Take control over resizing to enforce required invariants #383
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DavisVaughan
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DavisVaughan
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Aug 20, 2024
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| template <> | ||
| inline typename r_vector<double>::underlying_type const* r_vector<double>::get_const_p( | ||
| bool is_altrep, SEXP data) { | ||
| return REAL_OR_NULL(data); | ||
| } |
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Using utilities like REAL_OR_NULL() gives ALTREP types that can cheaply return a const pointer to an underlying buffer a chance to do so (like ALTREP views)
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| inline r_vector<T>::r_vector(const r_vector& rhs) | ||
| : cpp11::r_vector<T>(safe[Rf_shallow_duplicate](rhs)), capacity_(rhs.capacity_) {} | ||
| inline r_vector<T>::r_vector(const r_vector& rhs) { | ||
| // We don't want to just pass through to the read-only constructor because we'd | ||
| // have to convert to `SEXP` first, which could truncate, and then we'd still have | ||
| // to shallow duplicate after that to ensure we have a duplicate, which can result in | ||
| // too many copies (#369). | ||
| // | ||
| // Instead we take control of setting all fields to try and only duplicate 1 time. | ||
| // We try and reclaim unused capacity during the duplication by only reserving up to | ||
| // the `rhs.length_`. This is nice because if the user returns this object, the | ||
| // truncation has already been done and they don't have to pay for another allocation. | ||
| // Importantly, `reserve_data()` always duplicates even if there wasn't extra capacity, | ||
| // which ensures we have our own copy. | ||
| data_ = reserve_data(rhs.data_, rhs.is_altrep_, rhs.length_); | ||
| protect_ = detail::store::insert(data_); | ||
| is_altrep_ = ALTREP(data_); | ||
| data_p_ = get_p(is_altrep_, data_); | ||
| length_ = rhs.length_; | ||
| capacity_ = rhs.length_; | ||
| } |
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This avoids the double copy from #369
Before
> profmem::profmem(test_())
Rprofmem memory profiling of:
test_()
Memory allocations:
what bytes calls
1 alloc 400048 test_() -> .Call()
2 alloc 800048 test_() -> .Call()
3 alloc 400056 test_() -> .Call()
4 alloc 400056 test_() -> .Call() # <- this one is extra
total 2000208
After
> profmem::profmem(test_())
Rprofmem memory profiling of:
test_()
Memory allocations:
what bytes calls
1 alloc 400048 test_() -> .Call()
2 alloc 800048 test_() -> .Call()
3 alloc 400056 test_() -> .Call()
total 1600152
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| // Compared to `Rf_xlengthgets()`: | ||
| // - This always allocates, even if it is the same size, which is important when we use | ||
| // it in a constructor and need to ensure that it duplicates on the way in. | ||
| // - This copies over attributes with `Rf_copyMostAttrib()`, which is important when we | ||
| // use it in constructors and when we truncate right before returning from the `SEXP` | ||
| // operator. | ||
| // - This is more friendly to ALTREP `x`. | ||
| template <typename T> | ||
| inline SEXP r_vector<T>::reserve_data(SEXP x, bool is_altrep, R_xlen_t size) { |
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Important invariants of reserve_data() vs Rf_xlengthgets()
Useful for both:
- Usage in constructors, when we need to enforce a duplication to occur
- Usage during truncation, when we need to copy attributes too
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Closes #369