[PATCH] D62177: [ELF] Don't advance position in a memory region when assigning to the Dot

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

In D62177#1509729 <https://reviews.llvm.org/D62177#1509729>, @grimar wrote:

> I find the behavior of the GNU linkers wierd.
>
> If we use bfd and have:
>
>   .section .foo,"ax";
>   nop;
>  
>   .section .bar,"ax";
>   nop;
>
>
> and script:
>
>   MEMORY {
>     ram (ax) : ORIGIN = 0x42000, LENGTH = 0x100000
>   }
>   SECTIONS {
>     .foo : { *(.foo*) }
>     . += 0x2000;
>     .bar : { *(.bar*) }
>   }
>
>
> Then `.foo` section has address 0x42000 and `.bar` has 0x42001.
>  I.e. moving a dot does not change the address of the next section.
>
> But if I remove `MEMORY`:
>
>   SECTIONS {
>     .foo : { *(.foo*) }
>     . += 0x2000;
>     .bar : { *(.bar*) }
>   }
>
>
> Then `.foo` is `0x0` and `.bar` is `0x2001` (as I would expect to see)
>
> I think it is very strange that `MEMORY` command affects the Dot assignment behavior in that way.
>
> Seems that original script from PR had to use a hack to move the second output section:
>
>   aligned_dot = ALIGN(0x10 * 1024);  
>       
>   .data aligned_dot :
>   {
>     *(.data*)
>   }
>   


I agree... The code near `gold/script-sections.cc:2476` (in `Output_section_definition::set_section_addresses`) is responsible for this behavior

        vma_region = script_sections->find_memory_region(this, true, false, NULL);
        if (vma_region != NULL)  ////////// if it can find a matched memory region, dot_value will be ignored.
  	address = vma_region->get_current_address()->eval(symtab, layout,
  							  false);
        else
  	address = *dot_value;

Our code responsible for this behavior is:

  if (Ctx->MemRegion)
    Dot = Ctx->MemRegion->CurPos;


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D62177/new/

https://reviews.llvm.org/D62177