Add support to the alignment support header for conjuring a character

array of a suitable size and alignment for any of a number of different
types to be stored into the character array.

The mechanisms for producing an explicitly aligned type are fairly
complex because this operation is poorly supported on all compilers.
We've spent a fairly significant amount of time experimenting with
different implementations inside of Google, and the one using explicitly
expanded templates has been the most robust.

Credit goes to Nick Lewycky for writing the first 20 versions or so of
this logic we had inside of Google. I based this on the only one to
actually survive. In case anyone is worried, yes we are both explicitly
re-contributing and re-licensing it for LLVM. =]

Once the issues with actually specifying the alignment are finished, it
turns out that most compilers don't in turn align anything the way they
are instructed. Testing of this logic against both Clang and GCC
indicate that the alignment constraints are largely ignored by both
compilers! I've come up with and used a work-around by wrapping each
alignment-hinted type directly in a struct, and using that struct to
align the character array through a union. This elaborate hackery is
terrifying, but I've included testing that caught a terrifying number of
bugs in every other technique I've tried.

All of this in order to implement a poor C++98 programmers emulation of
C++11 unrestricted unions in classes such as SmallDenseMap.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158597 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/Support/AlignOf.h b/include/llvm/Support/AlignOf.h
index cebfa7982d6d0719c5a792a4e367d05eb33f6e56..b63e75fd662dd1ba40a7d76f98ed7c77fd345f9b 100644 (file)
--- a/include/llvm/Support/AlignOf.h
+++ b/include/llvm/Support/AlignOf.h
@@ -15,6 +15,9 @@
 #ifndef LLVM_SUPPORT_ALIGNOF_H
 #define LLVM_SUPPORT_ALIGNOF_H
 
+#include "llvm/Support/Compiler.h"
+#include <cstddef>
+
 namespace llvm {
 
 template <typename T>
@@ -56,5 +59,92 @@ struct AlignOf {
 template <typename T>
 static inline unsigned alignOf() { return AlignOf<T>::Alignment; }
 
+
+/// \brief Helper for building an aligned character array type.
+///
+/// This template is used to explicitly build up a collection of aligned
+/// character types. We have to build these up using a macro and explicit
+/// specialization to cope with old versions of MSVC and GCC where only an
+/// integer literal can be used to specify an alignment constraint. Once built
+/// up here, we can then begin to indirect between these using normal C++
+/// template parameters.
+template <size_t Alignment> struct AlignedCharArrayImpl {};
+template <> struct AlignedCharArrayImpl<0> {
+  typedef char type;
+};
+#if __cplusplus == 201103L || __has_feature(cxx_alignas)
+#define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
+  template <> struct AlignedCharArrayImpl<x> { \
+    typedef char alignas(x) type; \
+  }
+#elif defined(__clang__) || defined(__GNUC__)
+#define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
+  template <> struct AlignedCharArrayImpl<x> { \
+    typedef char type __attribute__((aligned(x))); \
+  }
+#elif defined(_MSC_VER)
+#define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
+  template <> struct AlignedCharArrayImpl<x> { \
+    typedef __declspec(align(x)) char type; \
+  }
+#else
+# error No supported align as directive.
+#endif
+
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(1);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(2);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(4);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(8);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(512);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(1024);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(2048);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(4096);
+LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(8192);
+// Any larger and MSVC complains.
+#undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT
+
+/// \brief This class template exposes a typedef for type containing a suitable
+/// aligned character array to hold elements of any of up to four types.
+///
+/// These types may be arrays, structs, or any other types. The goal is to
+/// produce a union type containing a character array which, when used, forms
+/// storage suitable to placement new any of these types over. Support for more
+/// than four types can be added at the cost of more boiler plate.
+template <typename T1,
+          typename T2 = char, typename T3 = char, typename T4 = char>
+class AlignedCharArray {
+  class AlignerImpl {
+    T1 t1; T2 t2; T3 t3; T4 t4;
+
+    AlignerImpl(); // Never defined or instantiated.
+  };
+  union SizerImpl {
+    char arr1[sizeof(T1)], arr2[sizeof(T2)], arr3[sizeof(T3)], arr4[sizeof(T4)];
+  };
+
+public:
+  // Sadly, Clang and GCC both fail to align a character array properly even
+  // with an explicit alignment attribute. To work around this, we union
+  // the character array that will actually be used with a struct that contains
+  // a single aligned character member. Tests seem to indicate that both Clang
+  // and GCC will properly register the alignment of a struct containing an
+  // aligned member, and this alignment should carry over to the character
+  // array in the union.
+  union union_type {
+    // This is the only member of the union which should be used by clients:
+    char buffer[sizeof(SizerImpl)];
+
+    // This member of the union only exists to force the alignment.
+    struct {
+      typename llvm::AlignedCharArrayImpl<AlignOf<AlignerImpl>::Alignment>::type
+        nonce_inner_member;
+    } nonce_member;
+  };
+};
+
 } // end namespace llvm
 #endif

