Richard Henderson <richard.hender...@linaro.org> writes:
> On 9/10/24 14:23, Richard Henderson wrote:
>> With tcg_last_op(), we always get the last op of the stream.
>> With TCGContext.emit_before_op, the most recently emitted op
>> is no longer the last op.
>> Instead, pass the op being emitted back from the allocator so
>> that we can link it to the label without needing to look it up.
>
> Oh, I meant to point out from whence this comes.
> The plugin uses a conditional
size_t n_insns = qemu_plugin_tb_n_insns(tb);
qemu_plugin_u64 quantum_insn =
qemu_plugin_scoreboard_u64_in_struct(vcpus, vCPUTime, quantum_insn);
/* count (and eventually trap) once per tb */
qemu_plugin_register_vcpu_tb_exec_inline_per_vcpu(
tb, QEMU_PLUGIN_INLINE_ADD_U64, quantum_insn, n_insns);
> ld_i32 tmp18,env,$0xffffffffffffdb10
> mul_i32 tmp18,tmp18,$0x18
> ext_i32_i64 tmp17,tmp18
> add_i64 tmp17,tmp17,$0x575410edadc8
qemu_plugin_register_vcpu_tb_exec_cond_cb(
tb, every_quantum_insn,
QEMU_PLUGIN_CB_NO_REGS, QEMU_PLUGIN_COND_GE,
quantum_insn, max_insn_per_quantum, NULL);
?
> ld_i64 tmp21,tmp17,$0x0
> brcond_i64 tmp21,$0x0,ltu,$L1
> ld_i32 tmp18,env,$0xffffffffffffdb10
> call plugin(0x79a2abfde66a),$0x1,$0,tmp18,$0x0
> set_label $L1
>
> Note that the branch is X < 0 (unsigned), which is always false, and
> thus the branch is optimized away.
I'm obviously missing something reading this. How can TCG know the state
of the scoreboard variables and optimise away the branch?
>
>
> r~
--
Alex Bennée
Virtualisation Tech Lead @ Linaro
