Re: [Qemu-devel] cpu_regs in target-i386

[Peter Maydell]

On 27 November 2011 23:07, Xin Tong <address@hidden> wrote:
> can you please show me where the (120) is used. we pushed a global
> TCGV to the gen_opparam_ptr. lets say the backend is i386 and the
> operation is an add.

So, you can find it like this:

(1) starting from tcg_global_mem_new_i64 we follow where that
offset went, and we see that tcg_global_mem_new_internal stores
it in the mem_offset field of a TCGTemp structure.
(2) So we grep for uses of mem_offset, and we find that tcg.c
uses it in the register allocator; basically there are a lot
of places where we say "if this is a TEMP_VAL_MEM then either
load or store the value". We do that load or store by calling
tcg_out_ld() or tcg_out_st(), passing the mem_offset value
(plus some other stuff like the base register and whatever
temporary register the allocator wants to use for it).
(3) grepping for tcg_out_ld() we find it's implemented in the
backends. Specifically, tcg/i386/tcg-target.c has the x86
version which writes out x86 code to do a load/store from
[reg+offset].

So we can deduce that TCG doesn't try to be particularly
clever about these values -- we just load them when we need
them and store them again when they are updated. (We don't
try to support issuing add instructions with memory operands
even if the host CPU architecture has them.)

I didn't particularly know this in advance -- I'm just looking
through the code and searching for uses of things and implementations
of functions. QEMU's code generation is not particularly
exotic:
 * guest-specific front end decodes instructions and writes
   intermediate representation
 * optimisation pass that works on IR
 * liveness analysis (ie work out when data values are no
   longer needed, so we can reuse the registers they're in)
 * turn IR into generated code by making calls to a handful
   of host-specific backend functions

(about the only slightly unexpected thing is that we do the
register allocation as we go along generating the final
code, rather than doing it as a separate pass.)
So if you can get a grasp of the general structure then
following through it to answer specific questions like this
should be straightforward.

-- PMM