Bug 1297804 - part 1 - eschew array bounds checking in nsTArray IPDL-generated code, reader-side; r=billm

For better or worse, the IPDL compiler, for protocols that send or
receive arrays, generates Read methods for transferring those arrays,
rather than going through the standard ParamTraits specializations.
These generated methods perform unnecessary bounds checks when accessing
elements; such checks can be safely bypassed because we know the exact
length of all the arrays involved.  (Some compilers are not smart enough
to eliminate the bounds checks on their own.)

ipc/ipdl/ipdl/lower.py

@@ -4331,6 +4331,16 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
         intype = _cxxConstRefType(arraytype, self.side)
         outtype = _cxxPtrToType(arraytype, self.side)
 
+        # We access elements directly in Read and Write to avoid array bounds
+        # checking.
+        directtype = _cxxBareType(arraytype.basetype, self.side)
+        if directtype.ptr:
+            typeinit = { 'ptrptr': 1 }
+        else:
+            typeinit = { 'ptr': 1 }
+        directtype = Type(directtype.name, **typeinit)
+        elemsvar = ExprVar('elems')
+
         write = MethodDefn(self.writeMethodDecl(intype, var))
         forwrite = StmtFor(init=ExprAssn(Decl(Type.UINT32, ivar.name),
                                          ExprLiteral.ZERO),
@@ -4354,10 +4364,13 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
                           cond=ExprBinary(ivar, '<', lenvar),
                           update=ExprPrefixUnop(ivar, '++'))
         forread.addstmt(
-            self.checkedRead(eltipdltype, ExprAddrOf(ExprIndex(favar, ivar)),
+            self.checkedRead(eltipdltype, ExprAddrOf(ExprIndex(elemsvar, ivar)),
                              msgvar, itervar, errfnRead,
                              '\'' + eltipdltype.name() + '[i]\'',
                              sentinelKey=arraytype.name()))
+        appendstmt = StmtDecl(Decl(directtype, elemsvar.name),
+                              init=ExprCall(ExprSelect(favar, '.', 'AppendElements'),
+                                            args=[ lenvar ]))
         read.addstmts([
             StmtDecl(Decl(_cxxArrayType(_cxxBareType(arraytype.basetype, self.side)), favar.name)),
             StmtDecl(Decl(Type.UINT32, lenvar.name)),
@@ -4366,7 +4379,7 @@ class _GenerateProtocolActorCode(ipdl.ast.Visitor):
                              [ arraytype.name() ],
                              sentinelKey=('length', arraytype.name())),
             Whitespace.NL,
-            StmtExpr(_callCxxArraySetLength(favar, lenvar)),
+            appendstmt,
             forread,
             StmtExpr(_callCxxSwapArrayElements(var, favar, '->')),
             StmtReturn.TRUE