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From: Richard Henderson <rth@twiddle.net>
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Date: Wed, 20 Apr 2016 08:04:34 -0700
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Subject: Re: [Qemu-devel] [PATCH 05/11] tcg/i386: Make direct jump patching
 thread-safe
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To: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Sergey Fedorov <sergey.fedorov@linaro.org>
Cc: qemu-devel@nongnu.org, Sergey Fedorov <serge.fdrv@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

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~