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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 assembler 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
(see section longcalls).