diff --git a/unittests/CMakeLists.txt b/unittests/CMakeLists.txt
index 11a5a5ac9757e95159b00a005373b8289ae58e92..bfb05edade20c41716bbd25a3fe84d83ba9d3bae 100644 (file)
--- a/unittests/CMakeLists.txt
+++ b/unittests/CMakeLists.txt
@@ -166,6 +166,7 @@ set(LLVM_LINK_COMPONENTS
   )
 
 add_llvm_unittest(Support
+  Support/AlignOfTest.cpp
   Support/AllocatorTest.cpp
   Support/Casting.cpp
   Support/CommandLineTest.cpp

diff --git a/unittests/Support/AlignOfTest.cpp b/unittests/Support/AlignOfTest.cpp
new file mode 100644 (file)
index 0000000..b86f2bd
--- /dev/null
+++ b/unittests/Support/AlignOfTest.cpp
@@ -0,0 +1,248 @@
+//===- llvm/unittest/Support/AlignOfTest.cpp - Alignment utility tests ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/AlignOf.h"
+#include "llvm/Support/Compiler.h"
+
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+// Define some fixed alignment types to use in these tests.
+#if __cplusplus == 201103L || __has_feature(cxx_alignas)
+typedef char alignas(1) A1;
+typedef char alignas(2) A2;
+typedef char alignas(4) A4;
+typedef char alignas(8) A8;
+#elif defined(__clang__) || defined(__GNUC__)
+typedef char A1 __attribute__((aligned(1)));
+typedef char A2 __attribute__((aligned(2)));
+typedef char A4 __attribute__((aligned(4)));
+typedef char A8 __attribute__((aligned(8)));
+#elif defined(_MSC_VER)
+typedef __declspec(align(1)) char A1;
+typedef __declspec(align(2)) char A2;
+typedef __declspec(align(4)) char A4;
+typedef __declspec(align(8)) char A8;
+#else
+# error No supported align as directive.
+#endif
+
+// Wrap the forced aligned types in structs to hack around compiler bugs.
+struct SA1 { A1 a; };
+struct SA2 { A2 a; };
+struct SA4 { A4 a; };
+struct SA8 { A8 a; };
+
+struct S1 {};
+struct S2 { char a; };
+struct S3 { int x; };
+struct S4 { double y; };
+struct S5 { A1 a1; A2 a2; A4 a4; A8 a8; };
+struct S6 { double f(); };
+struct D1 : S1 {};
+struct D2 : S6 { float g(); };
+struct D3 : S2 {};
+struct D4 : S2 { int x; };
+struct D5 : S3 { char c; };
+struct D6 : S2, S3 {};
+struct D7 : S1, S3 {};
+struct D8 : S1, D4, D5 { double x[2]; };
+struct D9 : S1, D1 { S1 s1; };
+struct V1 { virtual ~V1(); };
+struct V2 { int x; virtual ~V2(); };
+struct V3 : V1 { virtual ~V3(); };
+struct V4 : virtual V2 { int y; virtual ~V4(); };
+struct V5 : V4, V3 { double z; virtual ~V5(); };
+struct V6 : S1 { virtual ~V6(); };
+struct V7 : virtual V2, virtual V6 { virtual ~V7(); };
+struct V8 : V5, virtual V6, V7 { double zz; virtual ~V8(); };
+
+// Ensure alignment is a compile-time constant.
+char LLVM_ATTRIBUTE_UNUSED test_arr
+  [AlignOf<char>::Alignment]
+  [AlignOf<short>::Alignment]
+  [AlignOf<int>::Alignment]
+  [AlignOf<long>::Alignment]
+  [AlignOf<long long>::Alignment]
+  [AlignOf<float>::Alignment]
+  [AlignOf<double>::Alignment]
