[libc] [llvm] [libc] Improve qsort (PR #120450)

[Nick Desaulniers via llvm-commits]

================
@@ -27,11 +31,51 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 
+template <bool USE_QUICKSORT, typename F>
+void unstable_sort_impl(void *array, size_t array_len, size_t elem_size,
+                        const F &is_less) {
+  if (array == nullptr || array_len == 0 || elem_size == 0)
+    return;
+
+  if constexpr (USE_QUICKSORT) {
+    switch (elem_size) {
+    case 4: {
+      auto arr_fixed_size = internal::ArrayFixedSize<4>(array, array_len);
+      quick_sort(arr_fixed_size, is_less);
+      return;
+    }
+    case 8: {
+      auto arr_fixed_size = internal::ArrayFixedSize<8>(array, array_len);
+      quick_sort(arr_fixed_size, is_less);
+      return;
+    }
+    case 16: {
+      auto arr_fixed_size = internal::ArrayFixedSize<16>(array, array_len);
+      quick_sort(arr_fixed_size, is_less);
+      return;
+    }
+    default:
+      auto arr_generic_size =
+          internal::ArrayGenericSize(array, array_len, elem_size);
+      quick_sort(arr_generic_size, is_less);
+      return;
+    }
+  } else {
+    auto arr_generic_size =
+        internal::ArrayGenericSize(array, array_len, elem_size);
+    heap_sort(arr_generic_size, is_less);
+  }
+}
+
+template <typename F>
+void unstable_sort(void *array, size_t array_len, size_t elem_size,
+                   const F &is_less) {
 #if LIBC_QSORT_IMPL == LIBC_QSORT_QUICK_SORT
-constexpr auto sort = quick_sort;
+  unstable_sort_impl<true, F>(array, array_len, elem_size, is_less);
 #elif LIBC_QSORT_IMPL == LIBC_QSORT_HEAP_SORT
-constexpr auto sort = heap_sort;
+  unstable_sort_impl<false, F>(array, array_len, elem_size, is_less);
 #endif
----------------
nickdesaulniers wrote:

I think this can be expressed more concisely.
```c++
// Should we use qsort or heap sort?
#define USE_QSORT (LIBC_QSORT_IMPL) == (LIBC_QSORT_QUICK_SORT)
unstable_sort_impl<USE_QSORT, F>(array, array_len, elem_size, is_less);
```

https://github.com/llvm/llvm-project/pull/120450