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/Analysis/TargetLibraryInfo.h"
22 #include "llvm/IR/CallingConv.h"
23 #include "llvm/IR/DataLayout.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/IR/Module.h"
26 #include "llvm/IR/Type.h"
27 #include "llvm/InitializePasses.h"
28 #include "llvm/Transforms/IPO.h"
29 #include "llvm/Transforms/IPO/Inliner.h"
30 #include "llvm/Transforms/Utils/Cloning.h"
31 #include "llvm/Transforms/Utils/ModuleUtils.h"
32 
33 using namespace llvm;
34 
35 #define DEBUG_TYPE "inline"
36 
37 PreservedAnalyses AlwaysInlinerPass::run(Module &M,
38                                          ModuleAnalysisManager &MAM) {
39   // Add inline assumptions during code generation.
40   FunctionAnalysisManager &FAM =
41       MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
42   auto GetAssumptionCache = [&](Function &F) -> AssumptionCache & {
43     return FAM.getResult<AssumptionAnalysis>(F);
44   };
45   auto &PSI = MAM.getResult<ProfileSummaryAnalysis>(M);
46 
47   SmallSetVector<CallBase *, 16> Calls;
48   bool Changed = false;
49   SmallVector<Function *, 16> InlinedFunctions;
50   for (Function &F : M) {
51     // When callee coroutine function is inlined into caller coroutine function
52     // before coro-split pass,
53     // coro-early pass can not handle this quiet well.
54     // So we won't inline the coroutine function if it have not been unsplited
55     if (F.isPresplitCoroutine())
56       continue;
57 
58     if (!F.isDeclaration() && isInlineViable(F).isSuccess()) {
59       Calls.clear();
60 
61       for (User *U : F.users())
62         if (auto *CB = dyn_cast<CallBase>(U))
63           if (CB->getCalledFunction() == &F &&
64                 CB->hasFnAttr(Attribute::AlwaysInline) &&
65                 !CB->getAttributes().hasFnAttr(Attribute::NoInline))
66               Calls.insert(CB);
67 
68       for (CallBase *CB : Calls) {
69         Function *Caller = CB->getCaller();
70         OptimizationRemarkEmitter ORE(Caller);
71         auto OIC = shouldInline(
72             *CB,
73             [&](CallBase &CB) {
74               return InlineCost::getAlways("always inline attribute");
75             },
76             ORE);
77         assert(OIC);
78         emitInlinedIntoBasedOnCost(ORE, CB->getDebugLoc(), CB->getParent(), F,
79                                    *Caller, *OIC, false, DEBUG_TYPE);
80 
81         InlineFunctionInfo IFI(
82             /*cg=*/nullptr, GetAssumptionCache, &PSI,
83             &FAM.getResult<BlockFrequencyAnalysis>(*(CB->getCaller())),
84             &FAM.getResult<BlockFrequencyAnalysis>(F));
85 
86         InlineResult Res = InlineFunction(
87             *CB, IFI, &FAM.getResult<AAManager>(F), InsertLifetime);
88         assert(Res.isSuccess() && "unexpected failure to inline");
89         (void)Res;
90 
91         // Merge the attributes based on the inlining.
92         AttributeFuncs::mergeAttributesForInlining(*Caller, F);
93 
94         Changed = true;
95       }
96 
97       if (F.hasFnAttribute(Attribute::AlwaysInline)) {
98         // Remember to try and delete this function afterward. This both avoids
99         // re-walking the rest of the module and avoids dealing with any
100         // iterator invalidation issues while deleting functions.
101         InlinedFunctions.push_back(&F);
102       }
103     }
104   }
105 
106   // Remove any live functions.
107   erase_if(InlinedFunctions, [&](Function *F) {
108     F->removeDeadConstantUsers();
109     return !F->isDefTriviallyDead();
110   });
111 
112   // Delete the non-comdat ones from the module and also from our vector.
113   auto NonComdatBegin = partition(
114       InlinedFunctions, [&](Function *F) { return F->hasComdat(); });
115   for (Function *F : make_range(NonComdatBegin, InlinedFunctions.end())) {
116     M.getFunctionList().erase(F);
117     Changed = true;
118   }
119   InlinedFunctions.erase(NonComdatBegin, InlinedFunctions.end());
120 
121   if (!InlinedFunctions.empty()) {
122     // Now we just have the comdat functions. Filter out the ones whose comdats
123     // are not actually dead.
