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[RFC 21/28] [SFrame-V3] gas: sframe: add handling for .cfi_register for FP and RA

Indu Bhagat indu.bhagat@oracle.com
Tue Dec 9 09:07:46 GMT 2025 
Use SFrame FDE of type SFRAME_FDE_TYPE_FLEX_TOPMOST_FRAME.

When FP, RA were moved to a general-purpose register, the SFrame
generation previously warned and skipped the FDE (except on S390X). This
patch updates the translator to detect`.cfi_register` for RA (and FP) on
AMD64, tracks the destination register in the SFrame row entry, and
emits the register in the relevant FRE offsets in SFrame FDE type
SFRAME_FDE_TYPE_FLEX_TOPMOST_FRAME.

gas/
	* gen-sframe.c (output_sframe_row_entry_offsets): Emit register
	encoding for RA if tracked.
	(sframe_row_entry_initialize): Propagate ra_reg and ra_deref_p.
	(sframe_xlate_do_register): Handle .cfi_register for RA/FP on
	AMD64 by setting flex_topmost_p and recording the register.
	* gen-sframe.h (struct sframe_row_entry): Add ra_reg and
	ra_deref_p.
testsuite/gas/
	* cfi-sframe/cfi-sframe.exp: Run new test.
	* cfi-sframe/cfi-sframe-x86_64-6.d: New test.
	* cfi-sframe/cfi-sframe-x86_64-6.s: New test.
---
 gas/gen-sframe.c                              | 72 +++++++++++++++----
 gas/gen-sframe.h                              |  4 ++
 .../gas/cfi-sframe/cfi-sframe-x86_64-6.d      | 21 ++++++
 .../gas/cfi-sframe/cfi-sframe-x86_64-6.s      | 13 ++++
 gas/testsuite/gas/cfi-sframe/cfi-sframe.exp   |  1 +
 5 files changed, 96 insertions(+), 15 deletions(-)
 create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.d
 create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.s

diff --git a/gas/gen-sframe.c b/gas/gen-sframe.c
index 11cc828dbde..89675379d51 100644
--- a/gas/gen-sframe.c
+++ b/gas/gen-sframe.c
@@ -644,13 +644,25 @@ output_sframe_row_entry_offsets (const struct sframe_func_entry *sframe_fde,
 
       /* RA tracking enabled or not, emit two offsets for RA.
 	 Aside, emitting SFRAME_FRE_RA_OFFSET_INVALID is equivalent to emitting
-	 SFRAME_V3_FLEX_FDE_REG_ENCODE (0, 0, 0).
-	 FIXME - This may change later when we implement handling .cfi_register
-	 RA, reg.  Emit two 0 offsets for now.  */
-      fre_offset_func_map[idx].out_func (SFRAME_FRE_RA_OFFSET_INVALID);
-      /* FIXME Offset 0 for now.  */
-      fre_offset_func_map[idx].out_func (0);
-      fre_write_offsets += 2;
+	 SFRAME_V3_FLEX_FDE_REG_ENCODE (0, 0, 0).  */
+      if (sframe_fre->ra_loc == SFRAME_FRE_ELEM_LOC_STACK)
+	{
+	  /* Output RA related FRE offsets.  */
+	  reg_data = SFRAME_V3_FLEX_FDE_REG_ENCODE (sframe_fre->ra_reg,
+						    sframe_fre->ra_deref_p,
+						    1 /* reg_p.  */);
+	  offset_data = sframe_fre->ra_offset;
+	  fre_offset_func_map[idx].out_func (reg_data);
+	  fre_offset_func_map[idx].out_func (offset_data);
+	  fre_write_offsets += 2;
+	}
+      else
+	{
+	  fre_offset_func_map[idx].out_func (SFRAME_FRE_RA_OFFSET_INVALID);
+	  /* FIXME Offset 0 for now.  */
+	  fre_offset_func_map[idx].out_func (0);
+	  fre_write_offsets += 2;
+	}
 
       if (sframe_fre->fp_loc == SFRAME_FRE_ELEM_LOC_STACK)
 	{
@@ -1116,7 +1128,9 @@ sframe_row_entry_initialize (struct sframe_row_entry *cur_fre,
   cur_fre->fp_offset = prev_fre->fp_offset;
   cur_fre->fp_deref_p = prev_fre->fp_deref_p;
   cur_fre->ra_loc = prev_fre->ra_loc;
+  cur_fre->ra_reg = prev_fre->ra_reg;
   cur_fre->ra_offset = prev_fre->ra_offset;
+  cur_fre->ra_deref_p = prev_fre->ra_deref_p;
   /* Treat RA mangling as a sticky bit.  It retains its value until another
      .cfi_negate_ra_state is seen.  */
   cur_fre->mangled_ra_p = prev_fre->mangled_ra_p;
@@ -1440,6 +1454,18 @@ s390_sframe_xlate_do_register (struct sframe_xlate_ctx *xlate_ctx,
 }
 
