[PATCH] btrace: preserve call stack on function switch

[Markus Metzger <markus.t.metzger@intel.com>]

On 64-bit FC25, the _dl_runtime_resolve function uses a conditional branch to
'call' a particular variant optimized for that system:

    (gdb) disas _dl_runtime_resolve_avx_opt
    Dump of assembler code for function _dl_runtime_resolve_avx_opt:
       0x00007ffff7deeb60 <+0>: push   %rax
       0x00007ffff7deeb61 <+1>: push   %rcx
       0x00007ffff7deeb62 <+2>: push   %rdx
       0x00007ffff7deeb63 <+3>: mov    $0x1,%ecx
       0x00007ffff7deeb68 <+8>: xgetbv
       0x00007ffff7deeb6b <+11>: mov    %eax,%r11d
       0x00007ffff7deeb6e <+14>: pop    %rdx
       0x00007ffff7deeb6f <+15>: pop    %rcx
       0x00007ffff7deeb70 <+16>: pop    %rax
       0x00007ffff7deeb71 <+17>: and    $0x4,%r11d
       0x00007ffff7deeb75 <+21>: bnd je 0x7ffff7def4a0 <_dl_runtime_resolve_sse_vex>
    End of assembler dump.

When computing the function-level trace, btrace treats this as a switch from
_dl_runtime_resolve_avx_opt to _dl_runtime_resolve_sse_vex.  We know that we
switched functions but we can't really say in which caller/callee relationship
those two functions are.

In addition to preserving the indentaion level, also preserve the caller
information.  This is a heuristic since we don't really know.  But at least in
this case, this seems to be the right thing to do.

This fixes a fail in gdb.btrace/rn-dl-bind.exp on 64-bit FC25.

2017-02-01  Markus Metzger  <markus.t.metzger@intel.com>

	* btrace.c (ftrace_new_switch): Preserve up link and flags.
---
 gdb/btrace.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/gdb/btrace.c b/gdb/btrace.c
index 6d621e4..ddf6692 100644
--- a/gdb/btrace.c
+++ b/gdb/btrace.c
@@ -448,9 +448,11 @@ ftrace_new_switch (struct btrace_function *prev,
 {
   struct btrace_function *bfun;
 
-  /* This is an unexplained function switch.  The call stack will likely
-     be wrong at this point.  */
+  /* This is an unexplained function switch.  We can't really be sure about the
+     call stack, yet the best I can think of right now is to preserve it.  */
   bfun = ftrace_new_function (prev, mfun, fun);
+  bfun->up = prev->up;
+  bfun->flags = prev->flags;
 
   ftrace_debug (bfun, "new switch");
 
-- 
1.8.3.1