[llvm] r257284 - [ADT] Add an abstraction for embedding an integer within a pointer-like
Rafael Espíndola via llvm-commits
llvm-commits at lists.llvm.org
Wed Jan 13 12:12:36 PST 2016
This should also have an entry in the programmer's manual.
Thanks,
Rafael
On 10 January 2016 at 04:40, Chandler Carruth via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: chandlerc
> Date: Sun Jan 10 03:40:13 2016
> New Revision: 257284
>
> URL: http://llvm.org/viewvc/llvm-project?rev=257284&view=rev
> Log:
> [ADT] Add an abstraction for embedding an integer within a pointer-like
> type.
>
> This makes it easy and safe to use a set of flags as one elmenet of
> a tagged union with pointers. There is quite a bit of code that has
> historically done this by casting arbitrary integers to "pointers" and
> assuming that this was safe and reliable. It is neither, and has started
> to rear its head by triggering safety asserts in various abstractions
> like PointerLikeTypeTraits when the integers chosen are invariably poor
> choices for *some* platform and *some* situation. Not to mention the
> (hopefully unlikely) prospect of one of these integers actually getting
> allocated!
>
> With this, it will be straightforward to build type safe abstractions
> like this without being error prone. The abstraction itself is also
> remarkably simple thanks to the implicit conversion.
>
> This use case and pattern was also independently created by the folks
> working on Swift, and they're going to incrementally add any missing
> functionality they find.
>
> Differential Revision: http://reviews.llvm.org/D15844
>
> Added:
> llvm/trunk/include/llvm/ADT/PointerEmbeddedInt.h
> llvm/trunk/unittests/ADT/PointerEmbeddedIntTest.cpp
> Modified:
> llvm/trunk/unittests/ADT/CMakeLists.txt
>
> Added: llvm/trunk/include/llvm/ADT/PointerEmbeddedInt.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/PointerEmbeddedInt.h?rev=257284&view=auto
> ==============================================================================
> --- llvm/trunk/include/llvm/ADT/PointerEmbeddedInt.h (added)
> +++ llvm/trunk/include/llvm/ADT/PointerEmbeddedInt.h Sun Jan 10 03:40:13 2016
> @@ -0,0 +1,103 @@
> +//===- llvm/ADT/PointerEmbeddedInt.h ----------------------------*- C++ -*-===//
> +//
> +// The LLVM Compiler Infrastructure
> +//
> +// This file is distributed under the University of Illinois Open Source
> +// License. See LICENSE.TXT for details.
> +//
> +//===----------------------------------------------------------------------===//
> +
> +#ifndef LLVM_ADT_POINTEREMBEDDEDINT_H
> +#define LLVM_ADT_POINTEREMBEDDEDINT_H
> +
> +#include "llvm/ADT/DenseMapInfo.h"
> +#include "llvm/Support/PointerLikeTypeTraits.h"
> +#include <climits>
> +
> +namespace llvm {
> +
> +/// Utility to embed an integer into a pointer-like type. This is specifically
> +/// intended to allow embedding integers where fewer bits are required than
> +/// exist in a pointer, and the integer can participate in abstractions along
> +/// side other pointer-like types. For example it can be placed into a \c
> +/// PointerSumType or \c PointerUnion.
> +///
> +/// Note that much like pointers, an integer value of zero has special utility
> +/// due to boolean conversions. For example, a non-null value can be tested for
> +/// in the above abstractions without testing the particular active member.
> +/// Also, the default constructed value zero initializes the integer.
> +template <typename IntT, int Bits = sizeof(IntT) * CHAR_BIT>
> +class PointerEmbeddedInt {
> + uintptr_t Value;
> +
> + static_assert(Bits < sizeof(Value) * CHAR_BIT,
> + "Cannot embed more bits than we have in a pointer!");
> +
> + enum : uintptr_t {
> + // We shift as many zeros into the value as we can while preserving the
> + // number of bits desired for the integer.
> + Shift = sizeof(Value) * CHAR_BIT - Bits,
> +
> + // We also want to be able to mask out the preserved bits for asserts.
> + Mask = static_cast<uintptr_t>(-1) << Bits
> + };
> +
> + friend class PointerLikeTypeTraits<PointerEmbeddedInt>;
> +
> + explicit PointerEmbeddedInt(uintptr_t Value) : Value(Value) {}
> +
> +public:
> + PointerEmbeddedInt() : Value(0) {}
> +
> + PointerEmbeddedInt(IntT I) : Value(static_cast<uintptr_t>(I) << Shift) {
> + assert((I & Mask) == 0 && "Integer has bits outside those preserved!");
> + }
> +
> + PointerEmbeddedInt &operator=(IntT I) {
> + assert((I & Mask) == 0 && "Integer has bits outside those preserved!");
> + Value = static_cast<uintptr_t>(I) << Shift;
> + }
> +
> + // Note that this imilict conversion additionally allows all of the basic
> + // comparison operators to work transparently, etc.
