clang/lib/CodeGen/PatternInit.cpp

   1 //===--- PatternInit.cpp - Pattern Initialization -------------------------===//
   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 "PatternInit.h"
  10 #include "CodeGenModule.h"
  11 #include "clang/Basic/TargetInfo.h"
  12 #include "llvm/IR/Constant.h"
  13 #include "llvm/IR/Type.h"
  14
  15 llvm::Constant *clang::CodeGen::initializationPatternFor(CodeGenModule &CGM,
  16                                                          llvm::Type *Ty) {
  17   // The following value is a guaranteed unmappable pointer value and has a
  18   // repeated byte-pattern which makes it easier to synthesize. We use it for
  19   // pointers as well as integers so that aggregates are likely to be
  20   // initialized with this repeated value.
  21   // For 32-bit platforms it's a bit trickier because, across systems, only the
  22   // zero page can reasonably be expected to be unmapped. We use max 0xFFFFFFFF
  23   // assuming that memory access will overlap into zero page.
  24   const uint64_t IntValue =
  25       CGM.getContext().getTargetInfo().getMaxPointerWidth() < 64
  26           ? 0xFFFFFFFFFFFFFFFFull
  27           : 0xAAAAAAAAAAAAAAAAull;
  28   // Floating-point values are initialized as NaNs because they propagate. Using
  29   // a repeated byte pattern means that it will be easier to initialize
  30   // all-floating-point aggregates and arrays with memset. Further, aggregates
  31   // which mix integral and a few floats might also initialize with memset
  32   // followed by a handful of stores for the floats. Using fairly unique NaNs
  33   // also means they'll be easier to distinguish in a crash.
  34   constexpr bool NegativeNaN = true;
  35   constexpr uint64_t NaNPayload = 0xFFFFFFFFFFFFFFFFull;
  36   if (Ty->isIntOrIntVectorTy()) {
  37     unsigned BitWidth =
  38         cast<llvm::IntegerType>(Ty->getScalarType())->getBitWidth();
  39     if (BitWidth <= 64)
  40       return llvm::ConstantInt::get(Ty, IntValue);
  41     return llvm::ConstantInt::get(
  42         Ty, llvm::APInt::getSplat(BitWidth, llvm::APInt(64, IntValue)));
  43   }
  44   if (Ty->isPtrOrPtrVectorTy()) {
  45     auto *PtrTy = cast<llvm::PointerType>(Ty->getScalarType());
  46     unsigned PtrWidth =
  47         CGM.getDataLayout().getPointerSizeInBits(PtrTy->getAddressSpace());
  48     if (PtrWidth > 64)
  49       llvm_unreachable("pattern initialization of unsupported pointer width");
  50     llvm::Type *IntTy = llvm::IntegerType::get(CGM.getLLVMContext(), PtrWidth);
  51     auto *Int = llvm::ConstantInt::get(IntTy, IntValue);
  52     return llvm::ConstantExpr::getIntToPtr(Int, PtrTy);
  53   }
  54   if (Ty->isFPOrFPVectorTy()) {
  55     unsigned BitWidth = llvm::APFloat::semanticsSizeInBits(
  56         Ty->getScalarType()->getFltSemantics());
  57     llvm::APInt Payload(64, NaNPayload);
  58     if (BitWidth >= 64)
  59       Payload = llvm::APInt::getSplat(BitWidth, Payload);
  60     return llvm::ConstantFP::getQNaN(Ty, NegativeNaN, &Payload);
  61   }
  62   if (Ty->isArrayTy()) {
  63     // Note: this doesn't touch tail padding (at the end of an object, before
  64     // the next array object). It is instead handled by replaceUndef.
  65     auto *ArrTy = cast<llvm::ArrayType>(Ty);
  66     llvm::SmallVector<llvm::Constant *, 8> Element(
  67         ArrTy->getNumElements(),
  68         initializationPatternFor(CGM, ArrTy->getElementType()));
  69     return llvm::ConstantArray::get(ArrTy, Element);
  70   }
  71
  72   // Note: this doesn't touch struct padding. It will initialize as much union
  73   // padding as is required for the largest type in the union. Padding is
  74   // instead handled by replaceUndef. Stores to structs with volatile members
  75   // don't have a volatile qualifier when initialized according to C++. This is
  76   // fine because stack-based volatiles don't really have volatile semantics
  77   // anyways, and the initialization shouldn't be observable.
  78   auto *StructTy = cast<llvm::StructType>(Ty);
  79   llvm::SmallVector<llvm::Constant *, 8> Struct(StructTy->getNumElements());
  80   for (unsigned El = 0; El != Struct.size(); ++El)
  81     Struct[El] = initializationPatternFor(CGM, StructTy->getElementType(El));
  82   return llvm::ConstantStruct::get(StructTy, Struct);
  83 }