llvm-project

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//===--- PatternInit.cpp - Pattern Initialization -------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "PatternInit.h"
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#include "CodeGenModule.h"
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#include "clang/Basic/TargetInfo.h"
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#include "llvm/IR/Constant.h"
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#include "llvm/IR/Type.h"
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llvm::Constant *clang::CodeGen::initializationPatternFor(CodeGenModule &CGM,
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                                                         llvm::Type *Ty) {
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  // The following value is a guaranteed unmappable pointer value and has a
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  // repeated byte-pattern which makes it easier to synthesize. We use it for
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  // pointers as well as integers so that aggregates are likely to be
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  // initialized with this repeated value.
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  // For 32-bit platforms it's a bit trickier because, across systems, only the
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  // zero page can reasonably be expected to be unmapped. We use max 0xFFFFFFFF
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  // assuming that memory access will overlap into zero page.
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  const uint64_t IntValue =
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      CGM.getContext().getTargetInfo().getMaxPointerWidth() < 64
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          ? 0xFFFFFFFFFFFFFFFFull
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          : 0xAAAAAAAAAAAAAAAAull;
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  // Floating-point values are initialized as NaNs because they propagate. Using
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  // a repeated byte pattern means that it will be easier to initialize
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  // all-floating-point aggregates and arrays with memset. Further, aggregates
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  // which mix integral and a few floats might also initialize with memset
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  // followed by a handful of stores for the floats. Using fairly unique NaNs
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  // also means they'll be easier to distinguish in a crash.
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  constexpr bool NegativeNaN = true;
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  constexpr uint64_t NaNPayload = 0xFFFFFFFFFFFFFFFFull;
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  if (Ty->isIntOrIntVectorTy()) {
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    unsigned BitWidth =
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        cast<llvm::IntegerType>(Ty->getScalarType())->getBitWidth();
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    if (BitWidth <= 64)
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      return llvm::ConstantInt::get(Ty, IntValue);
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    return llvm::ConstantInt::get(
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        Ty, llvm::APInt::getSplat(BitWidth, llvm::APInt(64, IntValue)));
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  }
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  if (Ty->isPtrOrPtrVectorTy()) {
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    auto *PtrTy = cast<llvm::PointerType>(Ty->getScalarType());
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    unsigned PtrWidth =
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        CGM.getDataLayout().getPointerSizeInBits(PtrTy->getAddressSpace());
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    if (PtrWidth > 64)
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      llvm_unreachable("pattern initialization of unsupported pointer width");
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    llvm::Type *IntTy = llvm::IntegerType::get(CGM.getLLVMContext(), PtrWidth);
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    auto *Int = llvm::ConstantInt::get(IntTy, IntValue);
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    return llvm::ConstantExpr::getIntToPtr(Int, PtrTy);
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  }
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  if (Ty->isFPOrFPVectorTy()) {
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    unsigned BitWidth = llvm::APFloat::semanticsSizeInBits(
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        Ty->getScalarType()->getFltSemantics());
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    llvm::APInt Payload(64, NaNPayload);
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    if (BitWidth >= 64)
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      Payload = llvm::APInt::getSplat(BitWidth, Payload);
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    return llvm::ConstantFP::getQNaN(Ty, NegativeNaN, &Payload);
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  }
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  if (Ty->isArrayTy()) {
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    // Note: this doesn't touch tail padding (at the end of an object, before
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    // the next array object). It is instead handled by replaceUndef.
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    auto *ArrTy = cast<llvm::ArrayType>(Ty);
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    llvm::SmallVector<llvm::Constant *, 8> Element(
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        ArrTy->getNumElements(),
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        initializationPatternFor(CGM, ArrTy->getElementType()));
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    return llvm::ConstantArray::get(ArrTy, Element);
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  }
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  // Note: this doesn't touch struct padding. It will initialize as much union
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  // padding as is required for the largest type in the union. Padding is
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  // instead handled by replaceUndef. Stores to structs with volatile members
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  // don't have a volatile qualifier when initialized according to C++. This is
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  // fine because stack-based volatiles don't really have volatile semantics
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  // anyways, and the initialization shouldn't be observable.
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  auto *StructTy = cast<llvm::StructType>(Ty);
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  llvm::SmallVector<llvm::Constant *, 8> Struct(StructTy->getNumElements());
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  for (unsigned El = 0; El != Struct.size(); ++El)
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    Struct[El] = initializationPatternFor(CGM, StructTy->getElementType(El));
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  return llvm::ConstantStruct::get(StructTy, Struct);
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}
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