[glibc] hurd: document that gcc&gdb look at the trampoline code

[glibc] hurd: document that gcc&gdb look at the trampoline code

[Samuel Thibault]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=f112dcc506a6ec0aac5c34891736eec3c4f5dad6

commit f112dcc506a6ec0aac5c34891736eec3c4f5dad6
Author: Samuel Thibault <samuel.thibault@ens-lyon.org>
Date:   Mon Jun 8 14:41:57 2020 +0200

    hurd: document that gcc&gdb look at the trampoline code
    
    * sysdeps/mach/hurd/i386/trampoline.c (rpc_wait_trampoline): Document
    which gcc and gdb files look at the code of the trampoline.

Diff:
---
 sysdeps/mach/hurd/i386/trampoline.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sysdeps/mach/hurd/i386/trampoline.c b/sysdeps/mach/hurd/i386/trampoline.c
index e7eb316fce..8c7976919c 100644
--- a/sysdeps/mach/hurd/i386/trampoline.c
+++ b/sysdeps/mach/hurd/i386/trampoline.c
@@ -227,7 +227,11 @@ _hurd_setup_sighandler (struct hurd_sigstate *ss, __sighandler_t handler,
 }
 
 /* The trampoline code follows.  This used to be located inside
-   _hurd_setup_sighandler, but was optimized away by gcc 2.95.  */
+   _hurd_setup_sighandler, but was optimized away by gcc 2.95.
+
+   If you modify this, update
+   - in gcc: libgcc/config/i386/gnu-unwind.h x86_gnu_fallback_frame_state,
+   - in gdb: gdb/i386-gnu-tdep.c gnu_sigtramp_code.  */
 
 asm ("rpc_wait_trampoline:\n");
   /* This is the entry point when we have an RPC reply message to receive