On 7/30/23 10:56, Helge Deller wrote:
I'm quite unclear about translator_use_goto_tb() for qemu-user
emulation....(and in general).
Based on the function name, the function translator_use_goto_tb() shall
help to decide if a program should use goto_tb() and exit_tb() to jump
to the next instruction.
Currently, if the destination is on the same page, it returns true.
I wonder, if it shouldn't return false in this case instead, because
arches have code like this: (taken from target/hppa/translate.c):
if (... && translator_use_goto_tb(ctx, f)) {
tcg_gen_goto_tb(which);
tcg_gen_movi_reg(cpu_iaoq_f, f);
tcg_gen_movi_reg(cpu_iaoq_b, b);
tcg_gen_exit_tb(ctx->base.tb, which);
} else {
copy_iaoq_entry(cpu_iaoq_f, f, cpu_iaoq_b);
copy_iaoq_entry(cpu_iaoq_b, b, ctx->iaoq_n_var);
tcg_gen_lookup_and_goto_ptr();
}
Shouldn't, if the destination is on the same page, the (faster?)
path with tcg_gen_lookup_and_goto_ptr() be taken instead?
No, because tcg_gen_lookup_and_goto_ptr is not the faster path.
That always involves a lookup, then an indirect branch.
The goto_tb path is linked, so only requires a lookup once, and the branch may be direct
(depending on the host architecture).
r~
