242

Chapter 42. Xtensa Dependent Features

bnez.n

a2, M

j L

M:

(The

BNEZ.N

instruction would be used in this example only if the density option is available. Other 

wise,

BNEZ

would be used.)

42.4.2. Function Call Relaxation

Function calls may require relaxation because the Xtensa immediate call instructions (

CALL0

,

CALL4

,

CALL8

and

CALL12

) provide a PC relative offset of only 512 Kbytes in either direction. For larger

programs, it may be necessary to use indirect calls (

CALLX0

,

CALLX4

,

CALLX8

and

CALLX12

) where

the target address is specified in a register. The Xtensa assembler can automatically relax immediate

call instructions into indirect call instructions. This relaxation is done by loading the address of the

called function into the callee's return address register and then using a

CALLX

instruction. So, for

example:

call8 func

might be relaxed to:

.literal .L1, func

l32r

a8, .L1

callx8

a8

Because the addresses of targets of function calls are not generally known until link time, the assem 

bler must assume the worst and relax all the calls to functions in other source files, not just those that

really will be out of range. The linker can recognize calls that were unnecessarily relaxed, but it can

only partially remove the overhead introduced by the assembler.

Call relaxation has a negative effect on both code size and performance, so this relaxation is disabled

by default. If a program is too large and some of the calls are out of range, function call relaxation can

be enabled using the

 longcalls

command line option or the

longcalls

directive (Section 42.5.3

longcalls).

42.4.3. Other Immediate Field Relaxation

The

MOVI

machine instruction can only materialize values in the range from  2048 to 2047. Values

outside this range are best materialized with

L32R

instructions. Thus:

movi a0, 100000

is assembled into the following machine code:

.literal .L1, 100000

l32r a0, .L1