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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Redundant repz prefixes in generated amd64 code
Date: Fri, 16 Mar 2007 14:28:35 +0000
References: <200703161415.21283.jseward@acm.org>
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To: qemu-devel@nongnu.org

On Friday 16 March 2007 14:15, Julian Seward wrote:
> I'm seeing redundant repz (0xF3) prefixes in generated code, typically
> just before jumps:
>
> <code_gen_buffer+415>:  repz mov $0xe07f,%eax
> <code_gen_buffer+421>:  mov    %eax,0x20(%rbp)
> <code_gen_buffer+424>:  lea    -25168302(%rip),%ebx  # 0xaf0420 <tbs+96>
> <code_gen_buffer+430>:  retq
> <code_gen_buffer+431>:  mov    -25168245(%rip),%eax  # 0xaf0460 <tbs+160>
> <code_gen_buffer+437>:  jmpq   *%rax
> <code_gen_buffer+439>:  repz mov $0xe092,%eax
> <code_gen_buffer+445>:  mov    %eax,0x20(%rbp)
> <code_gen_buffer+448>:  lea    -25168325(%rip),%ebx   # 0xaf0421 <tbs+97>
> <code_gen_buffer+454>:  retq
>
> I assume these are something to do with translation chaining/unchaining but
> have been unable to figure out where they come from.

0000000000008b50 <op_goto_tb1>:
    8b50:       8b 05 00 00 00 00       mov    0(%rip),%eax 
                        8b52: R_X86_64_PC32     __op_param1+0x3c
    8b56:       ff e0                   jmpq   *%rax
    8b58:       f3 c3                   repz retq

qemu only strips the final ret off.
The prefixed ret is to avoid prefetch stalls on amd cpus.

Paul