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.

1 files changed, 15 insertions, 0 deletions

diff --git a/gas/doc/as.texi b/gas/doc/as.texi
index 7526b22..370f40f 100644
--- a/gas/doc/as.texi
+++ b/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