gas: add new command line option --scfi=experimental

When the command line option --scfi=experimenta is passed to the GNU
assembler, it will synthesize DWARF call frame information (CFI) for the
input assembly.

The option --scfi=experimental will also ignore most of the existing
.cfi_* directives, if already contained in the provided input file.
Only the following CFI directives will not be ignored:
  - .cfi_sections,
  - .cfi_label,
  - .cfi_signal_frame

To use SCFI, a target will need to:
    - define TARGET_USE_SCFI and TARGET_USE_GINSN, and other necessary
    definitions,
    - provide means to help GAS understand the target specific instruction
    semantics by creating ginsns.

The upcoming support for SCFI is inteded to be experimental, hence the
option --scfi=experimental.  The --scfi= may see more options like
--scfi=[all,none] added in future, once the SCFI support in GAS is
mature and robust.  The offering may also see for example, an
--scfi=inline option for dealing with inline asm may be added in the
future.  In --scfi=inline option, the GNU assembler may consume (and not
ignore) the compiler generated CFI for the code surrounding the inline
asm.

Also document the option.

gas/
        * as.c (show_usage): Add support for --scfi=experimental.
        (parse_args): Likewise.
        * as.h (enum synth_cfi_type): Define new type.
        * doc/as.texi: Document the new option.

gas/as.c

@@ -321,6 +321,11 @@ Options:\n\
                           generate GNU Build notes if none are present in the input\n"));
   fprintf (stream, _("\
   --gsframe               generate SFrame stack trace information\n"));
+# if defined (TARGET_USE_SCFI) && defined (TARGET_USE_GINSN)
+  fprintf (stream, _("\
+  --scfi=experimental     Synthesize DWARF CFI for hand-written asm\n\
+                          (experimental support)\n"));
+# endif
 #endif /* OBJ_ELF */
 
   fprintf (stream, _("\
@@ -511,7 +516,8 @@ parse_args (int * pargc, char *** pargv)
       OPTION_NOCOMPRESS_DEBUG,
       OPTION_NO_PAD_SECTIONS,
       OPTION_MULTIBYTE_HANDLING,  /* = STD_BASE + 40 */
-      OPTION_SFRAME
+      OPTION_SFRAME,
+      OPTION_SCFI
     /* When you add options here, check that they do
        not collide with OPTION_MD_BASE.  See as.h.  */
     };
@@ -543,7 +549,10 @@ parse_args (int * pargc, char *** pargv)
     ,{"sectname-subst", no_argument, NULL, OPTION_SECTNAME_SUBST}
     ,{"generate-missing-build-notes", required_argument, NULL, OPTION_ELF_BUILD_NOTES}
     ,{"gsframe", no_argument, NULL, OPTION_SFRAME}
-#endif
+# if defined (TARGET_USE_SCFI) && defined (TARGET_USE_GINSN)
+    ,{"scfi", required_argument, NULL, OPTION_SCFI}
+# endif
+#endif /* OBJ_ELF || OBJ_MAYBE_ELF.  */
     ,{"fatal-warnings", no_argument, NULL, OPTION_WARN_FATAL}
     ,{"gdwarf-2", no_argument, NULL, OPTION_GDWARF_2}
     ,{"gdwarf-3", no_argument, NULL, OPTION_GDWARF_3}
@@ -982,6 +991,16 @@ This program has absolutely no warranty.\n"));
 	  flag_execstack = 0;
 	  break;
 
+# if defined (TARGET_USE_SCFI) && defined (TARGET_USE_GINSN)
+	case OPTION_SCFI:
+	  if (optarg && strcasecmp (optarg, "experimental") == 0)
+	    flag_synth_cfi = SYNTH_CFI_EXPERIMENTAL;
+	  else
+	    as_fatal (_("Invalid --scfi= option: `%s'; suggested option: experimental"),
+		      optarg);
+	  break;
+# endif
+
 	case OPTION_SIZE_CHECK:
 	  if (strcasecmp (optarg, "error") == 0)
 	    flag_allow_nonconst_size = false;

gas/as.h

@@ -324,6 +324,14 @@ COMMON int flag_fatal_warnings; /* --fatal-warnings */
    are detected.  */
 COMMON unsigned char flag_always_generate_output; /* -Z */
 
+enum synth_cfi_type
+{
+  SYNTH_CFI_NONE = 0,
+  SYNTH_CFI_EXPERIMENTAL,
+};
+
+COMMON enum synth_cfi_type flag_synth_cfi;
+
 /* This is true if the assembler should output time and space usage.  */
 COMMON unsigned char flag_print_statistics;
 

gas/doc/as.texi

@@ -255,6 +255,7 @@ gcc(1), ld(1), and the Info entries for @file{binutils} and @file{ld}.
  [@b{--multibyte-handling=[allow|warn|warn-sym-only]}]
  [@b{--no-pad-sections}]
  [@b{-o} @var{objfile}] [@b{-R}]
+ [@b{--scfi=experimental}]
  [@b{--sectname-subst}]
  [@b{--size-check=[error|warning]}]
  [@b{--statistics}]
@@ -932,6 +933,20 @@ Ignored.  Supported for compatibility with tools that apss the same option to
 both the assembler and the linker.
 
 @ifset ELF
+@item --scfi=experimental
+This option controls whether the assembler should synthesize CFI for
+hand-written input.  If the input already contains some synthesizable CFI
+directives, the assembler ignores them and emits a warning.  Note that
+@code{--scfi=experimental} is not intended to be used for compiler-generated
+code, including inline assembly.  This experimental support is work in
+progress.  Only System V AMD64 ABI is supported.
+
+Each input function in assembly must begin with the @code{.type} directive, and
+should ideally be closed off using a @code{.size} directive.  When using SCFI,
+each @code{.type} directive prompts GAS to start a new FDE (a.k.a., Function
+Descriptor Entry).  This implies that with each @code{.type} directive, a
+previous block of instructions, if any, is finalised as a distinct FDE.
+
 @item --sectname-subst
 Honor substitution sequences in section names.
 @ifclear man