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 
82         // Merge the attributes based on the inlining.
83         AttributeFuncs::mergeAttributesForInlining(*Caller, F);
84 
85         Changed = true;
86       }
87 
88       // Remember to try and delete this function afterward. This both avoids
89       // re-walking the rest of the module and avoids dealing with any iterator
90       // invalidation issues while deleting functions.
91       InlinedFunctions.push_back(&F);
92     }
93 
94   // Remove any live functions.
95   erase_if(InlinedFunctions, [&](Function *F) {
96     F->removeDeadConstantUsers();
97     return !F->isDefTriviallyDead();
98   });
99 
100   // Delete the non-comdat ones from the module and also from our vector.
101   auto NonComdatBegin = partition(
102       InlinedFunctions, [&](Function *F) { return F->hasComdat(); });
103   for (Function *F : make_range(NonComdatBegin, InlinedFunctions.end()))
104     M.getFunctionList().erase(F);
105   InlinedFunctions.erase(NonComdatBegin, InlinedFunctions.end());
106 
107   if (!InlinedFunctions.empty()) {
108     // Now we just have the comdat functions. Filter out the ones whose comdats
109     // are not actually dead.
110     filterDeadComdatFunctions(M, InlinedFunctions);
111     // The remaining functions are actually dead.
112     for (Function *F : InlinedFunctions)
113       M.getFunctionList().erase(F);
114   }
115 
116   return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
117 }
118 
119 namespace {
120 
121 /// Inliner pass which only handles "always inline" functions.
122 ///
123 /// Unlike the \c AlwaysInlinerPass, this uses the more heavyweight \c Inliner
124 /// base class to provide several facilities such as array alloca merging.
125 class AlwaysInlinerLegacyPass : public LegacyInlinerBase {
126 
127 public:
128   AlwaysInlinerLegacyPass() : LegacyInlinerBase(ID, /*InsertLifetime*/ true) {
129     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
130   }
131 
132   AlwaysInlinerLegacyPass(bool InsertLifetime)
133       : LegacyInlinerBase(ID, InsertLifetime) {
134     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
135   }
136 
137   /// Main run interface method.  We override here to avoid calling skipSCC().
138   bool runOnSCC(CallGraphSCC &SCC) override { return inlineCalls(SCC); }
139 
140   static char ID; // Pass identification, replacement for typeid
141 
142   InlineCost getInlineCost(CallBase &CB) override;
143 
144   using llvm::Pass::doFinalization;
145   bool doFinalization(CallGraph &CG) override {
146     return removeDeadFunctions(CG, /*AlwaysInlineOnly=*/true);
147   }
148 };
149 }
150 
151 char AlwaysInlinerLegacyPass::ID = 0;
152 INITIALIZE_PASS_BEGIN(AlwaysInlinerLegacyPass, "always-inline",
153                       "Inliner for always_inline functions", false, false)
154 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
155 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
156 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
157 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
158 INITIALIZE_PASS_END(AlwaysInlinerLegacyPass, "always-inline",
159                     "Inliner for always_inline functions", false, false)
160 
161 Pass *llvm::createAlwaysInlinerLegacyPass(bool InsertLifetime) {
162   return new AlwaysInlinerLegacyPass(InsertLifetime);
163 }
164 
165 /// Get the inline cost for the always-inliner.
166 ///
167 /// The always inliner *only* handles functions which are marked with the
168 /// attribute to force inlining. As such, it is dramatically simpler and avoids
169 /// using the powerful (but expensive) inline cost analysis. Instead it uses
170 /// a very simple and boring direct walk of the instructions looking for
171 /// impossible-to-inline constructs.
172 ///
173 /// Note, it would be possible to go to some lengths to cache the information
174 /// computed here, but as we only expect to do this for relatively few and
175 /// small functions which have the explicit attribute to force inlining, it is
176 /// likely not worth it in practice.
177 InlineCost AlwaysInlinerLegacyPass::getInlineCost(CallBase &CB) {
178   Function *Callee = CB.getCalledFunction();
179 
180   // Only inline direct calls to functions with always-inline attributes
181   // that are viable for inlining.
182   if (!Callee)
183     return InlineCost::getNever("indirect call");
184 
185   // FIXME: We shouldn't even get here for declarations.
186   if (Callee->isDeclaration())
187     return InlineCost::getNever("no definition");
188 
189   if (!CB.hasFnAttr(Attribute::AlwaysInline))
190     return InlineCost::getNever("no alwaysinline attribute");
191 
192   auto IsViable = isInlineViable(*Callee);
193   if (!IsViable.isSuccess())
194     return InlineCost::getNever(IsViable.getFailureReason());
195 
196   return InlineCost::getAlways("always inliner");
197 }
198