[LLVMdev] llvm-qemu. (Was: Newbie)

Tilmann Scheller tilmann.scheller at googlemail.com
Sun Apr 6 09:46:59 PDT 2008

On Sun, Apr 6, 2008 at 6:32 PM, John Criswell <criswell at uiuc.edu> wrote:

> One of the things I noticed in the last message on llvm-qemu was that
> you were compiling with output from qemu with llvm-gcc?  Is this correct?

Not really, llvm-gcc is only invoked once when llvm-qemu is being built.

Qemu's code generator works like this: a source machine instruction is
decomposed into several simpler micro ops (basically the IR of qemu), for
every micro op there is a C implementation, which at build time is compiled
for the respective target architecture. At build time the actual code
generator is generated by the tool dyngen, which parses the compiled micro
op object file and generates a C function which receives a micro op stream
and generates the corresponding machine code by concatenating the respective
machine code for the micro ops and doing some additional patching (like
inserting parameters for the micro ops). What I did for llvm-qemu is to
write llvm-dyngen, which instead of an ELF file reads in a bitcode op.o
(generated by compiling op.c with llvm-gcc) and creates a function which
basically concatenates and patches the LLVM IR ops and after that JITs them.
Every source architecture has different micro ops, thus llvm-dyngen needs to
be used to create the code generator for a particular architecture.


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