73 
74   if (!InlinedFunctions.empty()) {
75     // Now we just have the comdat functions. Filter out the ones whose comdats
76     // are not actually dead.
77     filterDeadComdatFunctions(M, InlinedFunctions);
78     // The remaining functions are actually dead.
79     for (Function *F : InlinedFunctions)
80       M.getFunctionList().erase(F);
81   }
82 
83   return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
84 }
85 
86 namespace {
87 
88 /// Inliner pass which only handles "always inline" functions.
89 ///
90 /// Unlike the \c AlwaysInlinerPass, this uses the more heavyweight \c Inliner
91 /// base class to provide several facilities such as array alloca merging.
92 class AlwaysInlinerLegacyPass : public LegacyInlinerBase {
93 
94 public:
95   AlwaysInlinerLegacyPass() : LegacyInlinerBase(ID, /*InsertLifetime*/ true) {
96     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
97   }
98 
99   AlwaysInlinerLegacyPass(bool InsertLifetime)
100       : LegacyInlinerBase(ID, InsertLifetime) {
101     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
102   }
103 
104   /// Main run interface method.  We override here to avoid calling skipSCC().
105   bool runOnSCC(CallGraphSCC &SCC) override { return inlineCalls(SCC); }
106 
107   static char ID; // Pass identification, replacement for typeid
108 
109   InlineCost getInlineCost(CallSite CS) override;
110 
111   using llvm::Pass::doFinalization;
112   bool doFinalization(CallGraph &CG) override {
113     return removeDeadFunctions(CG, /*AlwaysInlineOnly=*/true);
114   }
115 };
116 }
117 
118 char AlwaysInlinerLegacyPass::ID = 0;
119 INITIALIZE_PASS_BEGIN(AlwaysInlinerLegacyPass, "always-inline",
120                       "Inliner for always_inline functions", false, false)
121 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
122 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
123 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
124 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
125 INITIALIZE_PASS_END(AlwaysInlinerLegacyPass, "always-inline",
126                     "Inliner for always_inline functions", false, false)
127 
128 Pass *llvm::createAlwaysInlinerLegacyPass(bool InsertLifetime) {
129   return new AlwaysInlinerLegacyPass(InsertLifetime);
130 }
131 
132 /// Get the inline cost for the always-inliner.
133 ///
134 /// The always inliner *only* handles functions which are marked with the
135 /// attribute to force inlining. As such, it is dramatically simpler and avoids
136 /// using the powerful (but expensive) inline cost analysis. Instead it uses
137 /// a very simple and boring direct walk of the instructions looking for
138 /// impossible-to-inline constructs.
139 ///
140 /// Note, it would be possible to go to some lengths to cache the information
141 /// computed here, but as we only expect to do this for relatively few and
142 /// small functions which have the explicit attribute to force inlining, it is
143 /// likely not worth it in practice.
144 InlineCost AlwaysInlinerLegacyPass::getInlineCost(CallSite CS) {
145   Function *Callee = CS.getCalledFunction();
146 
147   // Only inline direct calls to functions with always-inline attributes
148   // that are viable for inlining.
149   if (!Callee)
150     return InlineCost::getNever("indirect call");
151 
152   // FIXME: We shouldn't even get here for declarations.
153   if (Callee->isDeclaration())
154     return InlineCost::getNever("no definition");
155 
156   if (!CS.hasFnAttr(Attribute::AlwaysInline))
157     return InlineCost::getNever("no alwaysinline attribute");
158 
159   auto IsViable = isInlineViable(*Callee);
160   if (!IsViable)
161     return InlineCost::getNever(IsViable.message);
162 
163   return InlineCost::getAlways("always inliner");
164 }
165