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re PR target/50820 ([avr] Use EIND consistently)

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PR target/50820
	Port from 4.6 branch r180379
	* doc/invoke.texi (AVR Options): New subsubsection to explain EIND
	handling and indirect jump/calls on devices > 128k.

From-SVN: r180388
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Georg-Johann Lay 2011-10-24 14:49:47 +00:00 committed by Georg-Johann Lay
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@ -1,3 +1,10 @@
2011-10-24 Georg-Johann Lay <avr@gjlay.de>
PR target/50820
Port from 4.6 branch r180379
* doc/invoke.texi (AVR Options): New subsubsection to explain EIND
handling and indirect jump/calls on devices > 128k.
2011-10-24 Anatoly Sokolov <aesok@post.ru>
Georg-Johann Lay <avr@gjlay.de>

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gcc/doc/invoke.texi
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@ -10723,6 +10723,117 @@ sbiw r26, const
@end example
@end table
@subsubsection @code{EIND} and Devices with more than 128k Bytes of Flash
Pointers in the implementation are 16 bits wide.
The address of a function or label is represented as word address so
that indirect jumps and calls can address any code address in the
range of 64k words.
In order to faciliate indirect jump on devices with more than 128k
bytes of program memory space, there is a special function register called
@code{EIND} that serves as most significant part of the target address
when @code{EICALL} or @code{EIJMP} instructions are used.
Indirect jumps and calls on these devices are handled as follows and
are subject to some limitations:
@itemize @bullet
@item
The compiler never sets @code{EIND}.
@item
The startup code from libgcc never sets @code{EIND}.
Notice that startup code is a blend of code from libgcc and avr-libc.
For the impact of avr-libc on @code{EIND}, see the
@w{@uref{http://nongnu.org/avr-libc/user-manual,avr-libc user manual}}.
@item
The compiler uses @code{EIND} implicitely in @code{EICALL}/@code{EIJMP}
instructions or might read @code{EIND} directly.
@item
The compiler assumes that @code{EIND} never changes during the startup
code or run of the application. In particular, @code{EIND} is not
saved/restored in function or interrupt service routine
prologue/epilogue.
@item
It is legitimate for user-specific startup code to set up @code{EIND}
early, for example by means of initialization code located in
section @code{.init3}, and thus prior to general startup code that
initializes RAM and calls constructors.
@item
For indirect calls to functions and computed goto, the linker will
generate @emph{stubs}. Stubs are jump pads sometimes also called
@emph{trampolines}. Thus, the indirect call/jump will jump to such a stub.
The stub contains a direct jump to the desired address.
@item
Stubs will be generated automatically by the linker if
the following two conditions are met:
@itemize @minus
@item The address of a label is taken by means of the @code{gs} modifier
(short for @emph{generate stubs}) like so:
@example
LDI r24, lo8(gs(@var{func}))
LDI r25, hi8(gs(@var{func}))
@end example
@item The final location of that label is in a code segment
@emph{outside} the segment where the stubs are located.
@end itemize
@item
The compiler will emit such @code{gs} modifiers for code labels in the
following situations:
@itemize @minus
@item Taking address of a function or code label.
@item Computed goto.
@item If prologue-save function is used, see @option{-mcall-prologues}
command line option.
@item Switch/case dispatch tables. If you do not want such dispatch
tables you can specify the @option{-fno-jump-tables} command line option.
@item C and C++ constructors/destructors called during startup/shutdown.
@item If the tools hit a @code{gs()} modifier explained above.
@end itemize
@item
The default linker script is arranged for code with @code{EIND = 0}.
If code is supposed to work for a setup with @code{EIND != 0}, a custom
linker script has to be used in order to place the sections whose
name start with @code{.trampolines} into the segment where @code{EIND}
points to.
@item
Jumping to non-symbolic addresses like so is @emph{not} supported:
@example
int main (void)
@{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
@}
@end example
Instead, a stub has to be set up:
@example
int main (void)
@{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
@}
@end example
and the application be linked with @code{-Wl,--defsym,func_4=0x4}.
Alternatively, @code{func_4} can be defined in the linker script.
@end itemize
@node Blackfin Options
@subsection Blackfin Options
@cindex Blackfin Options