+  [AlignOf<long double>::Alignment]
+  [AlignOf<void *>::Alignment]
+  [AlignOf<int *>::Alignment]
+  [AlignOf<double (*)(double)>::Alignment]
+  [AlignOf<double (S6::*)()>::Alignment]
+  [AlignOf<A1>::Alignment]
+  [AlignOf<A2>::Alignment]
+  [AlignOf<A4>::Alignment]
+  [AlignOf<A8>::Alignment]
+  [AlignOf<SA1>::Alignment]
+  [AlignOf<SA2>::Alignment]
+  [AlignOf<SA4>::Alignment]
+  [AlignOf<SA8>::Alignment]
+  [AlignOf<S1>::Alignment]
+  [AlignOf<S2>::Alignment]
+  [AlignOf<S3>::Alignment]
+  [AlignOf<S4>::Alignment]
+  [AlignOf<S5>::Alignment]
+  [AlignOf<S6>::Alignment]
+  [AlignOf<D1>::Alignment]
+  [AlignOf<D2>::Alignment]
+  [AlignOf<D3>::Alignment]
+  [AlignOf<D4>::Alignment]
+  [AlignOf<D5>::Alignment]
+  [AlignOf<D6>::Alignment]
+  [AlignOf<D7>::Alignment]
+  [AlignOf<D8>::Alignment]
+  [AlignOf<D9>::Alignment]
+  [AlignOf<V1>::Alignment]
+  [AlignOf<V2>::Alignment]
+  [AlignOf<V3>::Alignment]
+  [AlignOf<V4>::Alignment]
+  [AlignOf<V5>::Alignment]
+  [AlignOf<V6>::Alignment]
+  [AlignOf<V7>::Alignment]
+  [AlignOf<V8>::Alignment];
+
+TEST(AlignOfTest, BasicAlignmentInvariants) {
+  // For a very strange reason, many compilers do not support this. Both Clang
+  // and GCC fail to align these properly.
+  EXPECT_EQ(1u, alignOf<A1>());
+#if 0
+  EXPECT_EQ(2u, alignOf<A2>());
+  EXPECT_EQ(4u, alignOf<A4>());
+  EXPECT_EQ(8u, alignOf<A8>());
+#endif
+
+  // But once wrapped in structs, the alignment is correctly managed.
+  EXPECT_LE(1u, alignOf<SA1>());
+  EXPECT_LE(2u, alignOf<SA2>());
+  EXPECT_LE(4u, alignOf<SA4>());
+  EXPECT_LE(8u, alignOf<SA8>());
+
+  EXPECT_EQ(1u, alignOf<char>());
+  EXPECT_LE(alignOf<char>(),   alignOf<short>());
+  EXPECT_LE(alignOf<short>(),  alignOf<int>());
+  EXPECT_LE(alignOf<int>(),    alignOf<long>());
+  EXPECT_LE(alignOf<long>(),   alignOf<long long>());
+  EXPECT_LE(alignOf<char>(),   alignOf<float>());
+  EXPECT_LE(alignOf<float>(),  alignOf<double>());
+  EXPECT_LE(alignOf<double>(), alignOf<long double>());
+  EXPECT_LE(alignOf<char>(),   alignOf<void *>());
+  EXPECT_EQ(alignOf<void *>(), alignOf<int *>());
+  EXPECT_LE(alignOf<char>(),   alignOf<S1>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<S2>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<S3>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<S4>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<S5>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<S6>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D1>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D2>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D3>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D4>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D5>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D6>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D7>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D8>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<D9>());
