[llvm] r337820 - ADT: Shrink SmallVector size 0 to 16B on 64-bit platforms

Tue Jul 24 04:32:14 PDT 2018

Author: dexonsmith
Date: Tue Jul 24 04:32:13 2018
New Revision: 337820

URL: http://llvm.org/viewvc/llvm-project?rev=337820&view=rev
Log:
ADT: Shrink SmallVector size 0 to 16B on 64-bit platforms

SmallVectorTemplateCommon wants to know the address of the first element
so it can detect whether it's in "small size" mode.

The old implementation split the small array, creating the storage for
the first element in SmallVectorTemplateCommon, and pulling the rest
into SmallVectorStorage where we know the size of the array.  This
bloats SmallVector size 0 by the larger of sizeof(void*) and sizeof(T),
and we're not even using the storage.

The new implementation leaves the full small storage to
SmallVectorStorage.  To calculate the offset of the first element in
SmallVectorTemplateCommon, we just need to know how far to jump, which
we can calculate out-of-band.  One subtlety is that we need
SmallVectorStorage to be properly aligned even when the size is 0, to be
sure that (for large alignments) we actually have the padding and it's
well defined to do the pointer math.

Modified:
    llvm/trunk/include/llvm/ADT/SmallVector.h
    llvm/trunk/lib/Support/SmallVector.cpp

Modified: llvm/trunk/include/llvm/ADT/SmallVector.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/SmallVector.h?rev=337820&r1=337819&r2=337820&view=diff
==============================================================================

--- llvm/trunk/include/llvm/ADT/SmallVector.h (original)
+++ llvm/trunk/include/llvm/ADT/SmallVector.h Tue Jul 24 04:32:13 2018
@@ -70,37 +70,42 @@ public:
   }
 };
 
+/// Figure out the offset of the first element.
+template <class T, typename = void> struct SmallVectorAlignmentAndSize {
+  AlignedCharArrayUnion<SmallVectorBase> Base;
+  AlignedCharArrayUnion<T> FirstEl;
+};
+
 /// This is the part of SmallVectorTemplateBase which does not depend on whether
 /// the type T is a POD. The extra dummy template argument is used by ArrayRef
 /// to avoid unnecessarily requiring T to be complete.
 template <typename T, typename = void>
 class SmallVectorTemplateCommon : public SmallVectorBase {
-private:
-  template <typename, unsigned> friend struct SmallVectorStorage;
-
-  // Allocate raw space for N elements of type T.  If T has a ctor or dtor, we
-  // don't want it to be automatically run, so we need to represent the space as
-  // something else.  Use an array of char of sufficient alignment.
-  using U = AlignedCharArrayUnion<T>;
-  U FirstEl;
+  /// Find the address of the first element.  For this pointer math to be valid
+  /// with small-size of 0 for T with lots of alignment, it's important that
+  /// SmallVectorStorage is properly-aligned even for small-size of 0.
+  void *getFirstEl() const {
+    return const_cast<void *>(reinterpret_cast<const void *>(
+        reinterpret_cast<const char *>(this) +
+        offsetof(SmallVectorAlignmentAndSize<T>, FirstEl)));
+  }
   // Space after 'FirstEl' is clobbered, do not add any instance vars after it.
 
 protected:
-  SmallVectorTemplateCommon(size_t Size) : SmallVectorBase(&FirstEl, Size) {}
+  SmallVectorTemplateCommon(size_t Size)
+      : SmallVectorBase(getFirstEl(), Size) {}
 
   void grow_pod(size_t MinCapacity, size_t TSize) {
-    SmallVectorBase::grow_pod(&FirstEl, MinCapacity, TSize);
+    SmallVectorBase::grow_pod(getFirstEl(), MinCapacity, TSize);
   }
 
   /// Return true if this is a smallvector which has not had dynamic
   /// memory allocated for it.
-  bool isSmall() const {
-    return BeginX == static_cast<const void*>(&FirstEl);
-  }
+  bool isSmall() const { return BeginX == getFirstEl(); }
 
   /// Put this vector in a state of being small.
   void resetToSmall() {
-    BeginX = &FirstEl;
+    BeginX = getFirstEl();
     Size = Capacity = 0; // FIXME: Setting Capacity to 0 is suspect.
   }
 
@@ -818,16 +823,17 @@ SmallVectorImpl<T> &SmallVectorImpl<T>::
   return *this;
 }
 
-/// Storage for the SmallVector elements which aren't contained in
-/// SmallVectorTemplateCommon. There are 'N-1' elements here. The remaining '1'
-/// element is in the base class. This is specialized for the N=1 and N=0 cases
+/// Storage for the SmallVector elements.  This is specialized for the N=0 case
 /// to avoid allocating unnecessary storage.
 template <typename T, unsigned N>
 struct SmallVectorStorage {
-  typename SmallVectorTemplateCommon<T>::U InlineElts[N - 1];
+  AlignedCharArrayUnion<T> InlineElts[N];
 };
-template <typename T> struct SmallVectorStorage<T, 1> {};
-template <typename T> struct SmallVectorStorage<T, 0> {};
+
+/// We need the storage to be properly aligned even for small-size of 0 so that
+/// the pointer math in \a SmallVectorTemplateCommon::getFirstEl() is
+/// well-defined.
+template <typename T> struct alignas(alignof(T)) SmallVectorStorage<T, 0> {};
 
 /// This is a 'vector' (really, a variable-sized array), optimized
 /// for the case when the array is small.  It contains some number of elements

Modified: llvm/trunk/lib/Support/SmallVector.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/SmallVector.cpp?rev=337820&r1=337819&r2=337820&view=diff
==============================================================================
--- llvm/trunk/lib/Support/SmallVector.cpp (original)
+++ llvm/trunk/lib/Support/SmallVector.cpp Tue Jul 24 04:32:13 2018
@@ -14,10 +14,29 @@
 #include "llvm/ADT/SmallVector.h"
 using namespace llvm;
 
-// Check that no bytes are wasted.
+// Check that no bytes are wasted and everything is well-aligned.
+namespace {
+struct Struct16B {
+  alignas(16) void *X;
+};
+struct Struct32B {
+  alignas(32) void *X;
+};
+}
+static_assert(sizeof(SmallVector<void *, 0>) ==
+                  sizeof(unsigned) * 2 + sizeof(void *),
+              "wasted space in SmallVector size 0");
+static_assert(alignof(SmallVector<Struct16B, 0>) >= alignof(Struct16B),
+              "wrong alignment for 16-byte aligned T");
+static_assert(alignof(SmallVector<Struct32B, 0>) >= alignof(Struct32B),
+              "wrong alignment for 32-byte aligned T");
+static_assert(sizeof(SmallVector<Struct16B, 0>) >= alignof(Struct16B),
+              "missing padding for 16-byte aligned T");
+static_assert(sizeof(SmallVector<Struct32B, 0>) >= alignof(Struct32B),
+              "missing padding for 32-byte aligned T");
 static_assert(sizeof(SmallVector<void *, 1>) ==
                   sizeof(unsigned) * 2 + sizeof(void *) * 2,
-              "wasted space in SmallVector size 1; missing EBO?");
+              "wasted space in SmallVector size 1");
 
 /// grow_pod - This is an implementation of the grow() method which only works
 /// on POD-like datatypes and is out of line to reduce code duplication.


