Re: [Qemu-devel] [PATCH 05/11] tcg/i386: Make direct jump patching thread-safe

[Richard Henderson]

On 04/20/2016 02:55 AM, Alex Bennée wrote:
> > +static void tcg_out_nopn(TCGContext *s, int n)
> > +{
> > +    static const uint8_t nop1[] = { 0x90 };
> > +    static const uint8_t nop2[] = { 0x66, 0x90 };
> > +    static const uint8_t nop3[] = { 0x8d, 0x76, 0x00 };
> > +    static const uint8_t *const nopn[] = { nop1, nop2, nop3 };
> > +    int i;
> > +    assert(n <= ARRAY_SIZE(nopn));
> > +    for (i = 0; i < n; ++i) {
> > +        tcg_out8(s, nopn[n - 1][i]);
> > +    }
> > +}
>
> *shudder* I recall x86 instruction encoding is weird. Maybe a comment
>   for the function to describe the 3 forms of NOP we have here?

I think I'd prefer to drop the tables and do

  /* Emit 1 or 2 operand size prefixes for the standard one byte nop,
     xchg %eax,%eax, forming xchg %ax,%ax.  All cores accept the
     duplicate prefix, and all of the interesting recent cores can
     decode and discard the duplicates in a single cycle.  */
  for (i = 1; i < n; ++i) {
    tcg_out8(s, 0x66);
  }
  tcg_out8(s, 0x90);


r~