|Subject:||[Qemu-devel] TCG flow vs dyngen|
|Date:||Fri, 10 Dec 2010 22:26:43 +0100|
|User-agent:||Mozilla/5.0 (X11; U; Linux i686; en-US; rv:188.8.131.52) Gecko/20101027 Thunderbird/3.1.6|
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!
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