replace std::aligned_storage

There's no reason to use std::aligned_storage when it's simpler to use
an array and alignas().  This way you don't have to remember whether the
template arguments are size-then-align or align-then-size, you don't
have to remember to use the _t variant or typename ... ::type, and
there's no risk to forgetting the alignment parameter entirely.

It doesn't look like this was deprecated, but I still think this paper
makes good arguments for why we shouldn't use it:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1413r1.pdf

Bug: skia:10921
Change-Id: Ia64a2e43c4cba9b4d64138a7474e353a8eaf01a6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/333258
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

Author: Mike Klein

bench/GrMemoryPoolBench.cpp

@@ -15,13 +15,17 @@
 namespace {
 
 // sizeof is a multiple of GrMemoryPool::kAlignment for 4, 8, or 16 byte alignment
-using Aligned = std::aligned_storage<32, GrMemoryPool::kAlignment>::type;
+struct alignas(GrMemoryPool::kAlignment) Aligned {
+    char buf[32];
+};
 static_assert(sizeof(Aligned) == 32);
 static_assert(sizeof(Aligned) % GrMemoryPool::kAlignment == 0);
 
 // sizeof is not a multiple of GrMemoryPool::kAlignment (will not be a multiple of max_align_t
 // if it's 4, 8, or 16, as desired).
-using Unaligned = std::aligned_storage<30, 2>::type;
+struct alignas(2) Unaligned {
+    char buf[30];
+};
 static_assert(sizeof(Unaligned) == 30);
 static_assert(sizeof(Unaligned) % GrMemoryPool::kAlignment != 0);
 

src/core/SkDescriptor.h

@@ -100,7 +100,7 @@ class SkAutoDescriptor {
               + 32;   // slop for occasional small extras
 
     SkDescriptor*   fDesc{nullptr};
-    std::aligned_storage<kStorageSize, alignof(uint32_t)>::type fStorage;
+    alignas(uint32_t) char fStorage[kStorageSize];
 };
 
 #endif  //SkDescriptor_DEFINED

src/core/SkTLazy.h

@@ -111,8 +111,8 @@ template <typename T> class SkTLazy {
     T* getMaybeNull() const { return fPtr; }
 
 private:
-    typename std::aligned_storage<sizeof(T), alignof(T)>::type fStorage;
-    T*                                                         fPtr{nullptr}; // nullptr or fStorage
+    alignas(T) char fStorage[sizeof(T)];
+    T*              fPtr{nullptr}; // nullptr or fStorage
 };
 
 /**

src/shaders/SkImageShader.cpp

@@ -337,7 +337,7 @@ std::unique_ptr<GrFragmentProcessor> SkImageShader::asFragmentProcessor(
                                                alignof(GrTextureAdjuster      ),
                                                alignof(GrImageTextureMaker    ),
                                                alignof(GrBitmapTextureMaker   )});
-    std::aligned_storage_t<kSize, kAlign> storage;
+    alignas(kAlign) char storage[kSize];
     GrTextureProducer* producer = nullptr;
     SkScopeExit destroyProducer([&producer]{ if (producer) { producer->~GrTextureProducer(); } });
 

tests/ArenaAllocTest.cpp

@@ -67,6 +67,10 @@ struct WithDtor {
     ~WithDtor() { }
 };
 
+struct alignas(8) OddAlignment {
+    char buf[10];
+};
+
 DEF_TEST(ArenaAlloc, r) {
 
     {
@@ -90,7 +94,7 @@ DEF_TEST(ArenaAlloc, r) {
         REPORTER_ASSERT(r, fooArray[4].y == -3.0f);
         REPORTER_ASSERT(r, created == 11);
         REPORTER_ASSERT(r, destroyed == 0);
-        arena.make<typename std::aligned_storage<10,8>::type>();
+        arena.make<OddAlignment>();
     }
     REPORTER_ASSERT(r, created == 11);
     REPORTER_ASSERT(r, destroyed == 11);
@@ -116,7 +120,7 @@ DEF_TEST(ArenaAlloc, r) {
         REPORTER_ASSERT(r, fooArray[4].y == -3.0f);
         REPORTER_ASSERT(r, created == 11);
         REPORTER_ASSERT(r, destroyed == 0);
-        arena.make<typename std::aligned_storage<10,8>::type>();
+        arena.make<OddAlignment>();
     }
     REPORTER_ASSERT(r, created == 11);
     REPORTER_ASSERT(r, destroyed == 11);
@@ -143,7 +147,7 @@ DEF_TEST(ArenaAlloc, r) {
         REPORTER_ASSERT(r, fooArray[4].y == -3.0f);
         REPORTER_ASSERT(r, created == 11);
         REPORTER_ASSERT(r, destroyed == 0);
-        arena.make<typename std::aligned_storage<10,8>::type>();
+        arena.make<OddAlignment>();
     }
     REPORTER_ASSERT(r, created == 11);
     REPORTER_ASSERT(r, destroyed == 11);