Update configure-flags.nix

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -27,6 +27,16 @@
 assert cloog != null -> stdenv.lib.versionOlder version "5";
 assert langJava -> stdenv.lib.versionOlder version "7";
 
+# Note [Windows Exception Handling]
+# sjlj (short jump long jump) exception handling makes no sense on x86_64,
+# it's forcably slowing programs down as it produces a constant overhead.
+# On x86_64 we have SEH (Structured Exception Handling) and we should use
+# that. On i686, we do not have SEH, and have to use sjlj with dwarf2.
+# Hence it's now conditional on x86_32 (i686 is 32bit).
+#
+# ref: https://stackoverflow.com/questions/15670169/what-is-difference-between-sjlj-vs-dwarf-vs-seh
+
+
 let
   inherit (stdenv)
     buildPlatform hostPlatform targetPlatform
@@ -60,7 +70,9 @@ let
       "--disable-debug"
       "--disable-win32-registry"
     ] ++ lib.optionals (crossMingw && targetPlatform.isx86_32) [
+      # See Note [Windows Exception Handling]
       "--enable-sjlj-exceptions"
+      "--with-dwarf2"
     ] else [
       (if crossDarwin then "--with-sysroot=${lib.getLib libcCross}/share/sysroot"
        else                "--with-headers=${lib.getDev libcCross}${libcCross.incdir or "/include"}")
@@ -87,9 +99,6 @@ let
       "--disable-nls"
       # To keep ABI compatibility with upstream mingw-w64
       "--enable-fully-dynamic-string"
-    ] ++ lib.optionals (crossMingw && targetPlatform.isx86_32) [
-      "--enable-sjlj-exceptions"
-      "--with-dwarf2"
     ] ++ lib.optional (targetPlatform.libc == "newlib") "--with-newlib"
       ++ lib.optional (targetPlatform.libc == "avrlibc") "--with-avrlibc"
     );