Clearing registers, Compares, Clearing registers -87 compares -87 – Intel ARCHITECTURE IA-32 User Manual

Page 159

General Optimization Guidelines

2-87

Clearing Registers

Pentium 4 processor provides special support to

xor

sub

, or

pxor

operations when executed within the same register. This recognizes that
clearing a register does not depend on the old value of the register. The

xorps

and

xorpd

instructions do not have this special support. They

cannot be used to break dependence chains.

In Intel Core Solo and Intel Core Duo processors; the

xor

sub

xorps

pxor

instructions can be used to clear execution dependencies on the

zero evaluation of the destination register.

Assembly/Compiler Coding Rule 51. (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.

Compares

Use

test

when comparing a value in a register with zero.

Test

essentially

and

s the operands together without writing to a destination

Test

is preferred over

and

because

and

produces an extra result

Test

is better than

cmp ..., 0

because the instruction size is

smaller.

Use

test

when comparing the result of a logical

and

with an immediate

constant for equality or inequality if the register is

eax

for cases such

as:

if (avar & 8) { }

The

test

instruction can also be used to detect rollover of modulo a

power of 2. For example, the C code:

if ( (avar % 16) == 0 ) { }

can be implemented using:

test eax, 0x0F

jnz AfterIf