+  EXPECT_LE(alignOf<S1>(),     alignOf<V1>());
+  EXPECT_LE(alignOf<V1>(),     alignOf<V2>());
+  EXPECT_LE(alignOf<V1>(),     alignOf<V3>());
+  EXPECT_LE(alignOf<V1>(),     alignOf<V4>());
+  EXPECT_LE(alignOf<V1>(),     alignOf<V5>());
+  EXPECT_LE(alignOf<V1>(),     alignOf<V6>());
+  EXPECT_LE(alignOf<V1>(),     alignOf<V7>());
+  EXPECT_LE(alignOf<V1>(),     alignOf<V8>());
+}
+
+TEST(AlignOfTest, BasicAlignedArray) {
+  // Note: this code exclusively uses the struct-wrapped arbitrarily aligned
+  // types because of the bugs mentioned above where GCC and Clang both
+  // disregard the arbitrary alignment specifier until the type is used to
+  // declare a member of a struct.
+  EXPECT_LE(1u, alignOf<AlignedCharArray<SA1>::union_type>());
+  EXPECT_LE(2u, alignOf<AlignedCharArray<SA2>::union_type>());
+  EXPECT_LE(4u, alignOf<AlignedCharArray<SA4>::union_type>());
+  EXPECT_LE(8u, alignOf<AlignedCharArray<SA8>::union_type>());
+
+  EXPECT_LE(1u, sizeof(AlignedCharArray<SA1>::union_type));
+  EXPECT_LE(2u, sizeof(AlignedCharArray<SA2>::union_type));
+  EXPECT_LE(4u, sizeof(AlignedCharArray<SA4>::union_type));
+  EXPECT_LE(8u, sizeof(AlignedCharArray<SA8>::union_type));
+
+  EXPECT_EQ(1u, (alignOf<AlignedCharArray<SA1>::union_type>()));
+  EXPECT_EQ(2u, (alignOf<AlignedCharArray<SA1, SA2>::union_type>()));
+  EXPECT_EQ(4u, (alignOf<AlignedCharArray<SA1, SA2, SA4>::union_type>()));
+  EXPECT_EQ(8u, (alignOf<AlignedCharArray<SA1, SA2, SA4, SA8>::union_type>()));
+
+  EXPECT_EQ(1u, sizeof(AlignedCharArray<SA1>::union_type));
+  EXPECT_EQ(2u, sizeof(AlignedCharArray<SA1, SA2>::union_type));
+  EXPECT_EQ(4u, sizeof(AlignedCharArray<SA1, SA2, SA4>::union_type));
+  EXPECT_EQ(8u, sizeof(AlignedCharArray<SA1, SA2, SA4, SA8>::union_type));
+
+  EXPECT_EQ(1u, (alignOf<AlignedCharArray<SA1[1]>::union_type>()));
+  EXPECT_EQ(2u, (alignOf<AlignedCharArray<SA1[2], SA2[1]>::union_type>()));
+  EXPECT_EQ(4u, (alignOf<AlignedCharArray<SA1[42], SA2[55],
+                                          SA4[13]>::union_type>()));
+  EXPECT_EQ(8u, (alignOf<AlignedCharArray<SA1[2], SA2[1],
+                                          SA4, SA8>::union_type>()));
+
+  EXPECT_EQ(1u,  sizeof(AlignedCharArray<SA1[1]>::union_type));
+  EXPECT_EQ(2u,  sizeof(AlignedCharArray<SA1[2], SA2[1]>::union_type));
+  EXPECT_EQ(4u,  sizeof(AlignedCharArray<SA1[3], SA2[2], SA4>::union_type));
+  EXPECT_EQ(16u, sizeof(AlignedCharArray<SA1, SA2[3],
+                                         SA4[3], SA8>::union_type));
+
+  // For other tests we simply assert that the alignment of the union mathes
+  // that of the fundamental type and hope that we have any weird type
