Chapter 42. Xtensa Dependent Features

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42.3.1. Using Density Instructions

The Xtensa instruction set has a code density option that provides 16 bit versions of some of the

most commonly used opcodes. Use of these opcodes can significantly reduce code size. When pos 

sible, the assembler automatically translates generic instructions from the core Xtensa instruction set

into equivalent instructions from the Xtensa code density option. This translation can be disabled by

using specific opcodes (Section 42.2.1 Opcode Names), by using the

 no density

command line

option (Section 42.1 Command Line Options), or by using the

no density

directive (Section 42.5.1

density).

It is a good idea not to use the density instructions directly. The assembler will automatically select

dense instructions where possible. If you later need to avoid using the code density option, you can

disable it in the assembler without having to modify the code.

42.3.2. Automatic Instruction Alignment

The Xtensa assembler will automatically align certain instructions, both to optimize performance and

to satisfy architectural requirements.

When the

 target align

command line option is enabled (Section 42.1 Command Line Options),

the assembler attempts to widen density instructions preceding a branch target so that the target in 

struction does not cross a 4 byte boundary. Similarly, the assembler also attempts to align each instruc 

tion following a call instruction. If there are not enough preceding safe density instructions to align a

target, no widening will be performed. This alignment has the potential to reduce branch penalties at

some expense in code size. The assembler will not attempt to align labels with the prefixes

.Ln

and

.LM

, since these labels are used for debugging information and are not typically branch targets.

The

LOOP

family of instructions must be aligned on either a 1 or 2 mod 4 byte boundary. The assembler

knows about this restriction and inserts the minimal number of 2 or 3 byte no op instructions to satisfy

it. When no op instructions are added, any label immediately preceding the original loop will be

moved in order to refer to the loop instruction, not the newly generated no op instruction.

Similarly, the

ENTRY

instruction must be aligned on a 0 mod 4 byte boundary. The assembler satisfies

this requirement by inserting zero bytes when required. In addition, labels immediately preceding the

ENTRY

instruction will be moved to the newly aligned instruction location.

42.4. Xtensa Relaxation

When an instruction operand is outside the range allowed for that particular instruction field,

as

can

transform the code to use a functionally equivalent instruction or sequence of instructions. This pro 

cess is known as relaxation. This is typically done for branch instructions because the distance of the

branch targets is not known until assembly time. The Xtensa assembler offers branch relaxation and

also extends this concept to function calls,

MOVI

instructions and other instructions with immediate

fields.

42.4.1. Conditional Branch Relaxation

When the target of a branch is too far away from the branch itself, i.e., when the offset from the branch

to the target is too large to fit in the immediate field of the branch instruction, it may be necessary to

replace the branch with a branch around a jump. For example,

beqz

a2, L

may result in: