为什么循环摘要在gprof的调用图输出中没有任何调用者?

gprof's documentation indicates :

index  % time    self  children called     name
----------------------------------------
                 1.77        0    1/1        main [2]
[3]     91.71    1.77        0    1+5    <cycle 1 as a whole> [3]
                 1.02        0    3          b <cycle 1> [4]
                 0.75        0    2          a <cycle 1> [5]
                    0        0    6/6        c [6]
----------------------------------------

<cycle as a whole> 条目中列出了所有呼叫者。它们都是这样的:

index  % time    self  children called             name
----------------------------------------------
[8]     65.6    259.55  5342.63  9334767+60122608 <cycle 2 as a whole> [8]
                133.28  2051.45  12043564+74015448    foo <cycle 2> [14]
                18.90   976.38   2379645              bar <cycle 2> [21]
...                                                                      
-----------------------------------------------

因为我的循环非常大,所以很难通过一个循环中的单个函数来跟踪调用者。

有人能告诉我为什么输出中缺少循环调用程序,以及如何让它们出现吗?