What do 'instruction prefixes' mean in modern x86

18
votes

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.

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.

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.

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?

2
assemblyx86prefix
Very good question. I'd like to read the answers of Peter Cordes and the other experts here. – zx485
@zx485: like everyone is saying, usually you'd only see a large number of prefixes when making a long NOP. One NOP takes the same time to execute regardless of length, other than code-size side-effects and frontend issues. (As Agner Fog's guide explains). You definitely don't want 14 NOPs wasting space in the uop cache on a CPU that uses a uop-cache. Other than that, well, the x32 ABI often uses address-size prefixes (so base+index*scale addressing modes don't accidentally go outside the 32bit address range). so lock inc word [edi + r10d*4] would need 4: lock op-sz addr-sz REX. – Peter Cordes
IIRC, Atom and Silvermont have a similar decoder limitation, but the prefix and escape bytes that are part of the encoding of SIMD instructions count. So they can bottleneck horribly on SSSE3 and later instructions with REX prefixes. – Peter Cordes

2 Answers

14
votes

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.

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.

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.

Examples:

  • 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
7
votes

The "four prefixes" deal comes from the "prefix groups":

  1. lock/rep/repne
  2. segment override
  3. operand size override
  4. address size override

You can repeat prefixes, but you cannot (you can, but the behaviour is undefined) use several different prefixes from the same group. Though that only applies to groups 1 and 2, the other groups have only 1 thing in them each.

Something like 66 66 66 66 66 66 66 66 90 is valid (but potentially slow to decode). 2E 3E 00 00 (mixing segment overrides) is not.

Stacking prefixes can be useful for code alignment when the bytes have to be executed, unlike padding with nop it doesn't cost execution time. Using too many at once may cost decoding time.