Re: [Qemu-devel] Profiling Qemu for speed?

[Jonas Maebe]

On 17 Apr 2005, at 10:21, John R. Hogerhuis wrote:

> One thought would be to have a peephole optimizer that looks back over
> the just translated basic block (or a state machine that matches such
> sequences as an on-line algorithm) and match against common, known
> primitive sequences, and replaces them with optimized versions.

Another thing I've thought about is checking what sequences of 
instructions often appear in x86 programs (such as e.g. "push %ebp; 
movl %esp, %ebp") and then creating C-functions which emulate such an 
antire block, so they can be optimized as a whole by gcc. That would 
give a similar performance gain on all supported targets, and not just 
on the one you created the peephole optimizer for (+ less work to 
debug).

The only possible downside is that you can't jump to a particular 
instruction in such a block (the same goes for several kinds of 
peephole optimizations though). I don't know yet how Qemu exactly keeps 
track of the translations it has already performed, whether it supports 
multiple existing translations of the same instruction and/or whether 
it can already automatically invalidate the old block in case it turns 
out it needs to be splitted and thus re-translated (I guess it should 
at least some of these things, since it theory an x86 could jump into 
the middle of an instruction in order to reinterpret the bytes as 
another instruction stream).


Jonas