124     filterDeadComdatFunctions(InlinedFunctions);
125     // The remaining functions are actually dead.
126     for (Function *F : InlinedFunctions) {
127       M.getFunctionList().erase(F);
128       Changed = true;
129     }
130   }
131 
132   return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
133 }
134 
135 namespace {
136 
137 /// Inliner pass which only handles "always inline" functions.
138 ///
139 /// Unlike the \c AlwaysInlinerPass, this uses the more heavyweight \c Inliner
140 /// base class to provide several facilities such as array alloca merging.
141 class AlwaysInlinerLegacyPass : public LegacyInlinerBase {
142 
143 public:
144   AlwaysInlinerLegacyPass() : LegacyInlinerBase(ID, /*InsertLifetime*/ true) {
145     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
146   }
147 
148   AlwaysInlinerLegacyPass(bool InsertLifetime)
149       : LegacyInlinerBase(ID, InsertLifetime) {
150     initializeAlwaysInlinerLegacyPassPass(*PassRegistry::getPassRegistry());
151   }
152 
153   /// Main run interface method.  We override here to avoid calling skipSCC().
154   bool runOnSCC(CallGraphSCC &SCC) override { return inlineCalls(SCC); }
155 
156   static char ID; // Pass identification, replacement for typeid
157 
158   InlineCost getInlineCost(CallBase &CB) override;
159 
160   using llvm::Pass::doFinalization;
161   bool doFinalization(CallGraph &CG) override {
162     return removeDeadFunctions(CG, /*AlwaysInlineOnly=*/true);
163   }
164 };
165 }
166 
167 char AlwaysInlinerLegacyPass::ID = 0;
168 INITIALIZE_PASS_BEGIN(AlwaysInlinerLegacyPass, "always-inline",
169                       "Inliner for always_inline functions", false, false)
170 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
171 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
172 INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
173 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
174 INITIALIZE_PASS_END(AlwaysInlinerLegacyPass, "always-inline",
175                     "Inliner for always_inline functions", false, false)
176 
177 Pass *llvm::createAlwaysInlinerLegacyPass(bool InsertLifetime) {
178   return new AlwaysInlinerLegacyPass(InsertLifetime);
179 }
180 
181 /// Get the inline cost for the always-inliner.
182 ///
183 /// The always inliner *only* handles functions which are marked with the
184 /// attribute to force inlining. As such, it is dramatically simpler and avoids
185 /// using the powerful (but expensive) inline cost analysis. Instead it uses
186 /// a very simple and boring direct walk of the instructions looking for
187 /// impossible-to-inline constructs.
188 ///
189 /// Note, it would be possible to go to some lengths to cache the information
190 /// computed here, but as we only expect to do this for relatively few and
191 /// small functions which have the explicit attribute to force inlining, it is
192 /// likely not worth it in practice.
193 InlineCost AlwaysInlinerLegacyPass::getInlineCost(CallBase &CB) {
194   Function *Callee = CB.getCalledFunction();
195 
196   // Only inline direct calls to functions with always-inline attributes
197   // that are viable for inlining.
198   if (!Callee)
199     return InlineCost::getNever("indirect call");
200 
201   // When callee coroutine function is inlined into caller coroutine function
202   // before coro-split pass,
203   // coro-early pass can not handle this quiet well.
204   // So we won't inline the coroutine function if it have not been unsplited
205   if (Callee->isPresplitCoroutine())
206     return InlineCost::getNever("unsplited coroutine call");
207 
208   // FIXME: We shouldn't even get here for declarations.
209   if (Callee->isDeclaration())
210     return InlineCost::getNever("no definition");
211 
212   if (!CB.hasFnAttr(Attribute::AlwaysInline))
213     return InlineCost::getNever("no alwaysinline attribute");
214 
215   if (Callee->hasFnAttribute(Attribute::AlwaysInline) && CB.isNoInline())
216     return InlineCost::getNever("noinline call site attribute");
217 
218   auto IsViable = isInlineViable(*Callee);
219   if (!IsViable.isSuccess())
220     return InlineCost::getNever(IsViable.getFailureReason());
221 
222   return InlineCost::getAlways("always inliner");
223 }
224