1 //===- InferFunctionAttrs.cpp - Infer implicit function attributes --------===//
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 #include "llvm/Transforms/IPO/InferFunctionAttrs.h"
10 #include "llvm/Analysis/TargetLibraryInfo.h"
11 #include "llvm/IR/Function.h"
12 #include "llvm/IR/LLVMContext.h"
13 #include "llvm/IR/Module.h"
14 #include "llvm/InitializePasses.h"
15 #include "llvm/Support/Debug.h"
16 #include "llvm/Support/raw_ostream.h"
17 #include "llvm/Transforms/Utils/BuildLibCalls.h"
18 using namespace llvm;
19 
20 #define DEBUG_TYPE "inferattrs"
21 
22 /// If we can infer one attribute from another on the declaration of a
23 /// function, explicitly materialize the maximal set for readability in the IR.
24 /// Doing this also allows our CGSCC inference to avoid needing to duplicate
25 /// this logic on all calls to declarations (as declarations aren't explicitly
26 /// visited by CGSCC passes in the new pass manager.)
27 static bool inferAttributesFromOthers(Function &F) {
28   // Note: We explicitly check for attributes rather than using cover functions
29   // because some of the cover functions include the logic being implemented.
30 
31   bool Changed = false;
32   // readnone + not convergent implies nosync
33   if (!F.hasFnAttribute(Attribute::NoSync) &&
34       F.doesNotAccessMemory() && !F.isConvergent()) {
35     F.setNoSync();
36     Changed = true;
37   }
38 
39   // readonly implies nofree
40   if (!F.hasFnAttribute(Attribute::NoFree) && F.onlyReadsMemory()) {
41     F.setDoesNotFreeMemory();
42     Changed = true;
43   }
44 
45   // willreturn implies mustprogress
46   if (!F.hasFnAttribute(Attribute::MustProgress) && F.willReturn()) {
47     F.setMustProgress();
48     Changed = true;
49   }
50 
51   // TODO: There are a bunch of cases of restrictive memory effects we
52   // can infer by inspecting arguments of argmemonly-ish functions.
53 
54   return Changed;
55 }
56 
57 static bool inferAllPrototypeAttributes(
58     Module &M, function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
59   bool Changed = false;
60 
61   for (Function &F : M.functions())
62     // We only infer things using the prototype and the name; we don't need
63     // definitions.
64     if (F.isDeclaration() && !F.hasOptNone()) {
65       Changed |= inferLibFuncAttributes(F, GetTLI(F));
66       Changed |= inferAttributesFromOthers(F);
67     }
68 
69   return Changed;
70 }
71 
72 PreservedAnalyses InferFunctionAttrsPass::run(Module &M,
73                                               ModuleAnalysisManager &AM) {
74   FunctionAnalysisManager &FAM =
75       AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
76   auto GetTLI = [&FAM](Function &F) -> TargetLibraryInfo & {
77     return FAM.getResult<TargetLibraryAnalysis>(F);
78   };
79 
80   if (!inferAllPrototypeAttributes(M, GetTLI))
81     // If we didn't infer anything, preserve all analyses.
82     return PreservedAnalyses::all();
83 
84   // Otherwise, we may have changed fundamental function attributes, so clear
85   // out all the passes.
86   return PreservedAnalyses::none();
87 }
88 
89 namespace {
90 struct InferFunctionAttrsLegacyPass : public ModulePass {
91   static char ID; // Pass identification, replacement for typeid
92   InferFunctionAttrsLegacyPass() : ModulePass(ID) {
93     initializeInferFunctionAttrsLegacyPassPass(
94         *PassRegistry::getPassRegistry());
95   }
96 
97   void getAnalysisUsage(AnalysisUsage &AU) const override {
98     AU.addRequired<TargetLibraryInfoWrapperPass>();
99   }
100 
101   bool runOnModule(Module &M) override {
102     if (skipModule(M))
103       return false;
104 
105     auto GetTLI = [this](Function &F) -> TargetLibraryInfo & {
106       return this->getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
107     };
108     return inferAllPrototypeAttributes(M, GetTLI);
109   }
110 };
111 }
112 
113 char InferFunctionAttrsLegacyPass::ID = 0;
114 INITIALIZE_PASS_BEGIN(InferFunctionAttrsLegacyPass, "inferattrs",
115                       "Infer set function attributes", false, false)
116 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
117 INITIALIZE_PASS_END(InferFunctionAttrsLegacyPass, "inferattrs",
118                     "Infer set function attributes", false, false)
119 
120 Pass *llvm::createInferFunctionAttrsLegacyPass() {
121   return new InferFunctionAttrsLegacyPass();
122 }
123