[PATCH] D117976: [SmallVector] Optimize move assignment operator for N==0 case

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: dexonsmith, lichray, Quuxplusone.
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Due to the SmallVector hierarchy, N==0 cannot be leveraged by functions defined
in base classes. This patch special cases N==0 for SmallVector to save code size
and be slightly more efficient.

In a Release build of x86 only clang, .text is -3.23KiB smaller. In lld .text is
6.42KiB smaller.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D117976

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


Index: llvm/include/llvm/ADT/SmallVector.h
===================================================================
--- llvm/include/llvm/ADT/SmallVector.h
+++ llvm/include/llvm/ADT/SmallVector.h
@@ -1226,7 +1226,25 @@
   }
 
   SmallVector &operator=(SmallVector &&RHS) {
-    SmallVectorImpl<T>::operator=(::std::move(RHS));
+    if (N == 0) {
+      // SmallVectorImpl<T>::operator= does not leverage N==0. Optimize the
+      // case.
+      if (this == &RHS)
+        return *this;
+      this->destroy_range(this->begin(), this->end());
+      if (RHS.empty()) {
+        this->Size = 0;
+      } else {
+        if (!this->isSmall())
+          free(this->begin());
+        this->BeginX = RHS.BeginX;
+        this->Size = RHS.Size;
+        this->Capacity = RHS.Capacity;
+        RHS.resetToSmall();
+      }
+    } else {
+      SmallVectorImpl<T>::operator=(::std::move(RHS));
+    }
     return *this;
   }
 


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