Release unused address space (#1186)

Use munmap() to release unused part of mmap()ed address space in
arena::initialize_semispace() to reduce the risk of running out of
mmap()able address space in a multithreaded situation.

Author: stevenmeker

runtime/alloc/arena.cpp

@@ -41,15 +41,31 @@ void arena::initialize_semispace() {
     abort();
   }
   //
+  //	std::align() may modify addr and request.
+  //
+  auto *start_block = reinterpret_cast<char *>(addr);
+  auto *end_block = start_block + request;
+  //
   //	We allocated 2 * HYPERBLOCK_SIZE worth of address space but we're only going to use 1, aligned on a
   //	HYPERBLOCK_SIZE boundry. This is so we can get end of the hyperblock by setting the low bits of any
   //	address within the space to 1.
-  //	We don't worry about unused address space either side of our aligned address space because there will be no
-  //	memory mapped to it.
   //
   current_addr_ptr = reinterpret_cast<char *>(
       std::align(HYPERBLOCK_SIZE, HYPERBLOCK_SIZE, addr, request));
   //
+  //	Release any unused address space at the start of the mmap()ed block.
+  //
+  if (size_t front_slop = current_addr_ptr - start_block) {
+    munmap(start_block, front_slop);
+  }
+  //
+  //	Release any unused address space at the end of the mmap()ed block.
+  //
+  auto *end_aligned = current_addr_ptr + HYPERBLOCK_SIZE;
+  if (size_t back_slop = end_block - end_aligned) {
+    munmap(end_aligned, back_slop);
+  }
+  //
   //	We put a semispace id in the last byte of the hyperblock so we can identify which semispace an address
   //	belongs to by setting the low bits to 1 to access this id.
   //