[Qemu-devel] TCG flow vs dyngen

[Stefano Bonifazi]

Hi all!
 From the technical documentation (http://www.usenix.org/publications/library/proceedings/usenix05/tech/freenix/bellard.html) I read:

The first step is to split each target CPU instruction into fewer simpler instructions called micro operations. Each micro operation is implemented by a small piece of C code. This small C source code is compiled by GCC to an object file. The micro operations are chosen so that their number is much smaller (typically a few hundreds) than all the combinations of instructions and operands of the target CPU. The translation from target CPU instructions to micro operations is done entirely with hand coded code.

A compile time tool called dyngen uses the object file containing the micro operations as input to generate a dynamic code generator. This dynamic code generator is invoked at runtime to generate a complete host function which concatenates several micro operations.

instead from wikipedia(http://en.wikipedia.org/wiki/QEMU) and other sources I read:

The Tiny Code Generator (TCG) aims to remove the shortcoming of relying on a particular version of GCC or any compiler, instead incorporating the compiler (code generator) into other tasks performed by QEMU in run-time. The whole translation task thus consists of two parts: blocks of target code (TBs) being rewritten in TCG ops - a kind of machine-independent intermediate notation, and subsequently this notation being compiled for the host's architecture by TCG. Optional optimisation passes are performed between them.

- So, I think that the technical documentation is now obsolete, isn't it?

- The "old way" used much offline (compile time) work compiling the micro operations into host machine code, while if I understand well, TCG does everything in run-time(please correct me if I am wrong!).. so I wonder, how can it be as fast as the previous method (or even faster)?

- If I understand well, TGC runtime flow is the following:
    - TCG takes the target binary, and splits it into target blocks
    - if the TB is not cached, TGC translates it (or better the target instructions it is composed by) into TCG micro ops,
    - TGC compiles TGC uops into host object code,
    - TGC caches the TB,
    - TGC tries to chain the block with others,
    - TGC copies the TB into the execution buffer
    - TGC runs it
Am I right? Please correct me, whether I am wrong, as I wanna use that flow scheme for trying to understand the code..
Thank you very much in advance!
Stefano B.