8067469: G1 ignores AlwaysPreTouch

Factor out pretouch code of the various virtual space management classes and use them everywhere including in G1.

Reviewed-by: stefank, ehelin, dholmes

hotspot/src/share/vm/gc_implementation/g1/g1PageBasedVirtualSpace.cpp

@@ -131,6 +131,9 @@ MemRegion G1PageBasedVirtualSpace::commit(uintptr_t start, size_t size_in_pages)
   _committed.set_range(start, start + size_in_pages);
 
   MemRegion result((HeapWord*)page_start(start), byte_size_for_pages(size_in_pages) / HeapWordSize);
+  if (AlwaysPreTouch) {
+    os::pretouch_memory((char*)result.start(), (char*)result.end());
+  }
   return result;
 }
 

hotspot/src/share/vm/gc_implementation/shared/mutableSpace.cpp

@@ -63,9 +63,7 @@ void MutableSpace::numa_setup_pages(MemRegion mr, bool clear_space) {
 }
 
 void MutableSpace::pretouch_pages(MemRegion mr) {
-  for (volatile char *p = (char*)mr.start(); p < (char*)mr.end(); p += os::vm_page_size()) {
+  os::pretouch_memory((char*)mr.start(), (char*)mr.end());
-    char t = *p; *p = t;
-  }
 }
 
 void MutableSpace::initialize(MemRegion mr,

hotspot/src/share/vm/runtime/os.cpp

@@ -1588,6 +1588,11 @@ bool os::release_memory(char* addr, size_t bytes) {
   return res;
 }
 
+void os::pretouch_memory(char* start, char* end) {
+  for (volatile char *p = start; p < end; p += os::vm_page_size()) {
+    *p = 0;
+  }
+}
 
 char* os::map_memory(int fd, const char* file_name, size_t file_offset,
                            char *addr, size_t bytes, bool read_only,

hotspot/src/share/vm/runtime/os.hpp

@@ -311,6 +311,12 @@ class os: AllStatic {
   static bool   uncommit_memory(char* addr, size_t bytes);
   static bool   release_memory(char* addr, size_t bytes);
 
+  // Touch memory pages that cover the memory range from start to end (exclusive)
+  // to make the OS back the memory range with actual memory.
+  // Current implementation may not touch the last page if unaligned addresses
+  // are passed.
+  static void   pretouch_memory(char* start, char* end);
+
   enum ProtType { MEM_PROT_NONE, MEM_PROT_READ, MEM_PROT_RW, MEM_PROT_RWX };
   static bool   protect_memory(char* addr, size_t bytes, ProtType prot,
                                bool is_committed = true);

hotspot/src/share/vm/runtime/virtualspace.cpp

@@ -615,19 +615,7 @@ bool VirtualSpace::expand_by(size_t bytes, bool pre_touch) {
   }
 
   if (pre_touch || AlwaysPreTouch) {
-    int vm_ps = os::vm_page_size();
+    os::pretouch_memory(previous_high, unaligned_new_high);
-    for (char* curr = previous_high;
-         curr < unaligned_new_high;
-         curr += vm_ps) {
-      // Note the use of a write here; originally we tried just a read, but
-      // since the value read was unused, the optimizer removed the read.
-      // If we ever have a concurrent touchahead thread, we'll want to use
-      // a read, to avoid the potential of overwriting data (if a mutator
-      // thread beats the touchahead thread to a page).  There are various
-      // ways of making sure this read is not optimized away: for example,
-      // generating the code for a read procedure at runtime.
-      *curr = 0;
-    }
   }
 
   _high += bytes;