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[V1 08/36] [SFrame-V3] sframe: gas: libsframe: use uint16_t for num_fres of FDE

Indu Bhagat indu.bhagat@oracle.com
Mon Dec 29 09:28:41 GMT 2025 
Reduce the size of the num_fres field in the Function Descriptor Entry
(FDE) from 32 bits to 16 bits.

The number of Frame Row Entries (FREs) for a single function is extremely
unlikely to exceed 65,535 in real-world scenarios. Reducing this field
saves 2 bytes per FDE, contributing to a smaller overall SFrame section size.

(BTW, these savings will be eaten up by a later commit which adds
support for text > 2 GiB by increasing an offset from int32_t to
int64_t).

Safety checks are added to the assembler to warn and skip SFrame FDE
generation if a function's FRE count exceeds UINT16_MAX.

Note regarding alignment: With the current patch, the members of
sframe_func_desc_entry_v3 are not at aligned boundaries anymore.  Recall
that all sframe_func_desc_entry_v3 entries are stored together in the
"SFrame FDE sub-section" forming an index.  Only after a later patch in
the series "[29/36] [SFrame-V3] include: gas: libsframe: split FDE into
desc and attr" will the alignment properties of SFrame index will be
restored.

include/
	* sframe.h (sframe_func_desc_entry_v3): Change sfde_func_num_fres
	type to uint16_t.
gas/
	* gen-sframe.c (output_sframe_funcdesc): Write 2 bytes for num_fres
	and assert it fits in uint16_t.
	(sframe_do_fde): Add check to skip FDE emission if num_fres exceeds
	UINT16_MAX.
libsframe/
	* sframe.c (sframe_encoder_write_fde): Cast num_fres to uint16_t
	to ensure correctly written out data.
	* testsuite/libsframe.decode/DATA2: Update binary test data.

---
[Changes from RFC]
  - Remove unnecessary typecast for num_fres [Jens].
  - Fixed up the commit log to reflect the minor shifting of the patch.
[End of changes from RFC]
---
 gas/gen-sframe.c                           |  19 +++++++++++++++++--
 include/sframe.h                           |   2 +-
 libsframe/sframe.c                         |   2 +-
 libsframe/testsuite/libsframe.decode/DATA2 | Bin 98 -> 94 bytes
 4 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
index a3ebd61e499..34bb92086a7 100644
--- a/gas/gen-sframe.c
+++ b/gas/gen-sframe.c
@@ -725,8 +725,9 @@ output_sframe_funcdesc (symbolS *start_of_fre_section,
 				      sfde_func_start_fre_off));
     }
 
-  /* Number of FREs.  */
-  out_four (sframe_fde->num_fres);
+  /* Number of FREs must fit uint16_t.  */
+  gas_assert (sframe_fde->num_fres <= UINT16_MAX);
+  out_two (sframe_fde->num_fres);
 
   /* SFrame FDE function info.  */
   unsigned char func_info;
@@ -2040,6 +2041,20 @@ sframe_do_fde (struct sframe_xlate_ctx *xlate_ctx,
 	= get_dw_fde_end_addrS (xlate_ctx->dw_fde);
     }
 
+  /* Number of FREs must fit uint16_t.  Check now, and do not emit the SFrame
+     FDE if it doesnt fit (although, it is not expected to happen for
+     real-world, useful programs).  The approach of truncating the FDE and
+     emitting multiple SFrame FDEs instead, is not a clearly preferable
+     handling either.  Its a divergence from the model where an SFrame FDE
+     encodes stack trace data between a .cfi_startproc and .cfi_endproc pair.
+     Further, some components (linkers, stack tracers) want to associate the
+     Start PC of a function to a known symbol in the file?  */
+  if (xlate_ctx->num_xlate_fres > UINT16_MAX)
+    {
+      as_warn (_("no SFrame FDE emitted; Number of FREs exceeds UINT16_MAX"));
+      return SFRAME_XLATE_ERR_NOTREPRESENTED;
+    }
+
   /* ABI/arch except s390x cannot represent FP without RA saved.  */
   if (sframe_ra_tracking_p ()
       && sframe_get_abi_arch () != SFRAME_ABI_S390X_ENDIAN_BIG)
diff --git a/include/sframe.h b/include/sframe.h
index e32a605186a..da6665148be 100644
--- a/include/sframe.h
+++ b/include/sframe.h
@@ -251,7 +251,7 @@ typedef struct sframe_func_desc_entry_v3
      beginning of the SFrame Frame Row Entry sub-section.  */
   uint32_t sfde_func_start_fre_off;
   /* Number of frame row entries for the function.  */
-  uint32_t sfde_func_num_fres;
+  uint16_t sfde_func_num_fres;
   /* Additional information for stack tracing from the function:
      - 4-bits: Identify the FRE type used for the function.
      - 1-bit: Identify the PC type of the function - mask or inc.
diff --git a/libsframe/sframe.c b/libsframe/sframe.c
index 4ad02f8c0ad..6fed48ab563 100644
--- a/libsframe/sframe.c
+++ b/libsframe/sframe.c
@@ -2131,7 +2131,7 @@ sframe_encoder_write_fde (const sframe_header *sfhp ATTRIBUTE_UNUSED,
   fdep->sfde_func_start_address = (int32_t)fde->func_start_pc_offset;
   fdep->sfde_func_size = fde->func_size;
   fdep->sfde_func_start_fre_off = fde->func_start_fre_off;
-  fdep->sfde_func_num_fres = fde->func_num_fres;
+  fdep->sfde_func_num_fres = (uint16_t)fde->func_num_fres;
   fdep->sfde_func_info = fde->func_info;
   fdep->sfde_func_rep_size = fde->func_rep_size;
   fdep->sfde_func_padding2 = 0;
diff --git a/libsframe/testsuite/libsframe.decode/DATA2 b/libsframe/testsuite/libsframe.decode/DATA2
index aa62398b2ca..7a218f86bcc 100644
Binary files a/libsframe/testsuite/libsframe.decode/DATA2 and b/libsframe/testsuite/libsframe.decode/DATA2 differ
-- 
2.43.0
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