Dylan McKay writes:
> Hello there,
>
> I am generating AVR executables with my own compiler, but linking them with
> AVR-GCC.
>
> Here is an example of a C file, which avr-as compiles fine. I made sure to
> first use my compiler to generate assembly from this file, and then
> assemble this using my compiler and gcc so that they are both using the
> same assembly source file.
>
> void foo();int bar = 2;int abc = 2;
> int jmp __attribute__((section(".vectors"))) = 0x940C;void (*ptr)()
> __attribute__((section(".vectors"))) = (&foo);
> void milan() { }void foo(){}void inter() { }
> int qwerty = 2;
>
> The .vectors section is generated fine. The correct jmp instruction is
> placed. Both compilers generate R_AVR_16 relocations for this.
>
> GCC however generates this (avr-objdump output):
>
> RELOCATION RECORDS FOR [.vectors]:
> OFFSET TYPE VALUE
> 00000004 R_AVR_16 .text+0x00000004
>
> Note that .text+0x00000004 does point to foo in the object file.
>
> Whereas my compiler generates this:
>
> RELOCATION RECORDS FOR [.vectors]:
> OFFSET TYPE VALUE
> 00000002 R_AVR_16 foo
>
> The problem is that GCC’s hardcoded section+offset works, but my symbol
> does not. avr-ld correctly relocates GCC code to foo in the executable, but
> relocates my code to _etext+0x2.
>
> Am I doing something wrong, or is not just not properly supported?
GAS has an adjust_reloc_syms method that does the symbol to
section+offset modification.
https://www.sourceware.org/ml/binutils/2002-05/msg00065.html
hints at why that's done.
As for your problem, I suspect you need R_AVR_16_PM (not R_AVR_16) - the
reloc with the PM suffix shifts the symbol value 1 bit to the right to
convert it from a byte address to a word address - which is what the JMP
instruction's encoding expects. GAS generates this reloc if it sees the
gs or pm modifier for an operand i.e.
.word gs(foo)
To reproduce your issue, I stripped off the gs modifier to make gas
generate a R_AVR_16 reloc. I also made it generate a reloc against foo
by declaring foo weak. But that still works:
$ ~/avr/install/bin/avr-as -mmcu=avr5 weak.s -o weak.o
$ ~/avr/install/bin/avr-objdump -r weak.o
weak.o: file format elf32-avr
RELOCATION RECORDS FOR [.vectors]:
OFFSET TYPE VALUE
00000002 R_AVR_16 foo
$ ~/avr/install/bin/avr-ld -mavr5 weak.o
$ ~/avr/install/bin/avr-objdump -S a.out
a.out: file format elf32-avr
Disassembly of section .text:
00000000 <jmp>:
0: 0c 94 ..
00000002 <ptr>:
2: 14 00 ..
00000004 <milan>:
4: cf 93 push r28
6: df 93 push r29
8: cd b7 in r28, 0x3d ; 61
a: de b7 in r29, 0x3e ; 62
c: 00 00 nop
e: df 91 pop r29
10: cf 91 pop r28
12: 08 95 ret
00000014 <foo>:
14: cf 93 push r28
...
I get the same result if the reloc is against the section name. ie.
$ ~/avr/install/bin/avr-as -mmcu=avr5 weak.s -o weak.o
$ ~/avr/install/bin/avr-objdump -r weak.o
weak.o: file format elf32-avr
RELOCATION RECORDS FOR [.vectors]:
OFFSET TYPE VALUE
00000002 R_AVR_16 .text+0x00000010
$ ~/avr/install/bin/avr-ld -mavr5 weak.o
$ ~/avr/install/bin/avr-objdump -S a.out
a.out: file format elf32-avr
Disassembly of section .text:
00000000 <jmp>:
0: 0c 94 ..
00000002 <ptr>:
2: 14 00 ..
00000004 <milan>:
4: cf 93 push r28
6: df 93 push r29
8: cd b7 in r28, 0x3d ; 61
a: de b7 in r29, 0x3e ; 62
c: 00 00 nop
e: df 91 pop r29
10: cf 91 pop r28
12: 08 95 ret
00000014 <foo>:
14: cf 93 push r28
Can you send the assembly file that produced the incorrect behavior?
Also, can you tell us the binutils version you are using?
Regards
Senthil
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