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 "llvm/IR/Constant.h"
12 #include "llvm/IR/Type.h"
13 
14 llvm::Constant *clang::CodeGen::initializationPatternFor(CodeGenModule &CGM,
15                                                          llvm::Type *Ty) {
16   // The following value is a guaranteed unmappable pointer value and has a
17   // repeated byte-pattern which makes it easier to synthesize. We use it for
18   // pointers as well as integers so that aggregates are likely to be
19   // initialized with this repeated value.
20   // For 32-bit platforms it's a bit trickier because, across systems, only the
21   // zero page can reasonably be expected to be unmapped. We use max 0xFFFFFFFF
22   // assuming that memory access will overlap into zero page.
23   const uint64_t IntValue =
24       CGM.getContext().getTargetInfo().getMaxPointerWidth() < 64
25           ? 0xFFFFFFFFFFFFFFFFull
26           : 0xAAAAAAAAAAAAAAAAull;
27   // Floating-point values are initialized as NaNs because they propagate. Using
28   // a repeated byte pattern means that it will be easier to initialize
29   // all-floating-point aggregates and arrays with memset. Further, aggregates
30   // which mix integral and a few floats might also initialize with memset
31   // followed by a handful of stores for the floats. Using fairly unique NaNs
32   // also means they'll be easier to distinguish in a crash.
33   constexpr bool NegativeNaN = true;
34   constexpr uint64_t NaNPayload = 0xFFFFFFFFFFFFFFFFull;
35   if (Ty->isIntOrIntVectorTy()) {
36     unsigned BitWidth = cast<llvm::IntegerType>(
37                             Ty->isVectorTy() ? Ty->getVectorElementType() : Ty)
38                             ->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>(
46         Ty->isVectorTy() ? Ty->getVectorElementType() : Ty);
47     unsigned PtrWidth = CGM.getContext().getTargetInfo().getPointerWidth(
48         PtrTy->getAddressSpace());
49     if (PtrWidth > 64)
50       llvm_unreachable("pattern initialization of unsupported pointer width");
51     llvm::Type *IntTy = llvm::IntegerType::get(CGM.getLLVMContext(), PtrWidth);
52     auto *Int = llvm::ConstantInt::get(IntTy, IntValue);
53     return llvm::ConstantExpr::getIntToPtr(Int, PtrTy);
54   }
55   if (Ty->isFPOrFPVectorTy()) {
56     unsigned BitWidth = llvm::APFloat::semanticsSizeInBits(
57         (Ty->isVectorTy() ? Ty->getVectorElementType() : Ty)
58             ->getFltSemantics());
59     llvm::APInt Payload(64, NaNPayload);
60     if (BitWidth >= 64)
61       Payload = llvm::APInt::getSplat(BitWidth, Payload);
62     return llvm::ConstantFP::getQNaN(Ty, NegativeNaN, &Payload);
63   }
64   if (Ty->isArrayTy()) {
65     // Note: this doesn't touch tail padding (at the end of an object, before
66     // the next array object). It is instead handled by replaceUndef.
67     auto *ArrTy = cast<llvm::ArrayType>(Ty);
68     llvm::SmallVector<llvm::Constant *, 8> Element(
69         ArrTy->getNumElements(),
70         initializationPatternFor(CGM, ArrTy->getElementType()));
71     return llvm::ConstantArray::get(ArrTy, Element);
72   }
73 
74   // Note: this doesn't touch struct padding. It will initialize as much union
75   // padding as is required for the largest type in the union. Padding is
76   // instead handled by replaceUndef. Stores to structs with volatile members
77   // don't have a volatile qualifier when initialized according to C++. This is
78   // fine because stack-based volatiles don't really have volatile semantics
79   // anyways, and the initialization shouldn't be observable.
80   auto *StructTy = cast<llvm::StructType>(Ty);
81   llvm::SmallVector<llvm::Constant *, 8> Struct(StructTy->getNumElements());
82   for (unsigned El = 0; El != Struct.size(); ++El)
83     Struct[El] = initializationPatternFor(CGM, StructTy->getElementType(El));
84   return llvm::ConstantStruct::get(StructTy, Struct);
85 }
86