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Re: Mingw-w64 exception handling configuration


On 05/12/2016 09:27 AM, Sandra Loosemore wrote:
I see that the default EH behavior for a biarch mingw-w64 target GCC is
to use SJLJ for the 32-bit multilib and SEH for the 64-bit one, but that
there are #errors in cygming.h and mingw32.h that prevent you from
configuring a biarch build with --disable-sjlj-exceptions to use DWARF-2
and SEH, respectively.  Is there a deep reason for this, or is this just
a case where nobody updated the configuration logic after SEH was
implemented?  I understand that before that, you had to use SJLJ for
64-bit targets because the DWARF-2 support is restricted to 32-bit
targets, so it was completely reasonable to error out.

Following up my own post....

I did some experimentation, and it looks like this is indeed just a result of bit-rotten configuration logic. I hacked up the attached patch against our local GCC 5.2 branch and built and tested it as a cross-compiler (C and C++ front ends only). Test results for the --disable-sjlj-exceptions case have a few differences compared to --enable-sjlj-exceptions but they're not particularly better or worse.

Maintainers, any guidance on getting this into trunk? E.g. I am not particularly interested in Cygwin target and have not tried to build that, but this affects code common to both Cygwin and Mingw-w64.

-Sandra

2016-05-13  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 464590)
+++ gcc/config/i386/cygming.h	(working copy)
@@ -347,16 +347,13 @@ 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 464590)
+++ 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 464590)
+++ libgcc/config.host	(working copy)
@@ -661,6 +661,10 @@ x86_64-*-cygwin*)
 	# This has to match the logic for DWARF2_UNWIND_INFO in gcc/config/i386/cygming.h
 	if test x$enable_sjlj_exceptions = xyes; then
 		tmake_eh_file="i386/t-sjlj-eh"
+	elif test "${libgcc_cv_lib_sjlj_exceptions}" = no \
+	     && test "${host_address}" = 32; then
+	        # biarch -m32 with --disable-sjlj-exceptions
+	     	tmake_eh_file="i386/t-dw2-eh"
 	else
 		tmake_eh_file="i386/t-seh-eh"
 	fi
@@ -713,6 +717,10 @@ x86_64-*-mingw*)
 	# This has to match the logic for DWARF2_UNWIND_INFO in gcc/config/i386/cygming.h
 	if test x$enable_sjlj_exceptions = xyes; then
 		tmake_eh_file="i386/t-sjlj-eh"
+	elif test "${libgcc_cv_lib_sjlj_exceptions}" = no \
+	     && test "${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

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