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
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