1 //===- AlwaysInliner.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/OptimizationRemarkEmitter.h"
20 #include "llvm/Analysis/ProfileSummaryInfo.h"
21 #include "llvm/IR/Module.h"
22 #include "llvm/InitializePasses.h"
23 #include "llvm/Transforms/IPO/Inliner.h"
24 #include "llvm/Transforms/Utils/Cloning.h"
25 #include "llvm/Transforms/Utils/ModuleUtils.h"
26 
27 using namespace llvm;
28 
29 #define DEBUG_TYPE "inline"
30 
31 PreservedAnalyses AlwaysInlinerPass::run(Module &M,
32                                          ModuleAnalysisManager &MAM) {
33   // Add inline assumptions during code generation.
34   FunctionAnalysisManager &FAM =
35       MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
36   auto GetAssumptionCache = [&](Function &F) -> AssumptionCache & {
37     return FAM.getResult<AssumptionAnalysis>(F);
38   };
39   auto &PSI = MAM.getResult<ProfileSummaryAnalysis>(M);
40 
41   SmallSetVector<CallBase *, 16> Calls;
42   bool Changed = false;
43   SmallVector<Function *, 16> InlinedFunctions;
44   for (Function &F : M) {
45     // When callee coroutine function is inlined into caller coroutine function
46     // before coro-split pass,
47     // coro-early pass can not handle this quiet well.
48     // So we won't inline the coroutine function if it have not been unsplited
49     if (F.isPresplitCoroutine())
50       continue;
51 
52     if (!F.isDeclaration() && isInlineViable(F).isSuccess()) {
53       Calls.clear();
54 
55       for (User *U : F.users())
56         if (auto *CB = dyn_cast<CallBase>(U))
57           if (CB->getCalledFunction() == &F &&
58                 CB->hasFnAttr(Attribute::AlwaysInline) &&
59                 !CB->getAttributes().hasFnAttr(Attribute::NoInline))
60               Calls.insert(CB);
61 
62       for (CallBase *CB : Calls) {
63         Function *Caller = CB->getCaller();
64         OptimizationRemarkEmitter ORE(Caller);
65         DebugLoc DLoc = CB->getDebugLoc();
66         BasicBlock *Block = CB->getParent();
67 
68         InlineFunctionInfo IFI(
69             /*cg=*/nullptr, GetAssumptionCache, &PSI,
70             &FAM.getResult<BlockFrequencyAnalysis>(*Caller),
71             &FAM.getResult<BlockFrequencyAnalysis>(F));
72 
73         InlineResult Res =
74             InlineFunction(*CB, IFI, /*MergeAttributes=*/true,
75                            &FAM.getResult<AAManager>(F), InsertLifetime);
76         if (!Res.isSuccess()) {
77           ORE.emit([&]() {
78             return OptimizationRemarkMissed(DEBUG_TYPE, "NotInlined", DLoc,
79                                             Block)
80                    << "'" << ore::NV("Callee", &F) << "' is not inlined into '"
81                    << ore::NV("Caller", Caller)
82                    << "': " << ore::NV("Reason", Res.getFailureReason());
83           });
84           continue;
85         }
86 
87         emitInlinedIntoBasedOnCost(
88             ORE, DLoc, Block, F, *Caller,
89             InlineCost::getAlways("always inline attribute"),
90             /*ForProfileContext=*/false, DEBUG_TYPE);
91 
92         Changed = true;
93       }
94 
95       if (F.hasFnAttribute(Attribute::AlwaysInline)) {
96         // Remember to try and delete this function afterward. This both avoids
97         // re-walking the rest of the module and avoids dealing with any
98         // iterator invalidation issues while deleting functions.
99         InlinedFunctions.push_back(&F);
100       }
101     }
102   }
103 
104   // Remove any live functions.
105   erase_if(InlinedFunctions, [&](Function *F) {
106     F->removeDeadConstantUsers();
107     return !F->isDefTriviallyDead();
108   });
109 
110   // Delete the non-comdat ones from the module and also from our vector.
111   auto NonComdatBegin = partition(
112       InlinedFunctions, [&](Function *F) { return F->hasComdat(); });
113   for (Function *F : make_range(NonComdatBegin, InlinedFunctions.end())) {
114     M.getFunctionList().erase(F);
115     Changed = true;
116   }
117   InlinedFunctions.erase(NonComdatBegin, InlinedFunctions.end());
118 
119   if (!InlinedFunctions.empty()) {
120     // Now we just have the comdat functions. Filter out the ones whose comdats
121     // are not actually dead.
