2017年6月19日 · The repz prefix (alternately spelled repe) means to increment rsi and rdi then repeat cmps as long as [rsi] and [rdi] compare equal. The rflags register will be set on each iteration; the final iteration where [rsi] ≠ [rdi] is what will …

2016年10月5日 · The issue of the repz ret has been covered here as well as in other sources [2, 3] quite satisfactorily. However, reading neither of these sources, I found answers to the following: What is the actual penalty in a quantitative comparison with ret or nop; ret? Especially in the latter case – is decoding one extra instruction (and an empty one ...

2017年9月29日 · 2) repeat and decrease ecx until ecx equal to 0 or ZF set in the CMPS,SCAS instructions and called repz or repe. The 0xF3 binary prefix is used as: 1) repeat and decrease ecx until ecx equal to 0 or ZF NOT set in the CMPS,SCAS instructions and called repnz or repne. Recently noticed that XACQUIRE/XRELEASE prefixes also have the same binary ...

repz ret: why all the hassle? has some extra details about the specific micro-architectural reasons why that gives K8 and Barcelona a hard time. Avoiding 1-byte ret as a possible branch target: AMD's optimization guide for K10 (Barcelona) recommends 3-byte ret 0 in those cases, which pops zero bytes from the stack as well as returning.

2010年1月19日 · The REPZ and REPNZ prefixes are synonymous forms of the REPE and REPNE prefixes, respectively.) Share.

2020年4月28日 · The repz is not needed for correct function at all. It's an optimization, because on some x86 microarchitectures, branching to a branch-instruction (like calling a ret, or jccing to another jcc) causes very poor performance (probably a pipeline flush or something like that, I forget the exact problem), and branching to an ignored prefix mitigates the problem.

Looking at the following assembly code: MOV ESI, DWORD PTR [EBP + C] MOV ECX, EDI MOV EAX, EAX SHR ECX, 2 LEA EDI, DWORD PTR[EBX + 18] REP MOVS DWORD PTR ES:[EDI], DWORD PTR [ESI] MOV ECX, EAX A...

Prefixes (other than lock) that don't apply to an instruction are ignored in practice by existing CPUs.. The documentation says using rep with instructions it doesn't apply to is "reserved and can cause unpredictable behaviour" because future CPUs might recognize it as part of …

2016年2月24日 · Let's talk about the prefix REPZ now. From vol. 1, §7.3.9.2: The following repeat prefixes can be used in conjunction with a count in the ECX register to cause a string instruction [hyphen removed] to repeat: REP — Repeat while the ECX register not zero. REPE/REPZ — Repeat while the ECX register not zero and the ZF flag is set.

2012年4月21日 · The REP prefix can be added to the INS, OUTS, MOVS, LODS, and STOS instructions, and the REPE, REPNE, REPZ, and REPNZ prefixes can be added to the CMPS and SCAS instructions. (The REPZ and REPNZ prefixes are synonymous forms of the REPE and REPNE prefixes, respectively.)