Re: Stack pointer is 0 in a bare metal AArch64 program

[Joel Sherrill <joel@rtems.org>]

On Sun, May 10, 2020 at 11:04 PM Orlando Arias via Newlib <
newlib@sourceware.org> wrote:

> Greetings,
>
> On 5/10/20 10:31 PM, Simon Marchi via Newlib wrote:
> > $ ./sim/aarch64/run --trace=on --trace-disasm=on ./sim/aarch64/a.out
> > memory:   ERROR: executable is too big: ffffffffffffffff
> > insn:      pc = 400168 instr = 58000281
> > disasm:   ldr   x1, 0x00000000004001b8
> > memory:   read of 0 (8 bytes) from 4001b8
> > insn:      pc = 40016c instr = 927cec20
> > disasm:   and   x0, x1, #0xfffffffffffffff0
> > insn:      pc = 400170 instr = 9100001f
> > disasm:   mov   sp, x0
> > insn:      pc = 400174 instr = d280001d
> > disasm:   mov   x29, #0x0                       // #0
> > insn:      pc = 400178 instr = a9bf77fd
> > disasm:   stp   x29, x29, [sp, #-16]!
>
> Within libgloss for Aarch64, the stack is initialized using a weak
> symbol with the value of 0:
>
> .macro GEN_DWORD name
> #if defined(__ILP32__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
>     .word \name
>     .word 0
> #elif defined(__ILP32__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
>     .word 0
>     .word \name
> #else
>     .dword \name
> #endif
> .endm
>
> .Lstack:
>     GEN_DWORD __stack
>     .weak __stack
>
>
> If the linker script you are utilizing does not define __stack, then the
> weak symbol is used.
>
> As an aside, this behavior in libgloss has always annoyed me. When
> working with Cortex-M cores [both ARMv7-M and ARMv8-M], msp is
> initialized using the value stored in address 0 of the memory map by the
> hardware. Yes, the application is free to change it afterwards, but it
> just seems redundant to me.
>

Then why isn't a linker script provided which provides this? This behavior
is in sharp contrast with other CPU-elf targets with libgloss support. They
can produce executables out of the box with no requirements like this.

--joel
RTEMS



>
> Cheers,
> Orlando.
>
>