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 }
84