[RFC/i385newframe] Fix sigtramp unwinder.

[Michal Ludvig <mludvig@suse.cz>]

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Hi,
this patch fixes x86_64_linux_sigcontext_addr() which returned wrong 
address, and consequently enables unwinding from signal handlers.
Detailed explanation of this change is in the comment in the patch.
OK to apply?

Michal Ludvig
-- 
* SuSE CR, s.r.o     * mludvig@suse.cz
* (+420) 296.545.373 * http://www.suse.cz

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2003-05-28  Michal Ludvig  <mludvig@suse.cz>

	* x86-64-linux-tdep.c (x86_64_linux_sigcontext_addr):
	Correctly determine sigcontext address.

Index: x86-64-linux-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/x86-64-linux-tdep.c,v
retrieving revision 1.17.18.1
diff -u -p -c -r1.17.18.1 x86-64-linux-tdep.c
*** x86-64-linux-tdep.c	17 May 2003 15:09:15 -0000	1.17.18.1
--- x86-64-linux-tdep.c	28 May 2003 14:32:12 -0000
*************** x86_64_linux_sigcontext_addr (struct fra
*** 110,124 ****
    CORE_ADDR sp, ucontext_addr;
    char buf[8];
  
    frame_unwind_register (next_frame, SP_REGNUM, buf);
    sp = extract_unsigned_integer (buf, 8);
  
!   /* The sigcontext structure is part of the user context.  A pointer
!      to the user context is passed as the third argument to the signal
!      handler.  */
!   read_memory (sp + 16, buf, 8);
!   ucontext_addr = extract_unsigned_integer (buf, 8);
!   return ucontext_addr + X86_64_LINUX_UCONTEXT_SIGCONTEXT_OFFSET;
  }
  \f
  
--- 110,126 ----
    CORE_ADDR sp, ucontext_addr;
    char buf[8];
  
+   /* The sigcontext structure is part of the user context.  A pointer
+      to the user context is passed as the third argument to the signal
+      handler, i.e. in register RDX. Unfortunately RDX isn't a callee 
+      saved register and so it's unwound value isn't reliable. 
+      Fortunately struct ucontext is right above the return address from 
+      the signal handler, i.e. also unwound RSP points to the user context 
+      and we can use it instead of RDX.  */
    frame_unwind_register (next_frame, SP_REGNUM, buf);
    sp = extract_unsigned_integer (buf, 8);
  
!   return sp + X86_64_LINUX_UCONTEXT_SIGCONTEXT_OFFSET;
  }
  \f