[SFrame-V3] gas: sframe: allow SFrame FDEs with 0 FREs for signal frames

Update the SFrame generation logic in GAS to emit Function Descriptor Entries
(FDEs) for signal frames even when no Frame Row Entries (FREs) could be
generated.

Previously, create_sframe_all () would discard any FDE that failed
translation or resulted in zero FREs. However, for signal frames (marked
with .cfi_signal_frame), preserving the FDE may be valuable even without
stack offsets. The presence of the SFrame FDE with the 'Signal'
attribute may allow stack tracers to identify the frame as a signal
trampoline and potentially apply fallback handling, rather than treating
the PC range as having no stack trace info at all.

The patch modifies create_sframe_all () to detect translation errors for
signal frames, effectively allowing the generation of an empty FDE (0 FREs)
marked with the 'S' attribute.

gas/
	* gen-sframe.c (sframe_fde_free): Add NULL check for safety.
	(create_sframe_all): Allow FDEs for signal frames even if translation
	encountered errors or produced no FREs.
gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-signal-1.d: New test ensuring
	signal frame FDE is emitted with 0 FREs.
	* gas/cfi-sframe/cfi-sframe-x86_64-signal-1.s: New test.

gas/gen-sframe.c

@@ -564,7 +564,12 @@ sframe_fde_alloc (void)
 static void
 sframe_fde_free (struct sframe_func_entry *sframe_fde)
 {
-  sframe_row_entry_free (sframe_fde->sframe_fres);
+  if (sframe_fde == NULL)
+    return;
+
+  if (sframe_fde->sframe_fres)
+    sframe_row_entry_free (sframe_fde->sframe_fres);
+
   XDELETE (sframe_fde);
 }
 
@@ -2098,12 +2103,15 @@ create_sframe_all (void)
       /* Initialize the translation context with information anew.  */
       sframe_xlate_ctx_init (xlate_ctx);
 
-      /* Process and link SFrame FDEs if no error.  Also skip adding an SFrame
-	 FDE if it does not contain any SFrame FREs.  There is little use of an
-	 SFrame FDE if there is no stack tracing information for the
-	 function.  */
+      /* Process and link SFrame FDEs if no error.  */
       int err = sframe_do_fde (xlate_ctx, dw_fde);
-      if (err || xlate_ctx->num_xlate_fres == 0)
+      if (err && get_dw_fde_signal_p (dw_fde))
+	{
+	  sframe_xlate_ctx_cleanup (xlate_ctx);
+	  err = SFRAME_XLATE_OK;
+	}
+
+      if (err)
 	{
 	  sframe_xlate_ctx_cleanup (xlate_ctx);
 	  sframe_fde_free (sframe_fde);

gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.d

@@ -0,0 +1,18 @@
+#as: --gsframe
+#warning: \.cfi\_escape with op \(0xf\)
+#objdump: --sframe=.sframe
+#name: Signal Frame with no FREs
+#...
+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: 0
+
+  Function Index :
+
+    func idx \[0\]: pc = 0x0, size = 3 bytes, attr = "S"
+    STARTPC + CFA + FP + RA +

gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-signal-1.s

@@ -0,0 +1,12 @@
+	.type	foo, @function
+foo:
+	.cfi_startproc
+	.cfi_signal_frame
+# DW_CFA_def_cfa_expression, length, DW_OP_breg9, (sleb)offset, DW_OP_deref
+	.cfi_escape 0xf,0x3,0x79,0x78,0x6
+	nop
+	pop	%rbp
+	.cfi_offset rbp, -16
+	ret
+	.cfi_endproc
+	.size	foo, .-foo

gas/testsuite/gas/cfi-sframe/cfi-sframe.exp

@@ -63,6 +63,7 @@ if { [istarget "x86_64-*-*"] && [gas_sframe_check] } then {
 	run_dump_test "cfi-sframe-x86_64-3"
 	run_dump_test "cfi-sframe-x86_64-pr33170"
 	run_dump_test "cfi-sframe-x86_64-pr33756"
+	run_dump_test "cfi-sframe-x86_64-signal-1"
 	run_dump_test "cfi-sframe-x86_64-empty-1"
 	run_dump_test "cfi-sframe-x86_64-empty-2"
 	run_dump_test "cfi-sframe-x86_64-empty-3"