Re: [PATCH 1/8] accel/tcg: cache the result of curr_cflags()

[BALATON Zoltan <balaton@eik.bme.hu>]

On Tue, 18 Aug 2026, Richard Henderson wrote:
> On 8/18/26 10:42, Matt Turner wrote:
>> curr_cflags() is called once per TB dispatch, from helper_lookup_tb_ptr()
>> and from the cpu_exec() loop. It recomputes the same value every time:
>>
>>      uint32_t cflags = cpu->tcg_cflags;
>>      if (unlikely(cpu_single_stepping(cpu))) { ... }
>>      else if (qatomic_read(&one_insn_per_tb)) { ... }
>>      else if (qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) { ... }
>> 
>> That is three loads and three branches on the hottest path in the
>> interpreter, for state that changes only when gdb enables single-step,
>> when one-insn-per-tb is toggled, or when the log mask changes.
>> 
>> Compute the value once into CPUState::tcg_curr_cflags and recompute it
>> from the four places that can change an input: tcg_cflags_set(),
>> cpu_single_step(), tcg_set_one_insn_per_tb() and qemu_set_log_internal().
>> curr_cflags() becomes a single load.
>
> Certainly one_insn_per_tb can only be set from the command-line, so that 
> should never have been handled along this path.

QEMU 11.1.50 monitor - type 'help' for more information
(qemu) help one-insn-per-tb
one-insn-per-tb [on|off] -- run emulation with one guest instruction per 
translation block

Regards,
BALATON Zoltan