 /* Translate DW_CFA_register into SFrame context.
+
+   This opcode indicates: Previous value of register1 is register2.  This is
+   not representable in SFrame stack trace format.  Detect the use of registers
+   interesting to SFrame (FP, RA for this opcode), and skip FDE generation
+   while warning the user.
+
+   Two exceptions apply though:
+     - for S390X, the stack offsets are used to carry register number in
+       default FDE types.  So invoke S390X specific handling.
+     - for AMD64, the flexible topmost frame encoding
+       SFRAME_FDE_TYPE_FLEX_TOPMOST_FRAME can be used for FP, RA registers.
+
    Return SFRAME_XLATE_OK if success.  */
 
 static int
@@ -1449,15 +1475,31 @@ sframe_xlate_do_register (struct sframe_xlate_ctx *xlate_ctx,
   /* Conditionally invoke S390-specific implementation.  */
   if (sframe_get_abi_arch () == SFRAME_ABI_S390X_ENDIAN_BIG)
     return s390_sframe_xlate_do_register (xlate_ctx, cfi_insn);
+  else if (sframe_get_abi_arch () == SFRAME_ABI_AMD64_ENDIAN_LITTLE)
+    {
+      struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;
 
-  /* Previous value of register1 is register2.  However, if the specified
-     register1 is not interesting (FP or RA reg), the current DW_CFA_register
-     instruction can be safely skipped without sacrificing the asynchronicity of
-     stack trace information.  */
-  if (cfi_insn->u.rr.reg1 == SFRAME_CFA_FP_REG
-      || cfi_insn->u.rr.reg1 == SFRAME_CFA_RA_REG
-      /* Ignore SP reg, as it can be recovered from the CFA tracking info.  */
-      )
+      if (cfi_insn->u.rr.reg1 == SFRAME_CFA_FP_REG)
+	{
+	  sframe_fre_set_fp_track (cur_fre, 0);
+	  cur_fre->fp_reg = cfi_insn->u.rr.reg2;
+	  cur_fre->fp_deref_p = false;
+	  cur_fre->merge_candidate = false;
+	  xlate_ctx->flex_topmost_p = true;
+	}
+      else if (cfi_insn->u.rr.reg1 == SFRAME_CFA_RA_REG)
+	{
+	  sframe_fre_set_ra_track (cur_fre, 0);
+	  cur_fre->ra_reg = cfi_insn->u.rr.reg2;
+	  cur_fre->ra_deref_p = false;
+	  cur_fre->merge_candidate = false;
+	  xlate_ctx->flex_topmost_p = true;
+	}
+    }
+  else if (cfi_insn->u.rr.reg1 == SFRAME_CFA_RA_REG
+	   /* Ignore SP reg, as it can be recovered from the CFA tracking
+	      info.  */
+	   || cfi_insn->u.rr.reg1 == SFRAME_CFA_FP_REG)
     {
       as_warn (_("no SFrame FDE emitted; %s register %u in .cfi_register"),
 	       sframe_register_name (cfi_insn->u.rr.reg1), cfi_insn->u.rr.reg1);
diff --git a/gas/gen-sframe.h b/gas/gen-sframe.h
index 4921e796a10..6c0699dec46 100644
--- a/gas/gen-sframe.h
+++ b/gas/gen-sframe.h
@@ -87,9 +87,13 @@ struct sframe_row_entry
   bool fp_deref_p;
 
   /* Track RA location.  Specify whether it is in register or memory.  */
+  unsigned int ra_reg;
   unsigned int ra_loc;
   /* If RA is stashed on stack, note the offset.  */
   offsetT ra_offset;
+ /* Whether RA recovery needs dereferencing.  This is tracked for
+    SFRAME_FDE_TYPE_FLEX_TOPMOST_FRAME SFrame FDE type.  */
+  bool ra_deref_p;
 };
 
 /* SFrame Function Description Entry.  */
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.d b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.d
new file mode 100644
index 00000000000..1906a0b5630
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.d
@@ -0,0 +1,21 @@
+#as: --gsframe
+#objdump: --sframe=.sframe
+#name: DW_CFA_register RA in flex FDE type
+#...
+
+Contents of the SFrame section .sframe:
+  Header :
+
+    Version: SFRAME_VERSION_3
+    Flags: SFRAME_F_FDE_FUNC_START_PCREL
+    CFA fixed RA offset: -8
+    Num FDEs: 1
+    Num FREs: 3
+
+  Function Index :
+
+    func idx \[0\]: pc = 0x0, size = 7 bytes
+    STARTPC +CFA +FP +RA +
+    0+0000 +sp\+8 +u +f +
+    0+0004 +sp\+40 +u +f +
+    0+0005 +sp\+0 +u +r2\+0 +
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.s b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.s
new file mode 100644
index 00000000000..69a3efb7e85
--- /dev/null
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-6.s
@@ -0,0 +1,13 @@
+	.p2align 4
+	.globl	foo
+	.type	foo, @function
+foo:
+	.cfi_startproc
+	sub	$0x20,%rsp
+	.cfi_adjust_cfa_offset 0x20
+	popq	%rcx
+	.cfi_register 16, 2
+	.cfi_def_cfa_offset 0
+	jmp	*%rcx
+	.cfi_endproc
+	.size	foo, .-foo
diff --git a/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp b/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
index 489b3ff003f..d7a2afbaf3d 100644
--- a/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
+++ b/gas/testsuite/gas/cfi-sframe/cfi-sframe.exp
@@ -64,6 +64,7 @@ if { [istarget "x86_64-*-*"] && [gas_sframe_check] } then {
 	run_dump_test "cfi-sframe-x86_64-3"
 	run_dump_test "cfi-sframe-x86_64-4"
 	run_dump_test "cfi-sframe-x86_64-5"
+	run_dump_test "cfi-sframe-x86_64-6"
 	run_dump_test "cfi-sframe-x86_64-pr33170"
 	run_dump_test "cfi-sframe-x86_64-signal-1"
 	run_dump_test "cfi-sframe-x86_64-empty-1"
-- 
2.43.0
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