Arm AARCH64 string alignment in .rodata

Arm AARCH64 string alignment in .rodata

[Bob Plantz]

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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?

Re: Arm AARCH64 string alignment in .rodata

[Xi Ruoyao]

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