[llvm] [WIP][ADT] Avoid slow size queries on append (PR #136365)

[Kazu Hirata via llvm-commits]

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@@ -679,13 +687,37 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
   void swap(SmallVectorImpl &RHS);
 
   /// Add the specified range to the end of the SmallVector.
-  template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
+  template <typename ItTy,
+            typename = detail::EnableIfConvertibleToInputIterator<ItTy>>
   void append(ItTy in_start, ItTy in_end) {
     this->assertSafeToAddRange(in_start, in_end);
-    size_type NumInputs = std::distance(in_start, in_end);
-    this->reserve(this->size() + NumInputs);
-    this->uninitialized_copy(in_start, in_end, this->end());
-    this->set_size(this->size() + NumInputs);
+    if constexpr (detail::IsOfIteratorCategory<
+                      ItTy, std::random_access_iterator_tag>) {
+      // Only reserve the required extra size upfront when the size calculation
+      // is guaranteed to be O(1).
+      size_type NumInputs = std::distance(in_start, in_end);
+      this->reserve(this->size() + NumInputs);
+      this->uninitialized_copy(in_start, in_end, this->end());
+      this->set_size(this->size() + NumInputs);
+    } else {
+      // Otherwise, append using `in_end` as the sentinel and reserve more space
+      // as necessary.
+      for (ItTy It = in_start; It != in_end;) {
+        iterator Dest = this->end();
+        size_type InitialSize = this->size();
+        size_type ExtraCap = this->capacity() - InitialSize;
+        size_type NumCopied = 0;
+        for (; NumCopied != ExtraCap && It != in_end; ++It, ++NumCopied) {
+          ::new ((void *)(Dest + NumCopied)) T(*It);
+        }
+        size_type NewSize = InitialSize + NumCopied;
+        this->set_size(NewSize);
+
+        if (It != in_end) {
+          this->reserve(NewSize + 1);
+        }
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kazutakahirata wrote:

nit: Drop curly braces.

https://github.com/llvm/llvm-project/pull/136365