122     filterDeadComdatFunctions(InlinedFunctions);
123     // The remaining functions are actually dead.
124     for (Function *F : InlinedFunctions) {
125       M.getFunctionList().erase(F);
126       Changed = true;
127     }
128   }
129 
130   return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
131 }
132 
133 namespace {
134 
135 /// Inliner pass which only handles "always inline" functions.
136 ///
137 /// Unlike the \c AlwaysInlinerPass, this uses the more heavyweight \c Inliner
138 /// base class to provide several facilities such as array alloca merging.
139 class AlwaysInlinerLegacyPass : public LegacyInlinerBase {
140 
141 public:
142   AlwaysInlinerLegacyPass() : LegacyInlinerBase(ID, /*InsertLifetime*/ true) {
143     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
144   }
145 
146   AlwaysInlinerLegacyPass(bool InsertLifetime)
147       : LegacyInlinerBase(ID, InsertLifetime) {
148     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
149   }
150 
151   /// Main run interface method.  We override here to avoid calling skipSCC().
152   bool runOnSCC(CallGraphSCC &SCC) override { return inlineCalls(SCC); }
153 
154   static char ID; // Pass identification, replacement for typeid
155 
156   InlineCost getInlineCost(CallBase &CB) override;
157 
158   using llvm::Pass::doFinalization;
159   bool doFinalization(CallGraph &CG) override {
160     return removeDeadFunctions(CG, /*AlwaysInlineOnly=*/true);
161   }
162 };
163 }
164 
165 char AlwaysInlinerLegacyPass::ID = 0;
166 INITIALIZE_PASS_BEGIN(AlwaysInlinerLegacyPass, "always-inline",
167                       "Inliner for always_inline functions", false, false)
168 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
169 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
170 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
171 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
172 INITIALIZE_PASS_END(AlwaysInlinerLegacyPass, "always-inline",
173                     "Inliner for always_inline functions", false, false)
174 
175 Pass *llvm::createAlwaysInlinerLegacyPass(bool InsertLifetime) {
176   return new AlwaysInlinerLegacyPass(InsertLifetime);
177 }
178 
179 /// Get the inline cost for the always-inliner.
180 ///
181 /// The always inliner *only* handles functions which are marked with the
182 /// attribute to force inlining. As such, it is dramatically simpler and avoids
183 /// using the powerful (but expensive) inline cost analysis. Instead it uses
184 /// a very simple and boring direct walk of the instructions looking for
185 /// impossible-to-inline constructs.
186 ///
187 /// Note, it would be possible to go to some lengths to cache the information
188 /// computed here, but as we only expect to do this for relatively few and
189 /// small functions which have the explicit attribute to force inlining, it is
190 /// likely not worth it in practice.
191 InlineCost AlwaysInlinerLegacyPass::getInlineCost(CallBase &CB) {
192   Function *Callee = CB.getCalledFunction();
193 
194   // Only inline direct calls to functions with always-inline attributes
195   // that are viable for inlining.
196   if (!Callee)
197     return InlineCost::getNever("indirect call");
198 
199   // When callee coroutine function is inlined into caller coroutine function
200   // before coro-split pass,
201   // coro-early pass can not handle this quiet well.
202   // So we won't inline the coroutine function if it have not been unsplited
203   if (Callee->isPresplitCoroutine())
204     return InlineCost::getNever("unsplited coroutine call");
205 
206   // FIXME: We shouldn't even get here for declarations.
207   if (Callee->isDeclaration())
208     return InlineCost::getNever("no definition");
209 
210   if (!CB.hasFnAttr(Attribute::AlwaysInline))
211     return InlineCost::getNever("no alwaysinline attribute");
212 
213   if (Callee->hasFnAttribute(Attribute::AlwaysInline) && CB.isNoInline())
214     return InlineCost::getNever("noinline call site attribute");
215 
216   auto IsViable = isInlineViable(*Callee);
217   if (!IsViable.isSuccess())
218     return InlineCost::getNever(IsViable.getFailureReason());
219 
220   return InlineCost::getAlways("always inliner");
221 }
222