Re: [Qemu-devel] TCG: AREG0 removal planning

[Paul Brook <paul@codesourcery.com>]

> TCG uses a fixed global register (AREG0) to which points to currently
> used CPUState, also known as 'env'. Using a fixed register has the
> downsides that the register must be reserved by TCG for generated code
> and by the compiler for compiling a few critical files (op_helper.c
> etc.). The latter also means that any calls to C library may be unsafe
> from those files.
> 
> Here are my sketches about transition to AREG0-less world.
> ...
> Translators/op helpers
> ...
> Comments? There are a few blank spots too.

I think a useful, and incremental goal is elimination of global cpu_env state 
in C code (i.e eliminate HELPER_CFLAGS and dyngen-exec.h).
We already have much of the infrastructure for this - op_helper v.s. helper.c 
and code_gen_prologue for transition in/out of "generated code" state.

In practice generated code probably accesses CPUState often enough that a 
dedicated register isn't a bad idea.  My guess is that eliminating it from C 
code gets us almost all of the useful benefit.  Removing it from the code 
generator (i.e. TCG_AREG0) may be more pain that it's worth.  For changes to 
the TCG side we want to consider how we can provide useful aliasing 
information, rather than a naive replacement of TCG_AREG0 with a variable.

Paul