Re: [PULL 19/20] tcg/ppc: Optimize 26-bit jumps

[Richard Henderson]

It's also has a race condition.

Please see 

20221206041715.314209-18-richard.henderson@linaro.org/">https://lore.kernel.org/qemu-devel/20221206041715.314209-18-richard.henderson@linaro.org/

r~

On Thu, 15 Dec 2022, 13:33 Michael Tokarev, <mjt@tls.msk.ru> wrote:

04.10.2022 22:52, Richard Henderson wrote:
> From: Leandro Lupori <leandro.lupori@eldorado.org.br>
>
> PowerPC64 processors handle direct branches better than indirect
> ones, resulting in less stalled cycles and branch misses.
>
> However, PPC's tb_target_set_jmp_target() was only using direct
> branches for 16-bit jumps, while PowerPC64's unconditional branch
> instructions are able to handle displacements of up to 26 bits.
> To take advantage of this, now jumps whose displacements fit in
> between 17 and 26 bits are also converted to direct branches.
>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Leandro Lupori <leandro.lupori@eldorado.org.br>
> [rth: Expanded some commentary.]
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   tcg/ppc/tcg-target.c.inc | 119 +++++++++++++++++++++++++++++----------
>   1 file changed, 88 insertions(+), 31 deletions(-)
>
> diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
> index 1cbd047ab3..e3dba47697 100644
> --- a/tcg/ppc/tcg-target.c.inc
> +++ b/tcg/ppc/tcg-target.c.inc
...

> +    /*
> +     * There's no convenient way to get the compiler to allocate a pair
> +     * of registers at an even index, so copy into r6/r7 and clobber.
> +     */
> +    asm("mr  %%r6, %1\n\t"
> +        "mr  %%r7, %2\n\t"
> +        "stq %%r6, %0"
> +        : "=Q"(*(__int128 *)rw) : "r"(p[0]), "r"(p[1]) : "r6", "r7");

This is the only place in qemu where __int128 is used (other places name
it __int128_t), and is used *unconditionally*.  Is it right?

In particular, this breaks compilation on powerpc:

cc -m32 -Ilibqemu-aarch64-softmmu.fa.p... -c ../../tcg/tcg.c
In file included from ../../tcg/tcg.c:432:
/<<PKGBUILDDIR>>/tcg/ppc/tcg-target.c.inc: In function ‘ppc64_replace4’:
/<<PKGBUILDDIR>>/tcg/ppc/tcg-target.c.inc:1885:18: error: expected _expression_ before ‘__int128’
  1885 |         : "=Q"(*(__int128 *)rw) : "r"(p[0]), "r"(p[1]) : "r6", "r7");
       |                  ^~~~~~~~
/<<PKGBUILDDIR>>/tcg/ppc/tcg-target.c.inc:1885:29: error: expected ‘)’ before ‘rw’
  1885 |         : "=Q"(*(__int128 *)rw) : "r"(p[0]), "r"(p[1]) : "r6", "r7");
       |               ~             ^~

Thanks,

/mjt