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goruntimedefer-keyword

Is the `runtime.deferreturn` ever called in go defer

At the bottom of a Go func with defer, the assembly looks like this:

   0x000000000047e1a0 <+256>:   callq  *%rsi
   0x000000000047e1a2 <+258>:   movb   $0x0,0x7(%rsp)
   0x000000000047e1a7 <+263>:   mov    0x748(%rsp),%rdx
   0x000000000047e1af <+271>:   mov    (%rdx),%rbx
   0x000000000047e1b2 <+274>:   callq  *%rbx
   0x000000000047e1b4 <+276>:   mov    0x750(%rsp),%rbp
   0x000000000047e1bc <+284>:   add    $0x758,%rsp
   0x000000000047e1c3 <+291>:   retq
   0x000000000047e1c4 <+292>:   callq  0x42ea40 <runtime.deferreturn>
   0x000000000047e1c9 <+297>:   mov    0x750(%rsp),%rbp
   0x000000000047e1d1 <+305>:   add    $0x758,%rsp
   0x000000000047e1d8 <+312>:   retq
   0x000000000047e1d9 <+313>:   callq  0x458900 <runtime.morestack>

As we can see, there is a retq before call runtime.deferreturn. So, it seems that the deferreturn will never be called. Do I understand this right?

Solution

  • As we can see, there is a retq before call runtime.deferreturn. So, it seems that the deferreturn will never be called. Do I understand this right?

    No. The code may be used by the Go runtime, for example, while panicking.