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      1 //==-LTOInternalize.cpp - LLVM Link Time Optimizer Internalization Utility -==//
      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 defines a helper to run the internalization part of LTO.
     10 //
     11 //===----------------------------------------------------------------------===//
     12 
     13 #include "llvm/LTO/legacy/UpdateCompilerUsed.h"
     14 #include "llvm/Analysis/TargetLibraryInfo.h"
     15 #include "llvm/CodeGen/TargetLowering.h"
     16 #include "llvm/CodeGen/TargetSubtargetInfo.h"
     17 #include "llvm/IR/LegacyPassManager.h"
     18 #include "llvm/IR/Mangler.h"
     19 #include "llvm/Transforms/Utils/ModuleUtils.h"
     20 #include "llvm/Target/TargetMachine.h"
     21 
     22 using namespace llvm;
     23 
     24 namespace {
     25 
     26 // Helper class that collects AsmUsed and user supplied libcalls.
     27 class PreserveLibCallsAndAsmUsed {
     28 public:
     29   PreserveLibCallsAndAsmUsed(const StringSet<> &AsmUndefinedRefs,
     30                              const TargetMachine &TM,
     31                              std::vector<GlobalValue *> &LLVMUsed)
     32       : AsmUndefinedRefs(AsmUndefinedRefs), TM(TM), LLVMUsed(LLVMUsed) {}
     33 
     34   void findInModule(Module &TheModule) {
     35     initializeLibCalls(TheModule);
     36     for (Function &F : TheModule)
     37       findLibCallsAndAsm(F);
     38     for (GlobalVariable &GV : TheModule.globals())
     39       findLibCallsAndAsm(GV);
     40     for (GlobalAlias &GA : TheModule.aliases())
     41       findLibCallsAndAsm(GA);
     42   }
     43 
     44 private:
     45   // Inputs
     46   const StringSet<> &AsmUndefinedRefs;
     47   const TargetMachine &TM;
     48 
     49   // Temps
     50   llvm::Mangler Mangler;
     51   StringSet<> Libcalls;
     52 
     53   // Output
     54   std::vector<GlobalValue *> &LLVMUsed;
     55 
     56   // Collect names of runtime library functions. User-defined functions with the
     57   // same names are added to llvm.compiler.used to prevent them from being
     58   // deleted by optimizations.
     59   void initializeLibCalls(const Module &TheModule) {
     60     TargetLibraryInfoImpl TLII(Triple(TM.getTargetTriple()));
     61     TargetLibraryInfo TLI(TLII);
     62 
     63     // TargetLibraryInfo has info on C runtime library calls on the current
     64     // target.
     65     for (unsigned I = 0, E = static_cast<unsigned>(LibFunc::NumLibFuncs);
     66          I != E; ++I) {
     67       LibFunc F = static_cast<LibFunc>(I);
     68       if (TLI.has(F))
     69         Libcalls.insert(TLI.getName(F));
     70     }
     71 
     72     SmallPtrSet<const TargetLowering *, 1> TLSet;
     73 
     74     for (const Function &F : TheModule) {
     75       const TargetLowering *Lowering =
     76           TM.getSubtargetImpl(F)->getTargetLowering();
     77 
     78       if (Lowering && TLSet.insert(Lowering).second)
     79         // TargetLowering has info on library calls that CodeGen expects to be
     80         // available, both from the C runtime and compiler-rt.
     81         for (unsigned I = 0, E = static_cast<unsigned>(RTLIB::UNKNOWN_LIBCALL);
     82              I != E; ++I)
     83           if (const char *Name =
     84                   Lowering->getLibcallName(static_cast<RTLIB::Libcall>(I)))
     85             Libcalls.insert(Name);
     86     }
     87   }
     88 
     89   void findLibCallsAndAsm(GlobalValue &GV) {
     90     // There are no restrictions to apply to declarations.
     91     if (GV.isDeclaration())
     92       return;
     93 
     94     // There is nothing more restrictive than private linkage.
     95     if (GV.hasPrivateLinkage())
     96       return;
     97 
     98     // Conservatively append user-supplied runtime library functions (supplied
     99     // either directly, or via a function alias) to llvm.compiler.used.  These
    100     // could be internalized and deleted by optimizations like -globalopt,
    101     // causing problems when later optimizations add new library calls (e.g.,
    102     // llvm.memset => memset and printf => puts).
    103     // Leave it to the linker to remove any dead code (e.g. with -dead_strip).
    104     GlobalValue *FuncAliasee = nullptr;
    105     if (isa<GlobalAlias>(GV)) {
    106       auto *A = cast<GlobalAlias>(&GV);
    107       FuncAliasee = dyn_cast<Function>(A->getAliasee());
    108     }
    109     if ((isa<Function>(GV) || FuncAliasee) && Libcalls.count(GV.getName())) {
    110       LLVMUsed.push_back(&GV);
    111       return;
    112     }
    113 
    114     SmallString<64> Buffer;
    115     TM.getNameWithPrefix(Buffer, &GV, Mangler);
    116     if (AsmUndefinedRefs.count(Buffer))
    117       LLVMUsed.push_back(&GV);
    118   }
    119 };
    120 
    121 } // namespace anonymous
    122 
    123 void llvm::updateCompilerUsed(Module &TheModule, const TargetMachine &TM,
    124                               const StringSet<> &AsmUndefinedRefs) {
    125   std::vector<GlobalValue *> UsedValues;
    126   PreserveLibCallsAndAsmUsed(AsmUndefinedRefs, TM, UsedValues)
    127       .findInModule(TheModule);
    128 
    129   if (UsedValues.empty())
    130     return;
    131 
    132   appendToCompilerUsed(TheModule, UsedValues);
    133 }
    134