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/AliasAnalysis.h"
17 #include "llvm/Analysis/AssumptionCache.h"
18 #include "llvm/Analysis/InlineCost.h"
19 #include "llvm/Analysis/ProfileSummaryInfo.h"
20 #include "llvm/Analysis/TargetLibraryInfo.h"
21 #include "llvm/IR/CallingConv.h"
22 #include "llvm/IR/DataLayout.h"
23 #include "llvm/IR/Instructions.h"
24 #include "llvm/IR/Module.h"
25 #include "llvm/IR/Type.h"
26 #include "llvm/InitializePasses.h"
27 #include "llvm/Transforms/IPO.h"
28 #include "llvm/Transforms/IPO/Inliner.h"
29 #include "llvm/Transforms/Utils/Cloning.h"
30 #include "llvm/Transforms/Utils/ModuleUtils.h"
31 
32 using namespace llvm;
33 
34 #define DEBUG_TYPE "inline"
35 
36 PreservedAnalyses AlwaysInlinerPass::run(Module &M,
37                                          ModuleAnalysisManager &MAM) {
38   // Add inline assumptions during code generation.
39   FunctionAnalysisManager &FAM =
40       MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
41   auto GetAssumptionCache = [&](Function &F) -> AssumptionCache & {
42     return FAM.getResult<AssumptionAnalysis>(F);
43   };
44   auto &PSI = MAM.getResult<ProfileSummaryAnalysis>(M);
45 
46   SmallSetVector<CallBase *, 16> Calls;
47   bool Changed = false;
48   SmallVector<Function *, 16> InlinedFunctions;
49   for (Function &F : M)
50     if (!F.isDeclaration() && F.hasFnAttribute(Attribute::AlwaysInline) &&
51         isInlineViable(F).isSuccess()) {
52       Calls.clear();
53 
54       for (User *U : F.users())
55         if (auto *CB = dyn_cast<CallBase>(U))
56           if (CB->getCalledFunction() == &F)
57             Calls.insert(CB);
58 
59       for (CallBase *CB : Calls) {
60         Function *Caller = CB->getCaller();
61         OptimizationRemarkEmitter ORE(Caller);
62         auto OIC = shouldInline(
63             *CB,
64             [&](CallBase &CB) {
65               return InlineCost::getAlways("always inline attribute");
66             },
67             ORE);
68         assert(OIC);
69         emitInlinedInto(ORE, CB->getDebugLoc(), CB->getParent(), F, *Caller,
70                         *OIC, false, DEBUG_TYPE);
71 
72         InlineFunctionInfo IFI(
73             /*cg=*/nullptr, GetAssumptionCache, &PSI,
74             &FAM.getResult<BlockFrequencyAnalysis>(*(CB->getCaller())),
75             &FAM.getResult<BlockFrequencyAnalysis>(F));
76 
77         InlineResult Res = InlineFunction(
78             *CB, IFI, &FAM.getResult<AAManager>(F), InsertLifetime);
79         assert(Res.isSuccess() && "unexpected failure to inline");
80         (void)Res;
81         Changed = true;
82       }
83 
84       // Remember to try and delete this function afterward. This both avoids
85       // re-walking the rest of the module and avoids dealing with any iterator
86       // invalidation issues while deleting functions.
87       InlinedFunctions.push_back(&F);
88     }
89 
90   // Remove any live functions.
91   erase_if(InlinedFunctions, [&](Function *F) {
92     F->removeDeadConstantUsers();
93     return !F->isDefTriviallyDead();
94   });
95 
96   // Delete the non-comdat ones from the module and also from our vector.
97   auto NonComdatBegin = partition(
98       InlinedFunctions, [&](Function *F) { return F->hasComdat(); });
99   for (Function *F : make_range(NonComdatBegin, InlinedFunctions.end()))
100     M.getFunctionList().erase(F);
101   InlinedFunctions.erase(NonComdatBegin, InlinedFunctions.end());
102 
103   if (!InlinedFunctions.empty()) {
104     // Now we just have the comdat functions. Filter out the ones whose comdats
105     // are not actually dead.
106     filterDeadComdatFunctions(M, InlinedFunctions);
107     // The remaining functions are actually dead.
108     for (Function *F : InlinedFunctions)
109       M.getFunctionList().erase(F);
110   }
111 
112   return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
113 }
114 
115 namespace {
116 
117 /// Inliner pass which only handles "always inline" functions.
118 ///
119 /// Unlike the \c AlwaysInlinerPass, this uses the more heavyweight \c Inliner
120 /// base class to provide several facilities such as array alloca merging.
121 class AlwaysInlinerLegacyPass : public LegacyInlinerBase {
122 
123 public:
124   AlwaysInlinerLegacyPass() : LegacyInlinerBase(ID, /*InsertLifetime*/ true) {
125     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
126   }
127 
128   AlwaysInlinerLegacyPass(bool InsertLifetime)
129       : LegacyInlinerBase(ID, InsertLifetime) {
130     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
131   }
132 
133   /// Main run interface method.  We override here to avoid calling skipSCC().
134   bool runOnSCC(CallGraphSCC &SCC) override { return inlineCalls(SCC); }
135 
136   static char ID; // Pass identification, replacement for typeid
137 
138   InlineCost getInlineCost(CallBase &CB) override;
139 
140   using llvm::Pass::doFinalization;
141   bool doFinalization(CallGraph &CG) override {
142     return removeDeadFunctions(CG, /*AlwaysInlineOnly=*/true);
143   }
144 };
145 }
146 
147 char AlwaysInlinerLegacyPass::ID = 0;
148 INITIALIZE_PASS_BEGIN(AlwaysInlinerLegacyPass, "always-inline",
149                       "Inliner for always_inline functions", false, false)
150 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
151 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
152 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
153 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
154 INITIALIZE_PASS_END(AlwaysInlinerLegacyPass, "always-inline",
155                     "Inliner for always_inline functions", false, false)
156 
157 Pass *llvm::createAlwaysInlinerLegacyPass(bool InsertLifetime) {
158   return new AlwaysInlinerLegacyPass(InsertLifetime);
159 }
160 
161 /// Get the inline cost for the always-inliner.
162 ///
163 /// The always inliner *only* handles functions which are marked with the
164 /// attribute to force inlining. As such, it is dramatically simpler and avoids
165 /// using the powerful (but expensive) inline cost analysis. Instead it uses
166 /// a very simple and boring direct walk of the instructions looking for
167 /// impossible-to-inline constructs.
168 ///
169 /// Note, it would be possible to go to some lengths to cache the information
170 /// computed here, but as we only expect to do this for relatively few and
171 /// small functions which have the explicit attribute to force inlining, it is
172 /// likely not worth it in practice.
173 InlineCost AlwaysInlinerLegacyPass::getInlineCost(CallBase &CB) {
174   Function *Callee = CB.getCalledFunction();
175 
176   // Only inline direct calls to functions with always-inline attributes
177   // that are viable for inlining.
178   if (!Callee)
179     return InlineCost::getNever("indirect call");
180 
181   // FIXME: We shouldn't even get here for declarations.
182   if (Callee->isDeclaration())
183     return InlineCost::getNever("no definition");
184 
185   if (!CB.hasFnAttr(Attribute::AlwaysInline))
186     return InlineCost::getNever("no alwaysinline attribute");
187 
188   auto IsViable = isInlineViable(*Callee);
189   if (!IsViable.isSuccess())
190     return InlineCost::getNever(IsViable.getFailureReason());
191 
192   return InlineCost::getAlways("always inliner");
193 }
194