[avr-libc-dev] [bug #35407] Missing multilib versions for tiny-stack targets

[Georg-Johann Lay]

URL:
  <http://savannah.nongnu.org/bugs/?35407>

                 Summary: Missing multilib versions for tiny-stack targets
                 Project: AVR C Runtime Library
            Submitted by: gjlayde
            Submitted on: Mo 30 Jan 2012 19:19:29 GMT
                Category: Feature Request
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Build system
                  Status: None
        Percent Complete: 0%
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 1.8.0
           Fixed Release: None

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Details:

avr-gcc-4.7 adds better support for targets with 8-bit stack pointer, see

http://gcc.gnu.org/PR51345

The outcome of the support is that there are new multilib variants needed for
targets with 8-bit SP:

avr2 is split into:

avr2             /* 16-bit SP targets */
avr2/tiny-stack  /* 8-bit SP targets */

avr25 is split into:

avr25             /* 16-bit SP targets */
avr25/tiny-stack  /* 8-bit SP targets */

avr-gcc will pick the right multilib version depending on -mmcu=MCU and
-mtiny-stack command line options as follows:

* If no 8-bit SP derivatives are available (e.g. for avr4) -mtiny-stack has no
effect on multilib selection.

* If the MCU has 8-bit SP, avr-gcc picks libraries from avr2*/tiny-stack

* If the MCU has 16-bit SP and there are 8-bit SP devices available for the
same architecture (i.e. avr2, avr25) then avr-gcc will pick libraries from
avr2* per default and from avr2*/tiny-stack if command line option
-mtiny-stack is on.

* If no library is found, the default from avr2 is used.

If avr-libc does not provide such libraries, the last point implies code size
increase for 8-bit SP devices in avr25 as default from avr2 (no MOVW, etc.) is
used as fallback.

For a proper implementation, avr-libc should provide multilib variants for

avr2/tiny-stack
avr25/tiny-stack

For a quick fix, avr2*/tiny-stack can be a copy of avr2*.

Notice that you can provide avr2/tiny-stack and avr25/tiny-stack with older
compiler versions, too. These libraries will be unused and increase the
installation size, but they won't hurt or disturb the compiler.






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