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