[libc] Bound the worst-case stack usage in qsort().

libc/src/stdlib/qsort_data.h

@@ -89,9 +89,15 @@ class Array {
   size_t size() const { return array_size; }
 
   // Make an Array starting at index |i| and size |s|.
-  Array make_array(size_t i, size_t s) const {
+  LIBC_INLINE Array make_array(size_t i, size_t s) const {
     return Array(get(i), s, elem_size, compare);
   }
+
+  // Reset this Array to point at a different interval of the same items.
+  LIBC_INLINE void reset_bounds(uint8_t *a, size_t s) {
+    array = a;
+    array_size = s;
+  }
 };
 
 using SortingRoutine = void(const Array &);

libc/src/stdlib/quick_sort.h

@@ -19,7 +19,7 @@ namespace LIBC_NAMESPACE_DECL {
 namespace internal {
 
 // A simple quicksort implementation using the Hoare partition scheme.
-static size_t partition(const Array &array) {
+LIBC_INLINE size_t partition(const Array &array) {
   const size_t array_size = array.size();
   size_t pivot_index = array_size / 2;
   uint8_t *pivot = array.get(pivot_index);
@@ -59,17 +59,32 @@ static size_t partition(const Array &array) {
   }
 }
 
-LIBC_INLINE void quick_sort(const Array &array) {
-  const size_t array_size = array.size();
-  if (array_size <= 1)
-    return;
-  size_t split_index = partition(array);
-  if (array_size <= 2) {
-    // The partition operation sorts the two element array.
-    return;
+LIBC_INLINE void quick_sort(Array array) {
+  while (true) {
+    const size_t array_size = array.size();
+    if (array_size <= 1)
+      return;
+    size_t split_index = partition(array);
+    if (array_size == 2)
+      // The partition operation sorts the two element array.
+      return;
+
+    // Make Arrays describing the two sublists that still need sorting.
+    Array left = array.make_array(0, split_index);
+    Array right = array.make_array(split_index, array.size() - split_index);
+
+    // Recurse to sort the smaller of the two, and then loop round within this
+    // function to sort the larger. This way, recursive call depth is bounded
+    // by log2 of the total array size, because every recursive call is sorting
+    // a list at most half the length of the one in its caller.
+    if (left.size() < right.size()) {
+      quick_sort(left);
+      array.reset_bounds(right.get(0), right.size());
+    } else {
+      quick_sort(right);
+      array.reset_bounds(left.get(0), left.size());
+    }
   }
-  quick_sort(array.make_array(0, split_index));
-  quick_sort(array.make_array(split_index, array.size() - split_index));
 }
 
 } // namespace internal