[llvm] f7bf28b - [SmallVector] Weaken the predicate for the memcpy optimization

[Benjamin Kramer via llvm-commits]

Author: Benjamin Kramer
Date: 2020-05-02T19:40:42+02:00
New Revision: f7bf28b2c02a4785d0d533fbbe4a7d8810b6dcf1

URL: https://github.com/llvm/llvm-project/commit/f7bf28b2c02a4785d0d533fbbe4a7d8810b6dcf1
DIFF: https://github.com/llvm/llvm-project/commit/f7bf28b2c02a4785d0d533fbbe4a7d8810b6dcf1.diff

LOG: [SmallVector] Weaken the predicate for the memcpy optimization

We don't require the type to be trivially assignable. While the standard
says that only is_trivially_copyable types may be memcpy'd, this seems
overly strict. We never assign the type, so there's no way for the type
to observe that the copy/move construction got elided. This is important
for std::pair<POD, POD>, which is not trivially assignable and probably
never will be because changing that would break ABI.

As a side-effect this no longer allows types with deleted copy/move
constructors in SmallVector. That's an unintended side-effect of
is_trivially_copyable anyways.

Shrinks Release+Asserts clang by 20k.

Added: 
    

Modified: 
    llvm/include/llvm/ADT/SmallVector.h

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
index 38aa1fa78a7e..6313fc563bea 100644
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -200,9 +200,17 @@ class SmallVectorTemplateCommon
   }
 };
 
-/// SmallVectorTemplateBase<TriviallyCopyable = false> - This is where we put method
-/// implementations that are designed to work with non-POD-like T's.
-template <typename T, bool = is_trivially_copyable<T>::value>
+/// SmallVectorTemplateBase<TriviallyCopyable = false> - This is where we put
+/// method implementations that are designed to work with non-trivial T's.
+///
+/// We approximate is_trivially_copyable with trivial move/copy construction and
+/// trivial destruction. While the standard doesn't specify that you're allowed
+/// copy these types with memcpy, there is no way for the type to observe this.
+/// This catches the important case of std::pair<POD, POD>, which is not
+/// trivially assignable.
+template <typename T, bool = (is_trivially_copy_constructible<T>::value) &&
+                             (is_trivially_move_constructible<T>::value) &&
+                             std::is_trivially_destructible<T>::value>
 class SmallVectorTemplateBase : public SmallVectorTemplateCommon<T> {
 protected:
   SmallVectorTemplateBase(size_t Size) : SmallVectorTemplateCommon<T>(Size) {}
@@ -290,7 +298,9 @@ void SmallVectorTemplateBase<T, TriviallyCopyable>::grow(size_t MinSize) {
 }
 
 /// SmallVectorTemplateBase<TriviallyCopyable = true> - This is where we put
-/// method implementations that are designed to work with POD-like T's.
+/// method implementations that are designed to work with trivially copyable
+/// T's. This allows using memcpy in place of copy/move construction and
+/// skipping destruction.
 template <typename T>
 class SmallVectorTemplateBase<T, true> : public SmallVectorTemplateCommon<T> {
 protected: