10. Pairing integer instructions (PPlain and PMMX)
The PPlain and PMMX have two pipelines for executing instructions, called the U-pipe and the V-pipe. Under certain conditions it is possible to execute two instructions simultaneously, one in the U-pipe and one in the V-pipe. This can almost double the speed. It is therefore advantageous to reorder your instructions to make them pair.
The following instructions are pairable in either pipe:
Two consecutive instructions will pair when the following conditions are met:
1. The first instruction is pairable in the U-pipe and the second instruction is pairable in the V-pipe.
2. The second instruction does not read or write a register which the first instruction writes to.
MOV EAX, EBX / MOV ECX, EAX ; read after write, do not pair MOV EAX, 1 / MOV EAX, 2 ; write after write, do not pair MOV EBX, EAX / MOV EAX, 2 ; write after read, pair OK MOV EBX, EAX / MOV ECX, EAX ; read after read, pair OK MOV EBX, EAX / INC EAX ; read and write after read, pair OK
3. In rule 2 partial registers are treated as full registers. Example:
MOV AL, BL / MOV AH, 0
writes to different parts of the same register, do not pair
4. Two instructions which both write to parts of the flags register can pair despite rule 2 and 3. Example:
SHR EAX, 4 / INC EBX ; pair OK
5. An instruction which writes to the flags can pair with a conditional jump despite rule 2. Example:
CMP EAX, 2 / JA LabelBigger ; pair OK
6. The following instruction combinations can pair despite the fact that they both modify the stack pointer:
PUSH + PUSH, PUSH + CALL, POP + POP
7. There are restrictions on the pairing of instructions with prefix. There are several types of prefixes:
On the PPlain, a prefixed instruction can only execute in the U-pipe, except for conditional near jumps.
On the PMMX, instructions with operand size, address size, or 0FH prefix can execute in either pipe, whereas instructions with segment, repeat, or lock prefix can only execute in the U-pipe.
8. An instruction which has both a displacement and immediate data is not pairable on the PPlain and only pairable in the U-pipe on the PMMX:
MOV DWORD PTR DS:, 0 ; not pairable or only in U-pipe CMP BYTE PTR [EBX+8], 1 ; not pairable or only in U-pipe CMP BYTE PTR [EBX], 1 ; pairable CMP BYTE PTR [EBX+8], AL ; pairable
(Another problem with instructions which have both a displacement and immediate data on the PMMX is that such instructions may be longer than 7 bytes, which means that only one instruction can be decoded per clock cycle, as explained in chapter 12.)
9. Both instructions must be preloaded and decoded. This is explained in chapter 8.
10. There are special pairing rules for MMX instructions on the PMMX: