[cfe-dev] AArch64 ASM / Custom Sections and Common Variables

[Oliver Stannard via cfe-dev]

What target (in particular OS, object file format) are you trying to
assemble for? Your example assembles without error for me with this command
line:
  clang --target=aarch64-none-eabi -c test.s -o test.o

That is generating ELF output, it's possible that other formats don't have
the necessary relocations to represent the first LDR instruction, or LLVM
doesn't know how to emit them.

It's also worth noting that a single LDR instruction only has a range
of +-1MiB, which might not be what you want. I often find the easiest way
to find reasonable assembly sequences is to compile a C file which does
what you want and look at the assembly output. For example, with this C
file:
  long FooValue = 0x0;

  __attribute((section(".text.foo1")))
  long foo1() {
    return FooValue;
  }

  __attribute((section(".text.foo2")))
  long foo2() {
    return FooValue;
  }

clang emits this assembly:
          .text
          .file   "test2.c"
          .section        .text.foo1,"ax", at progbits
          .globl  foo1                    // -- Begin function foo1
          .p2align        2
          .type   foo1, at function
  foo1:                                   // @foo1
  // %bb.0:                               // %entry
          adrp    x8, FooValue
          ldr     x0, [x8, :lo12:FooValue]
          ret
  .Lfunc_end0:
          .size   foo1, .Lfunc_end0-foo1
                                          // -- End function
          .section        .text.foo2,"ax", at progbits
          .globl  foo2                    // -- Begin function foo2
          .p2align        2
          .type   foo2, at function
  foo2:                                   // @foo2
  // %bb.0:                               // %entry
          adrp    x8, FooValue
          ldr     x0, [x8, :lo12:FooValue]
          ret
  .Lfunc_end1:
          .size   foo2, .Lfunc_end1-foo2
                                          // -- End function
          .type   FooValue, at object        // @FooValue
          .bss
          .globl  FooValue
          .p2align        3
  FooValue:
          .xword  0                       // 0x0
          .size   FooValue, 8

          .ident  "clang version 10.0.0 (git at github.com:llvm/llvm-project.git
36937ec7fb8e20b81adbac40bd48c6ed7ac6df61)"
          .section        ".note.GNU-stack","", at progbits
          .addrsig

Note the use of the ADRP/LDR pair of instructions, which has a range
of +-4GiB.

Oliver

On Sat, 9 Nov 2019 at 18:26, Joel Winarske via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> What is the proper way to have a variable common between two
> custom-defined sections?
>
> Example:
>
>   .global foo1
>   .section .text.foo1, "ax"
> foo1:
>   ldr w0, FooValue
>   ret
>
>   .global foo2
>   .section .text.foo2, "ax"
> foo2:
>   ldr w0, FooValue
>   ret
>
> FooValue: .word 0x00
>
> This generates "unsupported relocation type:
> fixup_aarch64_ldr_pcrel_imm19".  If I comment out the first ldr line, it
> builds, as FooValue is part of .text.foo2.
>
> If I define FooValue twice, once for each section it complains it's
> already defined.
>
>
> Joel
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20191111/ee9ebee0/attachment.html>