Thank you very much, Richard. This is precisely what I've been searching for!
--Bob
-----Original Message-----
From: Richard Earnshaw <Richard.Earnshaw@xxxxxxxxxxxx>
Sent: Wednesday, November 23, 2022 8:23 AM
To: Bob Plantz <rgplantz@xxxxxxxxxxx>; gcc-help@xxxxxxxxxxx
Subject: Re: :got: modifier in assembler
On 22/11/2022 19:12, Bob Plantz via Gcc-help wrote:
> I'm using the Arm A64 instruction set on a Raspberry Pi running Raspberry Pi 64-bit. Given the C code:
>
> extern int x, y, z;
>
> void addTwo(void)
> {
>
> z = x + y;
> }
>
> gcc produces the assembly language:
> .arch armv8-a
> .file "addTwoGlobal.c"
> .text
> .align 2
> .global addTwo
> .type addTwo, %function
> addTwo:
> adrp x0, :got:x
> ldr x0, [x0, #:got_lo12:x]
> ldr w1, [x0]
> adrp x0, :got:y
> ldr x0, [x0, #:got_lo12:y]
> ldr w0, [x0]
> add w1, w1, w0
> adrp x0, :got:z
> ldr x0, [x0, #:got_lo12:z]
> str w1, [x0]
> nop
> ret
> .size addTwo, .-addTwo
> .ident "GCC: (Debian 10.2.1-6) 10.2.1 20210110"
> .section .note.GNU-stack,"",@progbits
>
>
>
> I believe that the :got: and :got_lo12: modifiers tell the linker to use the address for x from the global offset table for the offset values in the
> adrp x0, :got:x
> ldr x0, [x0, #:got_lo12:x]
>
>
> Instructions.
>
> Using gdb, I can see that adrp loads the offset from the pc to the page number of x and that #:got_lo12:x is an immediate value that is the offset of x within that page.
>
> But I cannot find where :got: and :got_lo12: are documented.
>
>
>
>
>
The relocation directives should really be documented in GAS (the assembler), but the text there is pretty short and not very comprehensive [1]. The relocation names are essentially taken from the ELF specification for AArch64
(https://github.com/ARM-software/abi-aa/blob/main/aaelf64/aaelf64.rst)
and the relocation markers used by GAS largely follow the relocation names described in that document.
R.
[1]
https://sourceware.org/binutils/docs-2.39/as.html#AArch64_002dRelocations
