Indu Bhagat 8ab6e4c72a [SFrame-V3] include: gas: libsframe: split FDE into idx and attr ...

This patch introduces a structural change to the SFrame V3 format. It
shifts the SFrame Function Descriptor Entry (FDE) (a physical entity in
SFrame V2) into a conceptual one in SFrame V3, such that an FDE is now
split into two distinct parts to optimize the binary search table and
data organization:
 - FDE Index (sframe_func_desc_idx_v3): This structure contains the
   essential indexing information: the function start address offset,
   function size in bytes, and the offset to the SFrame FDE
   attribute/Frame Row Entries (FREs) area for the function.
 - FDE Attributes (sframe_func_desc_attr_v3): The metadata regarding the
   function (number and size of FREs, FDE type, and repetition block
   size etc.) is moved to a new structure.

On-Disk Layout: In V3, the "Attributes" are now stored immediately
preceding the SFrame FREs for that function. The sfde_func_start_fre_off
now points to the attr structure, and the actual FREs follow immediately
after.  IOW, the "Attributes" are now moved to the FRE sub-section,
located immediately preceding the FREs for the respective function.

The above layout has the advantage that:
  a) its cleaner with separation between the index elements vs other data
  b) the index has better cache locality (by virtue of it being smaller
     than the layout in SFrame V2).
  c) As the format evolves, the guarantees of alignment for FDE index
     are easier to maintain.  FDE attr, being in the SFrame FRE
     sub-section, carry no guarantees of alignment.

This had been previously suggested and communicated in an earlier
discussion on binutils mailling list
https://inbox.sourceware.org/binutils/29b1f7b0-61ea-410c-8aca-d5dd6115e668@oracle.com/

The read/write paths in sframe.c are updated to account for this split.

sframe_fde_tbl_init now requires access to the FRE buffer to populate
the internal FDE table, as the attributes are no longer resident in the
FDE section.

flip_sframe is refactored into version-specific handlers (_v2 and _v3)
because the endian-swapping logic now differs significantly. In V3, the
iterator must jump from the FDE table to the FRE section to swap the
attributes.

Lastly, the two entities generating SFrame sections (GAS and GNU ld)
both now must _not_ set the sfde_func_start_fre_off to zero, when the
number of FREs is zero.  This is because now there will be some valid
attr data at that location.

Backward Compatibility: Due to the need to support readelf/objdump for
SFrame V2 sections, the patch explicitly maintains V2 support via
separate code paths (e.g., flip_sframe_fdes_with_fres_v2)

Note about alignment: Now that the sframe_func_desc_idx_v3 is refactored
out of the conceptual SFrame FDE, SFrame FDE index member elements are
at aligned boundaries again.  The alignment property for SFrame FDE was
broken from an ealier patch "[08/36] sframe: gas: libsframe: use
uint16_t for num_fres of FDE" up until this one.

include/
	* sframe.h (sframe_func_desc_entry_v3): Remove sfde_func_num_fres,
	sfde_func_info, sfde_func_info2, and sfde_func_rep_size.  Rename
	to sframe_func_desc_idx_v3.
	(sframe_func_desc_attr_v3): New SFrame FDE attribute structure.
libsframe/
	* sframe.c (sframe_fde_tbl_init): Add argument for FRE buffer.
	Read attributes from the FRE section for V3.
	(flip_fde_desc): Rename from flip_fde. Check size against
	sframe_func_desc_entry_v3.
	(flip_fde_attr_v3): New function.
	(sframe_decode_fde_desc_v2): New function extracted from
	sframe_decode_fde.
	(sframe_decode_fde_desc_v3): New function.
	(sframe_decode_fde_attr_v3): New function.
	(flip_sframe_fdes_with_fres_v2): New function for V2 flipping.
	(flip_sframe_fdes_with_fres_v3): New function for V3 flipping.
	(flip_sframe): Dispatch to version-specific flip functions.
	(sframe_decode): Pass FRE buffer to sframe_fde_tbl_init.
	(sframe_decoder_get_offsetof_fde_start_addr): Adjust for subset
	of sframe_func_desc_entry_v3 restructured into
	sframe_func_desc_idx_v3.
	(sframe_encoder_get_offsetof_fde_start_addr): Likewise.
	(sframe_find_fre): Skip attribute size to find FREs in V3.
	(sframe_decoder_get_fre): Likewise.
	(sframe_decoder_get_fres_buf): Likewise.
	(sframe_encoder_add_fre): Add attribute size to byte count.
	(sframe_encoder_add_fres_buf): Read attributes from buffer.
	(sframe_encoder_write_fde): Write only FDE index fields.
	(sframe_encoder_write_func_attr): New function.
	(sframe_encoder_write_sframe): Write FDE attributes before FREs.
gas/
	* gen-sframe.c (output_sframe_funcdesc): Do not reset
	sfde_func_start_fre_off to zero when zero num FREs.
	(output_sframe_func_desc_attr): New refactored out function.
	(output_sframe_internal): Invoke output_sframe_func_desc_attr.
libsframe/testsuite/
	* libsframe.decode/DATA2: Regenerate data file.

2026-01-15 17:02:27 -08:00

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