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