AMD Typewriter x86 사용자 설명서

22007E/0—November 1999

by 26: 

use IMUL

by 27: 

LEA 

REG2, [REG1*4+REG1]     ;3 cycles

SHL

REG1, 5

SUB

REG1, REG2

by 28: 

MOV

REG2, REG1

;3 cycles

SHL 

REG1, 3

SUB 

REG1, REG2

SHL

REG1, 2

   
by 29: 

LEA 

REG2, [REG1*2+REG1] 

;3 cycles

SHL 

REG1, 5

SUB 

REG1, REG2

by 30: 

MOV

REG2, REG1

;3 cycles

SHL 

REG1, 4

SUB 

REG1, REG2

ADD

REG1, REG1

by 31: 

MOV 

REG2, REG1

;2 cycles

SHL 

REG1, 5

SUB 

REG1, REG2

by 32: 

SHL 

REG1, 5              

;1 cycle

In m any  prog ram s it  can  be  advantag eous to  use MMX
instructions to do integer-only work, especially if the function
already uses 3DNow!™ or MMX code. Using MMX instructions
relieves register pressure on the integer registers. As long as
data is simply loaded/stored, added, shifted, etc., MMX
instructions are good substitutes for integer instructions.
Integer registers are freed up with the following results:

■

May be able to reduce the number of integer registers to
saved/restored on function entry/edit.

■

Free up integer registers for pointers, loop counters, etc., so
that they do not have to be spilled to memory, which
reduces memory traffic and latency in dependency chains.

Be careful with regards to passing data between MMX and
integer registers and of creating mismatched store-to-load
forwarding cases. See “Unrolling Loops” on page 67.