[RFC, SCHEME_B 1/7] ld: sframe: emit function start addr as offset from FDE

[Jens Remus]

On 07.04.2025 02:25, Indu Bhagat wrote:
> Adopt the new semantics of sfde_func_start_address consistently.  For
> ld, this means even the in-memory contents of the FDE function start
> address (buffer passed to libsframe sframe_encoder_write () for writing
> out) are encoded in the new semantics.

> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c

> @@ -481,6 +481,13 @@ _bfd_elf_merge_section_sframe (bfd *abfd,
>   		address += sframe_read_value (abfd, contents,
>   					      pltn_r_offset, 4);
>   	      address += (sec->output_offset + r_offset);

It took me a while to understand why above does not get canceled out
by below and does not naturally fall into place if both were removed.
After all the SFrame FDE function start address field value should
now already be an offset to the function from the field itself.
IIUC address is an offset relative to the FDE in the "imaginary"
output .sframe section, as the linker assumes it to be, that is the
sections simply concatenated together, with alignment padding added
between concatenated input sections if needed.  Above makes address
an offset relative to the output .sframe section, which effectively
"removes" this "imaginary" linker view from the value.  Below then
makes it relative to the FDE of the planned output .sframe section,
as SFrame assumes it to be (prior to sorting of the FDEs), that is
one single header followed by all FDEs, followed by all FREs.

Is that correct?

> +	      /* SFrame FDE function start address is an offset from the
> +		 sfde_func_start_address field to the start PC.  The
> +		 calculation below is the distance of sfde_func_start_address
> +		 field from the start of the output SFrame section.  */
> +	      address -= (sframe_encoder_get_hdr_size (sfe_ctx)
> +			  + ((cur_fidx + num_enc_fidx) /* FDE index.  */
> +			     * sizeof (sframe_func_desc_entry)));

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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