With the release of GCC 4.7.1 I wanted to give Link Time Optimization
a try. I'm using this version:
http://sourceforge.net/projects/mobilechessboar/files/avr-gcc%20snapshots%20(Win32)
Part is Xmega128A1.
Adding -flto and -flto-report:
CFLAGS += -flto
CFLAGS += -flto-report
LDFLAGS += -flto
LDFLAGS += -flto-report
Reduced the project code size by a mind boggling ~52K. Would be nice,
alas it is not, traced the problem to this:
void reset_hardware( void ) __attribute__ ((__noreturn__)); /* Force
a system reset. We only return from this function by a hardware reset
*/
/* Force a Hardware Reset: */
void reset_hardware( void )
{
CCPWrite( &RST.CTRL, RST_SWRST_bm );
__builtin_unreachable();
}
With LTO all the code after __builtin_unreachable(); is gone, breaking
the project.
Replacing __builtin_unreachable():
void reset_hardware( void )
{
CCPWrite( &RST.CTRL, RST_SWRST_bm );
for(;;)
;
}
then LTO reduces the code by a more realistic 775 bytes.
A question I have is why do I have to add either
__builtin_unreachable(); or the empty for(;;) to suppress the
'noreturn function does return' warning message, when the function is declare
__noreturn__? Seems GCC and I don't agree on the usage of 'noreturn'
as marking the function that it will indeed never return.
Also LTO is incompatible with:
#include <avr/signature.h> /* Place the chip's signature into the .elf
output file. Simply including this file does the work. Must only
appear in the project *once* */
%.elf: $(OBJ)
$(OBJDUMP) -s -j .signature $(OBJDIR)/$@
LTO has removed .signature, resulting in avr-objdump giving an error
about .signature not being found.
Is there a way to mark .signature as precious at the source level?
Adding -flto to all source files but one complicates the Makefile.
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