[binutils-gdb] btrace: preserve call stack on function switch

[Markus Metzger]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=4c2c7ac69d7318d61a5c0e4f5ddcee1c1597f4e0

commit 4c2c7ac69d7318d61a5c0e4f5ddcee1c1597f4e0
Author: Markus Metzger <markus.t.metzger@intel.com>
Date:   Mon Jan 30 09:16:27 2017 +0100

    btrace: preserve call stack on function switch
    
    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.
    
    gdb/
    	* btrace.c (ftrace_new_switch): Preserve up link and flags.

Diff:
---
 gdb/ChangeLog | 4 ++++
 gdb/btrace.c  | 6 ++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e274191..ae33435 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2017-02-14  Markus Metzger  <markus.t.metzger@intel.com>
+
+	* btrace.c (ftrace_new_switch): Preserve up link and flags.
+
 2017-02-13  Luis Machado  <lgustavo@codesourcery.com>
 
 	* symfile (_initialize_symfile): Add usage text to the load command's
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");