Reimplement the .seh_scope directive

[Julian Waters <tanksherman27@gmail.com>]

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The .seh_scope directive was removed a little over a decade ago due to
being too Microsoft specific. This has proven to be a mistake as there is
now no easy way for reusable inline assembly to express Structured
Exception Handling scopes for Microsoft Windows targets. This patch
reimplements a simpler version of .seh_scope, with the proper semantics in
place

\
From cc9e143f8feb933ca8fc4b12deaab2985893b714 Mon Sep 17 00:00:00 2001
From: TheShermanTanker <tanksherman27@gmail.com>
Date: Sun, 2 Jul 2023 16:22:18 +0800
Subject: [PATCH] Reimplement .seh_scope

---
 gas/config/obj-coff-seh.c | 26 ++++++++++++++++++++++++--
 gas/config/obj-coff-seh.h |  5 ++++-
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/gas/config/obj-coff-seh.c b/gas/config/obj-coff-seh.c
index 7b4486a..b01a207 100644
--- a/gas/config/obj-coff-seh.c
+++ b/gas/config/obj-coff-seh.c
@@ -30,6 +32,7 @@ struct seh_seg_list {

 /* Local data.  */
 static seh_context *seh_ctx_cur = NULL;
+static unsigned int scope = 0;

 static htab_t seh_hash;

@@ -385,7 +388,21 @@ obj_coff_seh_handlerdata (int what ATTRIBUTE_UNUSED)
     return;
   demand_empty_rest_of_line ();

-  switch_xdata (seh_ctx_cur->subsection + 1, seh_ctx_cur->code_seg);
+  switch_xdata (seh_ctx_cur->subsection + (scope == 0) ? 1 : 2,
seh_ctx_cur->code_seg);
+}
+
+static void obj_coff_seh_scope (int what)
+{
+  if (!verify_context_and_target (".seh_scope", seh_kind_x64))
+    return;
+
+  segT seg = now_seg;
+  int subseg = now_subseg;
+
+  scope++;
+  switch_xdata (seh_ctx_cur->subsection + 2, seh_ctx_cur->code_seg);
+  s_rva (4);
+  subseg_set(seg, subseg);
 }

 /* Mark end of current context.  */
@@ -442,7 +459,7 @@ obj_coff_seh_proc (int what ATTRIBUTE_UNUSED)
     {
       x_segcur = seh_hash_find_or_make (seh_ctx_cur->code_seg, ".xdata");
       seh_ctx_cur->subsection = x_segcur->subseg;
-      x_segcur->subseg += 2;
+      x_segcur->subseg += 3;
     }

   SKIP_WHITESPACE ();
@@ -918,6 +935,11 @@ write_function_xdata (seh_context *c)

   seh_x64_write_function_xdata (c);

+  if (scope > 0) {
+    switch_xdata (c->subsection + 1, c->code_seg);
+    out_four (scope);
+  }
+
   subseg_set (save_seg, save_subseg);
 }

diff --git a/gas/config/obj-coff-seh.h b/gas/config/obj-coff-seh.h
index 8d77bac..feef17f 100644
--- a/gas/config/obj-coff-seh.h
+++ b/gas/config/obj-coff-seh.h
@@ -57,6 +57,7 @@
   .seh_savexmm
   .seh_pushframe
   .seh_code
+  .seh_scope
 */

 /* architecture specific pdata/xdata handling.  */
@@ -75,7 +76,8 @@
  {"seh_no32", obj_coff_seh_32, 0}, \
  {"seh_handler", obj_coff_seh_handler, 0}, \
  {"seh_code", obj_coff_seh_code, 0}, \
- {"seh_handlerdata", obj_coff_seh_handlerdata, 0},
+ {"seh_handlerdata", obj_coff_seh_handlerdata, 0}, \
+ {"seh_scope", obj_coff_seh_scope, 0},

 /* Type definitions.  */

@@ -151,6 +153,7 @@ static void obj_coff_seh_proc  (int);
 static void obj_coff_seh_handler (int);
 static void obj_coff_seh_handlerdata (int);
 static void obj_coff_seh_code (int);
+static void obj_coff_seh_scope (int);

 #define UNDSEC bfd_und_section_ptr

-- 
2.35.1.windows.2