A recent WebKit bug comment from Gavin Barraclough gives some insights into the rationale for the layered architecture and MacroAssembler used in the current JavaScript engine in WebKit. Some excerpts from that comment:

The abstract code generation layer (MacroAssembler interface down) is layered like a traditional compiler. In a traditional compiler, it is common to have an assembler layer completely independent of the compiler (often a separate application). The compiler takes a source code file, compiles it, and produces an output file of assembly code.…

Layering the compiler on top of an assembler in this fashion provides a number of benefits. For the compiler developer, layering the compiler on the assembler separates the instruction selection from the minutiae of machine instruction encoding. For clients of the compiler providing a well defined language for machine instruction generation is useful if the compiler provides facilities to bypass the higher level language, and directly emit a specific sequence of machine instructions…

The assembler interface within the JIT is designed to closely mimic that of the assembler layer in a traditional compiler…

If that sounds interesting, you can find a lot more details by reading the full text of the comment.