Re: [Qemu-devel] [PATCH 05/10] tcg: add jr opcode

[Richard Henderson]

On 04/11/2017 06:17 PM, Emilio G. Cota wrote:
> This will be used by TCG targets to implement a fast path
> for indirect branches.
>
> I only have implemented and tested this on an i386 host, so
> make this opcode optional and mark it as not implemented by
> other TCG backends.

I don't think this is quite the right abstraction.  In particular, if we can 
always return a valid address from the helper, we can eliminate a conditional 
branch.

I think this should work as follows:

(1) tb_ret_addr gets moved into TCGContext so that it's available for other 
code to see.

(2) Have a generic helper

void *HELPER(lookup_tb_ptr)(CPUArchState *env, target_ulong addr)
{
    TranslationBlock *tb = tb_from_jmp_cache(env, addr);
    return tb ? tb->tc_ptr : tcg_ctx.tb_ret_addr;
}

(3) Emit TCG opcodes like

        call t0,lookup_tb_ptr,env,addr
        jmp_tb t0

(4) Emit code for jmp_tb like

        mov     %rax,%rdx       // save target into new register
        xor     %eax,%eax       // set return value a-la exit_tb
        jmp     *%edx           // branch to tb or epilogue.

(5) There needs to be a convenience function in tcg/tcg-op.c.  If the host does 
not support jmp_tb, we should just generate exit_tb like we do now.  There 
should be no ifdefs inside target/*.



r~