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 = cast<llvm::IntegerType>(
38                             Ty->isVectorTy() ? Ty->getVectorElementType() : Ty)
39                             ->getBitWidth();
40     if (BitWidth <= 64)
41       return llvm::ConstantInt::get(Ty, IntValue);
42     return llvm::ConstantInt::get(
43         Ty, llvm::APInt::getSplat(BitWidth, llvm::APInt(64, IntValue)));
44   }
45   if (Ty->isPtrOrPtrVectorTy()) {
46     auto *PtrTy = cast<llvm::PointerType>(
47         Ty->isVectorTy() ? Ty->getVectorElementType() : Ty);
48     unsigned PtrWidth = CGM.getContext().getTargetInfo().getPointerWidth(
49         PtrTy->getAddressSpace());
50     if (PtrWidth > 64)
51       llvm_unreachable("pattern initialization of unsupported pointer width");
52     llvm::Type *IntTy = llvm::IntegerType::get(CGM.getLLVMContext(), PtrWidth);
53     auto *Int = llvm::ConstantInt::get(IntTy, IntValue);
54     return llvm::ConstantExpr::getIntToPtr(Int, PtrTy);
55   }
56   if (Ty->isFPOrFPVectorTy()) {
57     unsigned BitWidth = llvm::APFloat::semanticsSizeInBits(
58         (Ty->isVectorTy() ? Ty->getVectorElementType() : Ty)
59             ->getFltSemantics());
60     llvm::APInt Payload(64, NaNPayload);
61     if (BitWidth >= 64)
62       Payload = llvm::APInt::getSplat(BitWidth, Payload);
63     return llvm::ConstantFP::getQNaN(Ty, NegativeNaN, &Payload);
64   }
65   if (Ty->isArrayTy()) {
66     // Note: this doesn't touch tail padding (at the end of an object, before
67     // the next array object). It is instead handled by replaceUndef.
68     auto *ArrTy = cast<llvm::ArrayType>(Ty);
69     llvm::SmallVector<llvm::Constant *, 8> Element(
70         ArrTy->getNumElements(),
71         initializationPatternFor(CGM, ArrTy->getElementType()));
72     return llvm::ConstantArray::get(ArrTy, Element);
73   }
74 
75   // Note: this doesn't touch struct padding. It will initialize as much union
76   // padding as is required for the largest type in the union. Padding is
77   // instead handled by replaceUndef. Stores to structs with volatile members
78   // don't have a volatile qualifier when initialized according to C++. This is
79   // fine because stack-based volatiles don't really have volatile semantics
80   // anyways, and the initialization shouldn't be observable.
81   auto *StructTy = cast<llvm::StructType>(Ty);
82   llvm::SmallVector<llvm::Constant *, 8> Struct(StructTy->getNumElements());
83   for (unsigned El = 0; El != Struct.size(); ++El)
84     Struct[El] = initializationPatternFor(CGM, StructTy->getElementType(El));
85   return llvm::ConstantStruct::get(StructTy, Struct);
86 }
87