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<title>Debugging with GDB: Tail Call Frames</title>

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<a name="Tail-Call-Frames"></a>
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<hr>
<a name="Tail-Call-Frames-1"></a>
<h3 class="section">11.2 Tail Call Frames</h3>
<a name="index-tail-call-frames_002c-debugging"></a>

<p>Function <code>B</code> can call function <code>C</code> in its very last statement.  In
unoptimized compilation the call of <code>C</code> is immediately followed by return
instruction at the end of <code>B</code> code.  Optimizing compiler may replace the
call and return in function <code>B</code> into one jump to function <code>C</code>
instead.  Such use of a jump instruction is called <em>tail call</em>.
</p>
<p>During execution of function <code>C</code>, there will be no indication in the
function call stack frames that it was tail-called from <code>B</code>.  If function
<code>A</code> regularly calls function <code>B</code> which tail-calls function <code>C</code>,
then <small>GDB</small> will see <code>A</code> as the caller of <code>C</code>.  However, in
some cases <small>GDB</small> can determine that <code>C</code> was tail-called from
<code>B</code>, and it will then create fictitious call frame for that, with the
return address set up as if <code>B</code> called <code>C</code> normally.
</p>
<p>This functionality is currently supported only by DWARF 2 debugging format and
the compiler has to produce &lsquo;<samp>DW_TAG_GNU_call_site</samp>&rsquo; tags.  With
<small>GCC</small>, you need to specify <samp>-O -g</samp> during compilation, to get
this information.
</p>
<p><kbd>info frame</kbd> command (see <a href="Frame-Info.html#Frame-Info">Frame Info</a>) will indicate the tail call frame
kind by text <code>tail call frame</code> such as in this sample <small>GDB</small> output:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) x/i $pc - 2
   0x40066b &lt;b(int, double)+11&gt;: jmp 0x400640 &lt;c(int, double)&gt;
(gdb) info frame
Stack level 1, frame at 0x7fffffffda30:
 rip = 0x40066d in b (amd64-entry-value.cc:59); saved rip 0x4004c5
 tail call frame, caller of frame at 0x7fffffffda30
 source language c++.
 Arglist at unknown address.
 Locals at unknown address, Previous frame's sp is 0x7fffffffda30
</pre></div>

<p>The detection of all the possible code path executions can find them ambiguous.
There is no execution history stored (possible <a href="Reverse-Execution.html#Reverse-Execution">Reverse Execution</a> is never
used for this purpose) and the last known caller could have reached the known
callee by multiple different jump sequences.  In such case <small>GDB</small> still
tries to show at least all the unambiguous top tail callers and all the
unambiguous bottom tail calees, if any.
</p>
<dl compact="compact">
<dd><a name="set-debug-entry_002dvalues"></a></dd>
<dt><code>set debug entry-values</code></dt>
<dd><a name="index-set-debug-entry_002dvalues"></a>
<p>When set to on, enables printing of analysis messages for both frame argument
values at function entry and tail calls.  It will show all the possible valid
tail calls code paths it has considered.  It will also print the intersection
of them with the final unambiguous (possibly partial or even empty) code path
result.
</p>
</dd>
<dt><code>show debug entry-values</code></dt>
<dd><a name="index-show-debug-entry_002dvalues"></a>
<p>Show the current state of analysis messages printing for both frame argument
values at function entry and tail calls.
</p></dd>
</dl>

<p>The analysis messages for tail calls can for example show why the virtual tail
call frame for function <code>c</code> has not been recognized (due to the indirect
reference by variable <code>x</code>):
</p>
<div class="smallexample">
<pre class="smallexample">static void __attribute__((noinline, noclone)) c (void);
void (*x) (void) = c;
static void __attribute__((noinline, noclone)) a (void) { x++; }
static void __attribute__((noinline, noclone)) c (void) { a (); }
int main (void) { x (); return 0; }

Breakpoint 1, DW_OP_GNU_entry_value resolving cannot find
DW_TAG_GNU_call_site 0x40039a in main
a () at t.c:3
3	static void __attribute__((noinline, noclone)) a (void) { x++; }
(gdb) bt
#0  a () at t.c:3
#1  0x000000000040039a in main () at t.c:5
</pre></div>

<p>Another possibility is an ambiguous virtual tail call frames resolution:
</p>
<div class="smallexample">
<pre class="smallexample">int i;
static void __attribute__((noinline, noclone)) f (void) { i++; }
static void __attribute__((noinline, noclone)) e (void) { f (); }
static void __attribute__((noinline, noclone)) d (void) { f (); }
static void __attribute__((noinline, noclone)) c (void) { d (); }
static void __attribute__((noinline, noclone)) b (void)
{ if (i) c (); else e (); }
static void __attribute__((noinline, noclone)) a (void) { b (); }
int main (void) { a (); return 0; }

tailcall: initial: 0x4004d2(a) 0x4004ce(b) 0x4004b2(c) 0x4004a2(d)
tailcall: compare: 0x4004d2(a) 0x4004cc(b) 0x400492(e)
tailcall: reduced: 0x4004d2(a) |
(gdb) bt
#0  f () at t.c:2
#1  0x00000000004004d2 in a () at t.c:8
#2  0x0000000000400395 in main () at t.c:9
</pre></div>


<p>Frames #0 and #2 are real, #1 is a virtual tail call frame.
The code can have possible execution paths <code>main&rarr;a&rarr;b&rarr;c&rarr;d&rarr;f</code> or
<code>main&rarr;a&rarr;b&rarr;e&rarr;f</code>, <small>GDB</small> cannot find which one from the inferior state.
</p>
<p><code>initial:</code> state shows some random possible calling sequence <small>GDB</small>
has found.  It then finds another possible calling sequcen - that one is
prefixed by <code>compare:</code>.  The non-ambiguous intersection of these two is
printed as the <code>reduced:</code> calling sequence.  That one could have many
futher <code>compare:</code> and <code>reduced:</code> statements as long as there remain
any non-ambiguous sequence entries.
</p>
<p>For the frame of function <code>b</code> in both cases there are different possible
<code>$pc</code> values (<code>0x4004cc</code> or <code>0x4004ce</code>), therefore this frame is
also ambigous.  The only non-ambiguous frame is the one for function <code>a</code>,
therefore this one is displayed to the user while the ambiguous frames are
omitted.
</p>
<p>There can be also reasons why printing of frame argument values at function
entry may fail:
</p>
<div class="smallexample">
<pre class="smallexample">int v;
static void __attribute__((noinline, noclone)) c (int i) { v++; }
static void __attribute__((noinline, noclone)) a (int i);
static void __attribute__((noinline, noclone)) b (int i) { a (i); }
static void __attribute__((noinline, noclone)) a (int i)
{ if (i) b (i - 1); else c (0); }
int main (void) { a (5); return 0; }

(gdb) bt
#0  c (i=i@entry=0) at t.c:2
#1  0x0000000000400428 in a (DW_OP_GNU_entry_value resolving has found
function &quot;a&quot; at 0x400420 can call itself via tail calls
i=&lt;optimized out&gt;) at t.c:6
#2  0x000000000040036e in main () at t.c:7
</pre></div>

<p><small>GDB</small> cannot find out from the inferior state if and how many times did
function <code>a</code> call itself (via function <code>b</code>) as these calls would be
tail calls.  Such tail calls would modify thue <code>i</code> variable, therefore
<small>GDB</small> cannot be sure the value it knows would be right - <small>GDB</small>
prints <code>&lt;optimized out&gt;</code> instead.
</p>
<hr>
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