From: Bob Plantz <rgplantz@outlook.com>
To: "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
Subject: :got: modifier in assembler
Date: Tue, 22 Nov 2022 19:12:16 +0000 [thread overview]
Message-ID: <BYAPR04MB4661BF8FB9381F5AD81CDAE1D40D9@BYAPR04MB4661.namprd04.prod.outlook.com> (raw)
[-- Attachment #1: Type: text/plain, Size: 1340 bytes --]
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.
next reply other threads:[~2022-11-22 19:12 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-22 19:12 Bob Plantz [this message]
2022-11-23 16:22 ` Richard Earnshaw
2022-11-23 16:38 ` Bob Plantz
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