Re: [PATCH 3/8] accel/tcg: skip the can_do_io stores in user-only builds

On 8/18/26 10:42, Matt Turner wrote:
> Every translation block stores to cpu->neg.can_do_io twice: false before
> the first instruction, true before the last one. Nothing reads it in a
> user-only build. There is no memory-mapped I/O in linux-user, and every
> reader is in system_ss: cputlb.c, watchpoint.c, icount-common.c and
> tcg-accel-ops-icount.c.
> 
> Two stores per TB is not much on its own, but TBs are short. An emulated
> alpha gcc 16.2.0 compiling the SQLite 3.45.1 amalgamation (255k lines,
> -O2) executes 34.2 billion TBs at 6.04 guest instructions each, so this is
> 68 billion stores for nothing.
> 
> Measured on an x86-64 host, LTO build, on top of the preceding two
> patches:
> 
>      before: 1,469,729,281,442 instructions
>      after:  1,402,816,253,499 instructions   -4.55%
> 
>      before: 120.97s wall clock
>      after:  115.75s wall clock              -4.32%
> 
> The emulated compiler produces byte-identical output.
> 
> Signed-off-by: Matt Turner<[email protected]>
> ---
>   accel/tcg/translator.c | 7 +++++++
>   1 file changed, 7 insertions(+)

Good catch.

Reviewed-by: Richard Henderson <[email protected]>


r~