Re: [PATCH 0/2] tcg: Fix branch/label link during plugin expansion

["Alex Bennée" <alex.bennee@linaro.org>]

Richard Henderson <richard.henderson@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