Ensure pointer is 8-byte aligned in stack_alloc::alloc

stan/math/memory/stack_alloc.hpp

@@ -158,7 +158,9 @@ class stack_alloc {
    * Return a newly allocated block of memory of the appropriate
    * size managed by the stack allocator.
    *
-   * The allocated pointer will be 8-byte aligned.
+   * The allocated pointer will be 8-byte aligned. If the number
+   * of bytes requested is not a multiple of 8, the reserved space
+   * will be padded up to the next multiple of 8.
    *
    * This function may call C++'s <code>malloc()</code> function,
    * with any exceptions percolated through this function.
@@ -167,9 +169,11 @@ class stack_alloc {
    * @return A pointer to the allocated memory.
    */
   inline void* alloc(size_t len) {
+    size_t pad = len % 8 == 0 ? 0 : 8 - len % 8;
+
     // Typically, just return and increment the next location.
     char* result = next_loc_;
-    next_loc_ += len;
+    next_loc_ += len + pad;
     // Occasionally, we have to switch blocks.
     if (unlikely(next_loc_ >= cur_block_end_)) {
       result = move_to_next_block(len);

test/unit/math/memory/stack_alloc_test.cpp

@@ -91,6 +91,15 @@ TEST(stack_alloc, alloc) {
   allocator.recover_all();
 }
 
+TEST(stack_alloc, alloc_aligned) {
+  stan::math::stack_alloc allocator;
+  int* x = allocator.alloc_array<int>(3);
+
+  double* y = allocator.alloc_array<double>(4);
+  EXPECT_TRUE(stan::math::is_aligned(y, 8));
+  allocator.recover_all();
+}
+
 TEST(stack_alloc, in_stack) {
   stan::math::stack_alloc allocator;
 
@@ -113,7 +122,11 @@ TEST(stack_alloc, in_stack_second_block) {
   char* y = allocator.alloc_array<char>(1);
   EXPECT_TRUE(allocator.in_stack(x));
   EXPECT_TRUE(allocator.in_stack(y));
-  EXPECT_FALSE(allocator.in_stack(y + 1));
+  // effect of padding
+  EXPECT_TRUE(allocator.in_stack(y + 1));
+  EXPECT_TRUE(allocator.in_stack(y + 7));
+
+  EXPECT_FALSE(allocator.in_stack(y + 8));
 
   allocator.recover_all();
   EXPECT_FALSE(allocator.in_stack(x));