* Arm AARCH64 string alignment in .rodata
@ 2022-09-24 18:24 Bob Plantz
2022-09-24 18:55 ` Xi Ruoyao
0 siblings, 1 reply; 2+ messages in thread
From: Bob Plantz @ 2022-09-24 18:24 UTC (permalink / raw)
To: gcc-help
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On 64-bit Raspberry Pi OS, gcc aligns strings in the .rodata section on 64-bit boundaries:
.text
.section .rodata
.align 3
.LC0:
.string "Enter a number: "
.align 3
.LC1:
.string "%i"
.align 3
.LC2:
.string "Result: %i\n"
.text
.align 2
.global main
.type main, %function
main:
I understand the 32-bit boundary for the .text (32-bit instructions), but the element in a string is a byte. Arm documentation says that array addresses should be aligned at the element size. Why align the first character at a 64-bit boundary? Does this have to do with cache alignment?
^ permalink raw reply [flat|nested] 2+ messages in thread* Re: Arm AARCH64 string alignment in .rodata
2022-09-24 18:24 Arm AARCH64 string alignment in .rodata Bob Plantz
@ 2022-09-24 18:55 ` Xi Ruoyao
0 siblings, 0 replies; 2+ messages in thread
From: Xi Ruoyao @ 2022-09-24 18:55 UTC (permalink / raw)
To: Bob Plantz, gcc-help
On Sat, 2022-09-24 at 18:24 +0000, Bob Plantz via Gcc-help wrote:
> On 64-bit Raspberry Pi OS, gcc aligns strings in the .rodata section
> on 64-bit boundaries:
>
> .text
> .section .rodata
> .align 3
> .LC0:
> .string "Enter a number: "
> .align 3
> .LC1:
> .string "%i"
> .align 3
> .LC2:
> .string "Result: %i\n"
> .text
> .align 2
> .global main
> .type main, %function
> main:
>
> I understand the 32-bit boundary for the .text (32-bit instructions),
> but the element in a string is a byte. Arm documentation says that
> array addresses should be aligned at the element size. Why align the
> first character at a 64-bit boundary? Does this have to do with cache
> alignment?
In gcc/config/aarch64/aarch64.h:110:
/* Align definitions of arrays, unions and structures so that
initializations and copies can be made more efficient. This is not
ABI-changing, so it only affects places where we can see the
definition. Increasing the alignment tends to introduce padding,
so don't do this when optimizing for size/conserving stack space. */
If you use -Os, ".align 2" won't show up.
--
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University
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