Pack RegisterAtOffset harder.

<https://webkit.org/b/152501>

Reviewed by Michael Saboff.

Pack the register index and the offset into a single pointer-sized word instead of two.
This reduces memory consumption by 620 kB on mobile theverge.com.

The packing doesn't succeed on MSVC for some reason, so I've left out the static
assertion about class size in those builds.

* jit/RegisterAtOffset.cpp:
* jit/RegisterAtOffset.h:


Canonical link: https://commits.webkit.org/171355@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@195365 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Source/JavaScriptCore/ChangeLog

@@ -1,3 +1,19 @@
+2016-01-20  Andreas Kling  <akling@apple.com>
+
+        Pack RegisterAtOffset harder.
+        <https://webkit.org/b/152501>
+
+        Reviewed by Michael Saboff.
+
+        Pack the register index and the offset into a single pointer-sized word instead of two.
+        This reduces memory consumption by 620 kB on mobile theverge.com.
+
+        The packing doesn't succeed on MSVC for some reason, so I've left out the static
+        assertion about class size in those builds.
+
+        * jit/RegisterAtOffset.cpp:
+        * jit/RegisterAtOffset.h:
+
 2016-01-20  Per Arne Vollan  <peavo@outlook.com>
 
         [B3][Win64] Compile fix.

Source/JavaScriptCore/jit/RegisterAtOffset.cpp

@@ -30,6 +30,10 @@
 
 namespace JSC {
 
+#if !COMPILER(MSVC)
+static_assert(sizeof(RegisterAtOffset) == sizeof(ptrdiff_t), "RegisterAtOffset should be small.");
+#endif
+
 void RegisterAtOffset::dump(PrintStream& out) const
 {
     out.print(reg(), " at ", offset());

Source/JavaScriptCore/jit/RegisterAtOffset.h

@@ -70,7 +70,7 @@ class RegisterAtOffset {
 
 private:
     Reg m_reg;
-    ptrdiff_t m_offset;
+    ptrdiff_t m_offset : sizeof(ptrdiff_t) * 8 - sizeof(Reg) * 8;
 };
 
 } // namespace JSC