+  // productions that would trigger bugs.
+  EXPECT_EQ(alignOf<char>(), alignOf<AlignedCharArray<char>::union_type>());
+  EXPECT_EQ(alignOf<short>(), alignOf<AlignedCharArray<short>::union_type>());
+  EXPECT_EQ(alignOf<int>(), alignOf<AlignedCharArray<int>::union_type>());
+  EXPECT_EQ(alignOf<long>(), alignOf<AlignedCharArray<long>::union_type>());
+  EXPECT_EQ(alignOf<long long>(),
+            alignOf<AlignedCharArray<long long>::union_type>());
+  EXPECT_EQ(alignOf<float>(), alignOf<AlignedCharArray<float>::union_type>());
+  EXPECT_EQ(alignOf<double>(), alignOf<AlignedCharArray<double>::union_type>());
+  EXPECT_EQ(alignOf<long double>(),
+            alignOf<AlignedCharArray<long double>::union_type>());
+  EXPECT_EQ(alignOf<void *>(), alignOf<AlignedCharArray<void *>::union_type>());
+  EXPECT_EQ(alignOf<int *>(), alignOf<AlignedCharArray<int *>::union_type>());
+  EXPECT_EQ(alignOf<double (*)(double)>(),
+            alignOf<AlignedCharArray<double (*)(double)>::union_type>());
+  EXPECT_EQ(alignOf<double (S6::*)()>(),
+            alignOf<AlignedCharArray<double (S6::*)()>::union_type>());
+  EXPECT_EQ(alignOf<S1>(), alignOf<AlignedCharArray<S1>::union_type>());
+  EXPECT_EQ(alignOf<S2>(), alignOf<AlignedCharArray<S2>::union_type>());
+  EXPECT_EQ(alignOf<S3>(), alignOf<AlignedCharArray<S3>::union_type>());
+  EXPECT_EQ(alignOf<S4>(), alignOf<AlignedCharArray<S4>::union_type>());
+  EXPECT_EQ(alignOf<S5>(), alignOf<AlignedCharArray<S5>::union_type>());
+  EXPECT_EQ(alignOf<S6>(), alignOf<AlignedCharArray<S6>::union_type>());
+  EXPECT_EQ(alignOf<D1>(), alignOf<AlignedCharArray<D1>::union_type>());
+  EXPECT_EQ(alignOf<D2>(), alignOf<AlignedCharArray<D2>::union_type>());
+  EXPECT_EQ(alignOf<D3>(), alignOf<AlignedCharArray<D3>::union_type>());
+  EXPECT_EQ(alignOf<D4>(), alignOf<AlignedCharArray<D4>::union_type>());
+  EXPECT_EQ(alignOf<D5>(), alignOf<AlignedCharArray<D5>::union_type>());
+  EXPECT_EQ(alignOf<D6>(), alignOf<AlignedCharArray<D6>::union_type>());
+  EXPECT_EQ(alignOf<D7>(), alignOf<AlignedCharArray<D7>::union_type>());
+  EXPECT_EQ(alignOf<D8>(), alignOf<AlignedCharArray<D8>::union_type>());
+  EXPECT_EQ(alignOf<D9>(), alignOf<AlignedCharArray<D9>::union_type>());
+  EXPECT_EQ(alignOf<V1>(), alignOf<AlignedCharArray<V1>::union_type>());
+  EXPECT_EQ(alignOf<V2>(), alignOf<AlignedCharArray<V2>::union_type>());
+  EXPECT_EQ(alignOf<V3>(), alignOf<AlignedCharArray<V3>::union_type>());
+  EXPECT_EQ(alignOf<V4>(), alignOf<AlignedCharArray<V4>::union_type>());
+  EXPECT_EQ(alignOf<V5>(), alignOf<AlignedCharArray<V5>::union_type>());
+  EXPECT_EQ(alignOf<V6>(), alignOf<AlignedCharArray<V6>::union_type>());
+  EXPECT_EQ(alignOf<V7>(), alignOf<AlignedCharArray<V7>::union_type>());
+  EXPECT_EQ(alignOf<V8>(), alignOf<AlignedCharArray<V8>::union_type>());
+}
+
+}