Intel ARCHITECTURE IA-32 User Manual

Page 177

General Optimization Guidelines

2-105

Assembly/Compiler Coding Rule 42. (M impact, H generality)

inc

and

dec

instructions should be replaced with an

add

sub

instruction,

because

add

and

sub

overwrite all flags, whereas inc and dec do not,

therefore creating false dependencies on earlier instructions that set the
flags. 2-73

Assembly/Compiler Coding Rule 43. (ML impact, L generality) Avoid

rotate

by register or

rotate

by immediate instructions. If possible,

replace with a

rotate

by 1 instruction. 2-74

Assembly/Compiler Coding Rule 44. (ML impact, L generality) Use
simple instructions that are less than eight bytes in length. 2-76

Assembly/Compiler Coding Rule 45. (M impact, MH generality)
Avoid using prefixes to change the size of immediate and displacement.
2-76

Assembly/Compiler Coding Rule 46. (M impact, MH generality)
Break dependences on portions of registers between instructions by
operating on 32-bit registers instead of partial registers. For moves, this
can be accomplished with 32-bit moves or by using

movzx

. 2-76

Assembly/Compiler Coding Rule 47. (M impact, M generality) Try to
use zero extension or operate on 32-bit operands instead of using moves
with sign extension. 2-77

Assembly/Compiler Coding Rule 48. (ML impact, M generality)
Avoid placing instructions that use 32-bit immediates which cannot be
encoded as a sign-extended 16-bit immediate near each other. Try to
schedule µops that have no immediate immediately before or after µops
with 32-bit immediates. 2-77

Assembly/Compiler Coding Rule 49. (M impact, ML generality) Use

xor

sub

, or

pxor

to set a register to

, or to break a false dependence

chain resulting from re-use of registers. In contexts where the condition
codes must be preserved, move

into the register instead. This requires

more code space than using

xor

and

sub

, but avoids setting the condition

codes. 2-78

Assembly/Compiler Coding Rule 50. (ML impact, M generality) Use
the

test

instruction instead of

and

when the result of the logical

and

not used. This saves uops in execution. Use a

test

if a register with itself