“指令前缀"在现代x86中是什么意思 [英] What do 'instruction prefixes' mean in modern x86

问题描述

要了解为什么推土机不如我，我一直在看Agner Fog出色的微体系结构书，在第178页的推土机下有此段落.

To get an understanding on why Bulldozer was subpar I've been looking at Agner Fog's excellent microarchitecture book, in it on page 178 under bulldozer it has this paragraph.

具有三个前缀的指令可以在一个时钟周期内解码.如果指令超过三个，则处以非常大的罚款 前缀.带4-7前缀的指令需要额外的14-15个时钟周期 解码.带8-11前缀的指令需要20-22个时钟周期 额外，带有12-14前缀的指令需要27-28个时钟周期 额外的.因此，不建议延长NOP指令的时间 具有三个以上的前缀.此规则的前缀计数包括 操作数大小，地址大小，段，重复，锁定，REX和XOP 前缀.三字节的VEX前缀计为1，而两字节的 VEX前缀不计算在内.转义码(0F，0F38、0F3A)不计算在内.

Instructions with up to three prefixes can be decoded in one clock cycle. There is a very large penalty for instructions with more than three prefixes. Instructions with 4-7 prefixes take 14-15 clock cycles extra to decode. Instructions with 8-11 prefixes take 20-22 clock cycles extra, and instructions with 12-14 prefixes take 27 - 28 clock cycles extra. It is therefore not recommended to make NOP instructions longer with more than three prefixes. The prefix count for this rule includes operand size, address size, segment, repeat, lock, REX and XOP prefixes. A three-bytes VEX prefix counts as one, while a two-bytes VEX prefix does not count. Escape codes (0F, 0F38, 0F3A) do not count.

当我搜索前缀时，我遇到了非常技术性的定义，这远远超出了我的能力范围.或者，建议每条指令将其限制为4个，这与上述摘录有冲突.

When I searched for prefixes I was hit with very technical definitions far and away beyond my abilities. Or, suggested that they were limited to 4 per instruction which conflicts with the above extract.

因此，简单地说，有人可以解释他们是什么/做什么，以及为什么您可能希望将最多14个以上的内容添加到一条指令上而不是将其分解?

So in simple terms, can someone explain what they are/do and why you might want to tack on up to 14+ onto an instruction instead of breaking it up?

推荐答案

通常，您会根据需要使用尽可能多的内容，并使用预期的指令和操作数来确定.汇编器会自动发出某些前缀，而其他一些则可以手动使用.

Normally you use as many as needed, with the intended instruction and operands determining that. The assembler issues some of the prefixes automatically, while others you get to use manually.

他们提到的情况是针对多字节NOP，该字节传统上用于对齐填充，其思想是使用一条但适当长的指令来节省资源.显然，事实证明，使用更多前缀只是为了保持一条指令可能比使用两条前缀更少的指令要差.

The case they mention is for multi-byte NOP which is traditionally used for alignment padding where the idea is to use a single but appropriately long instruction to conserve resources. Apparently it turns out that using more prefixes just to keep it a single instruction may be worse performer than using two instructions with less prefixes.

此规则的前缀计数包括操作数大小，地址大小，段，重复，锁定，REX和XOP前缀.三字节的VEX前缀计为1，而两字节的VEX前缀不计为.

The prefix count for this rule includes operand size, address size, segment, repeat, lock, REX and XOP prefixes. A three-bytes VEX prefix counts as one, while a two-bytes VEX prefix does not count.

示例:

操作数大小:可以在32位和16位寄存器之间切换，例如mov ax, [foo]的编码与mov eax, [foo]相同，但前缀为
• 地址大小:可以在32/16或64/32位地址大小之间切换，例如mov [eax], foo的编码与mov [rax], foo相同，但前缀为67h(在64位模式下)
• 细分:可以覆盖所使用的细分，例如mov [fs:eax], foo的编码与mov [eax], foo相同，但前缀为64h.
• repeat:与字符串指令一起重复使用，例如rep cmpsb的编码与cmpsb相同，但前缀为f3h
• lock:与某些指令结合使用以使其原子化，例如lock add [foo], 1的编码与add [foo], 1相同，但前缀为f0h
• REX.W:用于切换到64位操作数大小，例如add rax, 1的编码与add eax, 1相同，但前缀为48h
• REX.R，B，X:用作modr/m字节的扩展名以访问额外的寄存器，例如add r8d, 1与add eax, 1相同，但前缀为41h
• XOP，VEX:与向量指令子集一起使用

• operand size: can switch between 32 and 16 bit registers, e.g. mov ax, [foo] is encoded the same as mov eax, [foo] but with the prefix 66h
• address size: can switch between 32/16 or 64/32 bit address sizes, e.g. mov [eax], foo is encoded the same as mov [rax], foo but with the prefix 67h (in 64 bit mode)
• segment: can override the segment used, e.g. mov [fs:eax], foo is encoded the same as mov [eax], foo but with the prefix 64h.
• repeat: used with string instructions for repeating, e.g. rep cmpsb is the encoded the same as cmpsb but with the prefix f3h
• lock: used with certain instructions to make them atomic, e.g. lock add [foo], 1 is encoded the same as add [foo], 1 but with the prefix f0h
• REX.W: used to switch to 64 bit operand size, e.g. add rax, 1 is encoded the same as add eax, 1 but with the prefix 48h
• REX.R,B,X: used as extensions of the modr/m byte to access extra registers, e.g. add r8d, 1 is the same as add eax, 1 but with the prefix 41h
• XOP, VEX: used with vector instruction subsets

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