[patch] Allow configuration with --disable-sjlj-exceptions on biarch x86 Windows targets

[Sandra Loosemore <sandra at codesourcery dot com>]

This is a slightly revised version of the WIP patch against GCC 5.1 I 
previously posted here:

https://gcc.gnu.org/ml/gcc/2016-05/msg00135.html

To recap, I needed a biarch x86_64 mingw-w64 target compiler that uses 
DWARF-2 exception handling in 32-bit mode (for compatibility with an 
older i686-mingw32 toolchain configured with --disable-sjlj-exceptions). 
 But, the configuration machinery was rejecting this.  It used to be 
that SJLJ exceptions were the only choice for 64-bit mode so it was 
correct to reject --disable-sjlj-exceptions in a biarch toolchain, but 
now the default for 64-bit mode is SEH instead.  With this patch, 
configuring with --disable-sjlj-exceptions selects DWARF-2 EH in 32-bit 
mode and SEH in 64-bit mode.

I tested this in a cross-compiler configured to build C and C++ only for 
x86_64 mingw-w64 target, and test results look about the same as the 
default configuration, which uses SJLJ for 32-bit mode.  (If it's 
relevant, I also used the compiler built with the 5.1 version of the 
patch to build complete Windows-host toolchains for nios2-elf and 
nios2-linux-gnu, which I tested by running the GDB testsuite.)  There 
are no actual code changes here so I'd expect the 32-bit DWARF-2 EH 
support to work exactly the same as in a 32-bit-only x86 configuration.

OK to commit?

-Sandra

2016-05-19  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/
	* config/i386/cygming.h (DWARF2_UNWIND_INFO): Allow 
	--disable-sjlj-exceptions for TARGET_BI_ARCH to select DWARF-2 EH
	for 32-bit mode and SEH for 64-bit.
	* config/i386/mingw32.h (SHARED_LIBGCC_UNDEFS_SPEC): Handle
	TARGET_64BIT_DEFAULT.

	libgcc/
	* config.host [x86_64-*-cygwin*]: Handle tmake_eh_file for mixed 
	dw2/seh configuration.
	[x86_64-*-mingw*]: Likewise.

Index: gcc/config/i386/cygming.h
===================================================================
--- gcc/config/i386/cygming.h	(revision 236256)
+++ gcc/config/i386/cygming.h	(working copy)
@@ -339,16 +339,13 @@ do {						\
 #define ASM_COMMENT_START " #"
 
 #ifndef DWARF2_UNWIND_INFO
-/* If configured with --disable-sjlj-exceptions, use DWARF2, else
-   default to SJLJ.  */
+/* If configured with --disable-sjlj-exceptions, use DWARF2 for 32-bit
+   mode else default to SJLJ.  64-bit code uses SEH unless you request
+   SJLJ.  */
 #if  (defined (CONFIG_SJLJ_EXCEPTIONS) && !CONFIG_SJLJ_EXCEPTIONS)
 /* The logic of this #if must be kept synchronised with the logic
-   for selecting the tmake_eh_file fragment in config.gcc.  */
+   for selecting the tmake_eh_file fragment in libgcc/config.host.  */
 #define DWARF2_UNWIND_INFO 1
-/* If multilib is selected break build as sjlj is required.  */
-#if defined (TARGET_BI_ARCH)
-#error For 64-bit windows and 32-bit based multilib version of gcc just SJLJ exceptions are supported.
-#endif
 #else
 #define DWARF2_UNWIND_INFO 0
 #endif
Index: gcc/config/i386/mingw32.h
===================================================================
--- gcc/config/i386/mingw32.h	(revision 236256)
+++ gcc/config/i386/mingw32.h	(working copy)
@@ -100,10 +100,12 @@ along with GCC; see the file COPYING3.  
 #if DWARF2_UNWIND_INFO
 /* DW2-unwind is just available for 32-bit mode.  */
 #if TARGET_64BIT_DEFAULT
-#error DW2 unwind is not available for 64-bit.
-#endif
+#define SHARED_LIBGCC_UNDEFS_SPEC \
+  "%{m32: %{shared-libgcc: -u ___register_frame_info -u ___deregister_frame_info}}"
+#else
 #define SHARED_LIBGCC_UNDEFS_SPEC \
  "%{shared-libgcc: -u ___register_frame_info -u ___deregister_frame_info}"
+#endif
 #else
 #define SHARED_LIBGCC_UNDEFS_SPEC ""
 #endif
Index: libgcc/config.host
===================================================================
--- libgcc/config.host	(revision 236256)
+++ libgcc/config.host	(working copy)
@@ -678,6 +678,9 @@ x86_64-*-cygwin*)
 	# This has to match the logic for DWARF2_UNWIND_INFO in gcc/config/i386/cygming.h
 	if test x$ac_cv_sjlj_exceptions = xyes; then
 		tmake_eh_file="i386/t-sjlj-eh"
+	elif test "${host_address}" = 32; then
+	        # biarch -m32 with --disable-sjlj-exceptions
+	     	tmake_eh_file="i386/t-dw2-eh"
 	else
 		tmake_eh_file="i386/t-seh-eh"
 	fi
@@ -730,6 +733,10 @@ x86_64-*-mingw*)
 	# This has to match the logic for DWARF2_UNWIND_INFO in gcc/config/i386/cygming.h
 	if test x$ac_cv_sjlj_exceptions = xyes; then
 		tmake_eh_file="i386/t-sjlj-eh"
+	elif test "${host_address}" = 32; then
+	        # biarch -m32 with --disable-sjlj-exceptions
+	     	tmake_eh_file="i386/t-dw2-eh"
+		md_unwind_header=i386/w32-unwind.h
 	else
 		tmake_eh_file="i386/t-seh-eh"
 	fi