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

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 9/13/24 03:23, Alex Bennée wrote:
> 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?
> 

The constant against which we compare scoreboard entry value is known at 
translation time.

>>
>>
>> r~
>