> + operator IntT() const { return static_cast<IntT>(Value >> Shift); }
> +};
> +
> +// Provide pointer like traits to support use with pointer unions and sum
> +// types.
> +template <typename IntT, int Bits>
> +class PointerLikeTypeTraits<PointerEmbeddedInt<IntT, Bits>> {
> + typedef PointerEmbeddedInt<IntT, Bits> T;
> +
> +public:
> + static inline void *getAsVoidPointer(const T &P) {
> + return reinterpret_cast<void *>(P.Value);
> + }
> + static inline T getFromVoidPointer(void *P) {
> + return T(reinterpret_cast<uintptr_t>(P));
> + }
> + static inline T getFromVoidPointer(const void *P) {
> + return T(reinterpret_cast<uintptr_t>(P));
> + }
> +
> + enum { NumLowBitsAvailable = T::Shift };
> +};
> +
> +// Teach DenseMap how to use PointerEmbeddedInt objects as keys if the Int type
> +// itself can be a key.
> +template <typename IntT, int Bits>
> +struct DenseMapInfo<PointerEmbeddedInt<IntT, Bits>> {
> + typedef PointerEmbeddedInt<IntT, Bits> T;
> +
> + typedef DenseMapInfo<IntT> IntInfo;
> +
> + static inline T getEmptyKey() { return IntInfo::getEmptyKey(); }
> + static inline T getTombstoneKey() { return IntInfo::getTombstoneKey(); }
> + static unsigned getHashValue(const T &Arg) {
> + return IntInfo::getHashValue(Arg);
> + }
> + static bool isEqual(const T &LHS, const T &RHS) { return LHS == RHS; }
> +};
> +}
> +
> +#endif
>
> Modified: llvm/trunk/unittests/ADT/CMakeLists.txt
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/ADT/CMakeLists.txt?rev=257284&r1=257283&r2=257284&view=diff
> ==============================================================================
> --- llvm/trunk/unittests/ADT/CMakeLists.txt (original)
> +++ llvm/trunk/unittests/ADT/CMakeLists.txt Sun Jan 10 03:40:13 2016
> @@ -25,6 +25,7 @@ set(ADTSources
> MapVectorTest.cpp
> OptionalTest.cpp
> PackedVectorTest.cpp
> + PointerEmbeddedIntTest.cpp
> PointerIntPairTest.cpp
> PointerSumTypeTest.cpp
> PointerUnionTest.cpp
>
> Added: llvm/trunk/unittests/ADT/PointerEmbeddedIntTest.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/ADT/PointerEmbeddedIntTest.cpp?rev=257284&view=auto
> ==============================================================================
> --- llvm/trunk/unittests/ADT/PointerEmbeddedIntTest.cpp (added)
> +++ llvm/trunk/unittests/ADT/PointerEmbeddedIntTest.cpp Sun Jan 10 03:40:13 2016
> @@ -0,0 +1,46 @@
> +//===- llvm/unittest/ADT/PointerEmbeddedIntTest.cpp -----------------------===//
> +//
> +// The LLVM Compiler Infrastructure
> +//
> +// This file is distributed under the University of Illinois Open Source
> +// License. See LICENSE.TXT for details.
> +//
> +//===----------------------------------------------------------------------===//
> +
> +#include "gtest/gtest.h"
> +#include "llvm/ADT/PointerEmbeddedInt.h"
> +using namespace llvm;
> +
> +namespace {
> +
> +TEST(PointerEmbeddedIntTest, Basic) {
> + PointerEmbeddedInt<int, CHAR_BIT> I = 42, J = 43;
> +
> + EXPECT_EQ(42, I);
> + EXPECT_EQ(43, I + 1);
> + EXPECT_EQ(sizeof(uintptr_t) * CHAR_BIT - CHAR_BIT,
> + PointerLikeTypeTraits<decltype(I)>::NumLowBitsAvailable);
> +
> + EXPECT_FALSE(I == J);
> + EXPECT_TRUE(I != J);
> + EXPECT_TRUE(I < J);
> + EXPECT_FALSE(I > J);
> + EXPECT_TRUE(I <= J);
> + EXPECT_FALSE(I >= J);
> +
> + EXPECT_FALSE(I == 43);
> + EXPECT_TRUE(I != 43);
> + EXPECT_TRUE(I < 43);
> + EXPECT_FALSE(I > 43);
> + EXPECT_TRUE(I <= 43);
> + EXPECT_FALSE(I >= 43);
> +
> + EXPECT_FALSE(42 == J);
> + EXPECT_TRUE(42 != J);
> + EXPECT_TRUE(42 < J);
> + EXPECT_FALSE(42 > J);
> + EXPECT_TRUE(42 <= J);
> + EXPECT_FALSE(42 >= J);
> +}
> +
> +} // end anonymous namespace
>
>
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