Switch to Dwarf2 exception handling

Closes: #540782

debian/NEWS

+gcc-mingw-w64 (24) UNRELEASED; urgency=medium
+
+  * This release of gcc-mingw-w64 changes the 32-bit exception handling
+    mechanism from SJLJ to Dwarf2, to match MSYS2, Fedora, and other
+    toolchains, and to allow the Rust toolchain to provide 32-bit
+    cross-compilers.
+    This will require rebuilding all artifacts built with previous
+    releases of the toolchain. Shared binaries will need to be shipped
+    with the dw2 DLL instead of the sjlj DLL.
+
+ -- Stephen Kitt <skitt@debian.org>  Sat, 21 Nov 2020 14:55:37 +0100

debian/changelog

+gcc-mingw-w64 (24) UNRELEASED; urgency=medium
+
+  * Following MSYS2 and Fedora’s example (among others), switch from SJLJ
+    exception handling to Dwarf2; this will help the Rust toolchain in
+    particular. Closes: #540782.
+
+ -- Stephen Kitt <skitt@debian.org>  Thu, 19 Nov 2020 09:00:39 +0100
+
 gcc-mingw-w64 (23) unstable; urgency=medium
 
   * Enable gnat on architectures where gnat-10 is available.

debian/rules

@@ -24,7 +24,9 @@
 export NO_PKG_MANGLE=1
 
 target_version := 10
-targets := i686-w64-mingw32 x86_64-w64-mingw32
+target32 := i686-w64-mingw32
+target64 := x86_64-w64-mingw32
+targets := $(target32) $(target64)
 threads := posix win32
 gnat_arches := alpha amd64 arm64 armel armhf hppa i386 mips64el mipsel ppc64 ppc64el riscv64 s390x sh4 sparc64 x32
 
@@ -294,6 +296,7 @@ mingw-w64-patch-stamp: $(patch_stamp)
 		xargs -0r touch --no-dereference --date='@$(SOURCE_DATE_EPOCH)'
 	touch $@
 
+# 32- and 64-bit targets are split: 32-bit uses Dwarf2, 64-bit the default SEH
 configure: configure-stamp
 configure-stamp: debian/control mingw-w64-patch-stamp
 	dh_testdir
@@ -302,23 +305,31 @@ configure-stamp: debian/control mingw-w64-patch-stamp
 	echo ======================================================================
 ifeq ($(filter stage1,$(DEB_BUILD_PROFILES)),)
 	set -e; \
-	for target in $(targets); do \
-		for threads in $(threads); do \
-			echo $(base_version)-$$threads > $(upstream_dir)/gcc/BASE-VER && \
-			dh_auto_configure \
-				-B$(build_dir)/$$target-$$threads \
-				-D$(upstream_dir) -- \
-				$(CONFFLAGS); \
-		done; \
-	done
-else
-	set -e; \
-	for target in $(targets); do \
+	for threads in $(threads); do \
+		echo $(base_version)-$$threads > $(upstream_dir)/gcc/BASE-VER; \
+		target=$(target32); \
+		dh_auto_configure \
+			-B$(build_dir)/$(target32)-$$threads \
+			-D$(upstream_dir) -- \
+			$(CONFFLAGS) --disable-sjlj-exceptions --with-dwarf2; \
+		target=$(target64); \
 		dh_auto_configure \
-			-B$(build_dir)/$$target \
+			-B$(build_dir)/$(target64)-$$threads \
 			-D$(upstream_dir) -- \
 			$(CONFFLAGS); \
 	done
+else
+	set -e; \
+	target=$(target32); \
+	dh_auto_configure \
+		-B$(build_dir)/$(target32) \
+		-D$(upstream_dir) -- \
+		$(CONFFLAGS) --disable-sjlj-exceptions --with-dwarf2; \
+	target=$(target64); \
+	dh_auto_configure \
+		-B$(build_dir)/$(target64) \
+		-D$(upstream_dir) -- \
+		$(CONFFLAGS)
 endif
 	touch $@