Add comment better explaining the calling-stubs-from-stubs issue.

Review URL: http://codereview.chromium.org/7031046

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8059 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/code-stubs.h

@@ -34,8 +34,7 @@
 namespace v8 {
 namespace internal {
 
-// List of code stubs used on all platforms. The order in this list is important
-// as only the stubs up to and including Instanceof allows nested stub calls.
+// List of code stubs used on all platforms.
 #define CODE_STUB_LIST_ALL_PLATFORMS(V)  \
   V(CallFunction)                        \
   V(UnaryOp)                             \
@@ -43,12 +42,17 @@ namespace internal {
   V(StringAdd)                           \
   V(SubString)                           \
   V(StringCompare)                       \
-  V(SmiOp)                               \
   V(Compare)                             \
-  V(CompareIC)                           \
   V(MathPow)                             \
   V(TranscendentalCache)                 \
   V(Instanceof)                          \
+  /* All stubs above this line only exist in a few versions, which are  */  \
+  /* generated ahead of time.  Therefore compiling a call to one of     */  \
+  /* them can't cause a new stub to be compiled, so compiling a call to */  \
+  /* them is GC safe.  The ones below this line exist in many variants  */  \
+  /* so code compiling a call to one can cause a GC.  This means they   */  \
+  /* can't be called from other stubs, since stub generation code is    */  \
+  /* not GC safe.                                                       */  \
   V(ConvertToDouble)                     \
   V(WriteInt32ToHeapNumber)              \
   V(StackCheck)                          \
@@ -195,6 +199,7 @@ class CodeStub BASE_EMBEDDED {
            MajorKeyBits::encode(MajorKey());
   }
 
+  // See comment above, where Instanceof is defined.
   bool AllowsStubCalls() { return MajorKey() <= Instanceof; }
 
   class MajorKeyBits: public BitField<uint32_t, 0, kMajorBits> {};