hurd: libgcc unwinding support over signal trampolines

* libgcc/config.host (md_unwind_header): Set to i386/gnu-unwind.h on
i[34567]86-*-gnu*.
* src/libgcc/config/i386/gnu-unwind.h: New file.

diff --git a/libgcc/config.host b/libgcc/config.host
index 2cd42097167..044b34d53cc 100644
--- a/libgcc/config.host
+++ b/libgcc/config.host
@@ -734,11 +734,17 @@ i[34567]86-*-linux*)
 	tm_file="${tm_file} i386/elf-lib.h"
 	md_unwind_header=i386/linux-unwind.h
 	;;
-i[34567]86-*-kfreebsd*-gnu | i[34567]86-*-gnu* | i[34567]86-*-kopensolaris*-gnu)
+i[34567]86-*-kfreebsd*-gnu | i[34567]86-*-kopensolaris*-gnu)
 	extra_parts="$extra_parts crtprec32.o crtprec64.o crtprec80.o crtfastmath.o"
 	tmake_file="${tmake_file} i386/t-crtpc t-crtfm i386/t-crtstuff t-dfprules"
 	tm_file="${tm_file} i386/elf-lib.h"
 	;;
+i[34567]86-*-gnu*)
+	extra_parts="$extra_parts crtprec32.o crtprec64.o crtprec80.o crtfastmath.o"
+	tmake_file="${tmake_file} i386/t-crtpc t-crtfm i386/t-crtstuff t-dfprules"
+	tm_file="${tm_file} i386/elf-lib.h"
+	md_unwind_header=i386/gnu-unwind.h
+	;;
 x86_64-*-linux*)
 	extra_parts="$extra_parts crtprec32.o crtprec64.o crtprec80.o crtfastmath.o"
 	tmake_file="${tmake_file} i386/t-crtpc t-crtfm i386/t-crtstuff t-dfprules"
diff --git a/src/libgcc/config/i386/gnu-unwind.h b/src/libgcc/config/i386/gnu-unwind.h
new file mode 100644
index 00000000000..db47f0ac1d4
--- /dev/null
+++ b/src/libgcc/config/i386/gnu-unwind.h
@@ -0,0 +1,107 @@
+/* DWARF2 EH unwinding support for GNU Hurd: x86.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   Contributed by Samuel Thibault <samuel.thibault@gnu.org>
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3, or (at your option)
+any later version.
+
+GCC is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+Under Section 7 of GPL version 3, you are granted additional
+permissions described in the GCC Runtime Library Exception, version
+3.1, as published by the Free Software Foundation.
+
+You should have received a copy of the GNU General Public License and
+a copy of the GCC Runtime Library Exception along with this program;
+see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+<http://www.gnu.org/licenses/>.  */
+
+/* Do code reading to identify a signal frame, and set the frame
+   state data appropriately.  See unwind-dw2.c for the structs. */
+
+#ifndef inhibit_libc
+
+#include <signal.h>
+
+#define MD_FALLBACK_FRAME_STATE_FOR x86_gnu_fallback_frame_state
+
+static _Unwind_Reason_Code
+x86_gnu_fallback_frame_state
+(struct _Unwind_Context *context, _Unwind_FrameState *fs)
+{
+  struct handler_args {
+    int signo;
+    int sigcode;
+    struct sigcontext *scp;
+  } *handler_args;
+  struct sigcontext *scp;
+  unsigned long usp;
+
+/*
+ * i386 sigtramp frame we are looking for follows.
+ * (see glibc/sysdeps/mach/hurd/i386/trampoline.c assembly)
+ *
+ * rpc_wait_trampoline:
+ *   0:	b8 e7 ff ff ff       	mov    $-25,%eax       mach_msg_trap
+ *   5:	9a 00 00 00 00 07 00 	lcall  $7,$0
+ *  12:	89 01                	movl   %eax, (%ecx)
+ *  14:	89 dc                	movl   %ebx, %esp      switch to signal stack
+ *
+ * trampoline:
+ *  16:	ff d2                	call   *%edx           call the handler function
+ * RA HERE
+ *  18:	83 c4 0c             	addl   $12, %esp       pop its args
+ *  21:	c3                   	ret                    return to sigreturn
+ *
+ * firewall:
+ *  22:	f4                   	hlt
+ */
+
+  if (!(   *(unsigned int   *)(context->ra     ) == 0xc30cc483
+        && *(unsigned char  *)(context->ra +  4) ==       0xf4
+
+        && *(unsigned int   *)(context->ra -  4) == 0xd2ffdc89
+        && *(unsigned int   *)(context->ra -  8) == 0x01890007
+        && *(unsigned int   *)(context->ra - 12) == 0x00000000
+        && *(unsigned int   *)(context->ra - 16) == 0x9affffff
+        && *(unsigned short *)(context->ra - 18) ==     0xe7b8))
+    return _URC_END_OF_STACK;
+
+  handler_args = context->cfa;
+  scp = handler_args->scp;
+  usp = scp->sc_uesp;
+
+  fs->regs.cfa_how = CFA_REG_OFFSET;
+  fs->regs.cfa_reg = 4;
+  fs->regs.cfa_offset = usp - (unsigned long) context->cfa;
+
+  fs->regs.reg[0].how = REG_SAVED_OFFSET;
+  fs->regs.reg[0].loc.offset = (unsigned long)&scp->sc_eax - usp;
+  fs->regs.reg[1].how = REG_SAVED_OFFSET;
+  fs->regs.reg[1].loc.offset = (unsigned long)&scp->sc_ecx - usp;
+  fs->regs.reg[2].how = REG_SAVED_OFFSET;
+  fs->regs.reg[2].loc.offset = (unsigned long)&scp->sc_edx - usp;
+  fs->regs.reg[3].how = REG_SAVED_OFFSET;
+  fs->regs.reg[3].loc.offset = (unsigned long)&scp->sc_ebx - usp;
+  fs->regs.reg[5].how = REG_SAVED_OFFSET;
+  fs->regs.reg[5].loc.offset = (unsigned long)&scp->sc_ebp - usp;
+  fs->regs.reg[6].how = REG_SAVED_OFFSET;
+  fs->regs.reg[6].loc.offset = (unsigned long)&scp->sc_esi - usp;
+  fs->regs.reg[7].how = REG_SAVED_OFFSET;
+  fs->regs.reg[7].loc.offset = (unsigned long)&scp->sc_edi - usp;
+  fs->regs.reg[8].how = REG_SAVED_OFFSET;
+  fs->regs.reg[8].loc.offset = (unsigned long)&scp->sc_eip - usp;
+  fs->retaddr_column = 8;
+  fs->signal_frame = 1;
+
+  return _URC_NO_REASON;
+}
+
+#endif /* ifndef inhibit_libc */