>Is the frame pointer guaranteed to be preserved from the time it is set in
>.init2 until it is used in main() [...]
The *stack pointer* is initialized in .init2 but the *frame pointer*
is not. Rather, the frame pointer is set up in the prologue for each
function that needs it (i.e. has local storage or takes the address of
one or more parameters). The "naked" attribute tells the compiler to
omit the function prologue and epilogue which, apparently, also causes
the frame pointer setup to be omitted as well. It would be useful to
add a caveat to the documentation about using local
variables in naked functions.
One way to work around the problem is to create a separate helper
function to do the work and then call it from the .init8 function.
You'll need to ensure that the helper function is *not* static to
avoid having the compiler inline the code. You may also have to
adjust the compiler options to prevent the compiler from inlining the
code. The output below (resulting from code derived from your
example) was obtained using the default call cost and inline size
option settings.
Don Kinzer
--- Example Code ---
void id_filter_init8( void ) __attribute__ ((naked))
__attribute__((section(".init8")));
void id_filter_init8( void )
{
98: 07 d0 rcall .+14 ; 0xa8 <bar>
9a: 03 d0 rcall .+6 ; 0xa2 <main>
9c: 2a c0 rjmp .+84 ; 0xf2 <_exit>
...
void bar(void)
{
a8: df 93 push r29
aa: cf 93 push r28
ac: cd b7 in r28, 0x3d ; 61
ae: de b7 in r29, 0x3e ; 62
b0: c0 58 subi r28, 0x80 ; 128
b2: d0 40 sbci r29, 0x00 ; 0
b4: 0f b6 in r0, 0x3f ; 63
b6: f8 94 cli
b8: de bf out 0x3e, r29 ; 62
ba: 0f be out 0x3f, r0 ; 63
bc: cd bf out 0x3d, r28 ; 61
be: fe 01 movw r30, r28
c0: 31 96 adiw r30, 0x01 ; 1
uint8_t ff_u8[128U];
/* memset() should really be used here */
for( uint16_t idx_u16 = 0U; idx_u16 < (uint16_t) sizeof( ff_u8 ); idx_u16++)
c2: ce 01 movw r24, r28
c4: 8f 57 subi r24, 0x7F ; 127
c6: 9f 4f sbci r25, 0xFF ; 255
{
ff_u8[ idx_u16 ] = 0U;
c8: 11 92 st Z+, r1
void bar(void)
{
uint8_t ff_u8[128U];
/* memset() should really be used here */
for( uint16_t idx_u16 = 0U; idx_u16 < (uint16_t) sizeof( ff_u8 ); idx_u16++)
ca: e8 17 cp r30, r24
cc: f9 07 cpc r31, r25
ce: e1 f7 brne .-8 ; 0xc8 <bar+0x20>
{
ff_u8[ idx_u16 ] = 0U;
}
foo(ff_u8[ val ]); // <-- this is to prevent the code above from
being optimized away
d0: 80 91 00 01 lds r24, 0x0100
d4: fe 01 movw r30, r28
d6: e8 0f add r30, r24
d8: f1 1d adc r31, r1
da: 81 81 ldd r24, Z+1 ; 0x01
dc: e1 df rcall .-62 ; 0xa0 <foo>
}
de: c0 58 subi r28, 0x80 ; 128
e0: df 4f sbci r29, 0xFF ; 255
e2: 0f b6 in r0, 0x3f ; 63
e4: f8 94 cli
e6: de bf out 0x3e, r29 ; 62
e8: 0f be out 0x3f, r0 ; 63
ea: cd bf out 0x3d, r28 ; 61
ec: cf 91 pop r28
ee: df 91 pop r29
f0: 08 95 ret
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