Re: [Qemu-devel] [patch] gcc4 host support

[Paul Brook <paul@codesourcery.com>]

On Thursday 19 May 2005 19:14, Thomas Steffen wrote:
> On 5/19/05, Paul Brook <paul@codesourcery.com> wrote:
> > I don't think GNU lightning really gains us much. We'd still have to do
> > the hard bits (optimization, register allocation, assigning stack slots,
> > etc). The actual native code generation (which is what lightning gives
> > you) is only a few hundred lines of code.
>
> I have the feeling that will apply to most alternatives. The problem
> is to turn the C code into machine code, right? That is a difficult
> problem, which is why a C compiler is a complex piece of software.
> Using a different tool is not going to turn it into a simple problem.

No. The problem is to turn machine code into (a different form of) machine 
code. A lot of the complexity in a compiler is involved with with turning the 
high-level language constructs into simple low-level machine operations.

With qemu we're just translating from one simple form to another, so I'd argue 
that all you really need is a clever way of papering over the differences 
between the host and the guest.

What we have now (dyngen) is basically just an assembler. It maps qemu micro 
ops directly into blocks host code. The only reason dyngen uses gcc is to 
avoid having to hand write host encodings for all the ops. 

Paul