linux-kernel - Re: [PATCH] Fix jump parsing for C++ code.

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Date:   Thu, 11 Feb 2021 14:16:06 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Martin Liška <mliska@...e.cz>
Cc:     linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] Fix jump parsing for C++ code.

Em Thu, Feb 11, 2021 at 01:37:55PM +0100, Martin Liška escreveu:
> Considering the following testcase:
> 
> int
> foo(int a, int b)
> {
>   for (unsigned i = 0; i < 1000000000; i++)
>     a += b;
>   return a;
> }
> 
> int main()
> {
>   foo (3, 4);
>   return 0;
> }
> 
> perf annotate displays:
>  86.52 │40055e: → ja   40056c <foo(int, int)+0x26>
>  13.37 │400560:   mov  -0x18(%rbp),%eax
>        │400563:   add  %eax,-0x14(%rbp)
>        │400566:   addl $0x1,-0x4(%rbp)
>   0.11 │40056a: → jmp  400557 <foo(int, int)+0x11>
>        │40056c:   mov  -0x14(%rbp),%eax
>        │40056f:   pop  %rbp
> 
> and the 'ja 40056c' does not link to the location in the function.
> It's caused by fact that comma is wrongly parsed, it's part
> of function signature.
> 
> With my patch I see:
> 
>  86.52 │   ┌──ja   26
>  13.37 │   │  mov  -0x18(%rbp),%eax
>        │   │  add  %eax,-0x14(%rbp)
>        │   │  addl $0x1,-0x4(%rbp)
>   0.11 │   │↑ jmp  11
>        │26:└─→mov  -0x14(%rbp),%eax
> 
> and 'o' output prints:
>  86.52 │4005┌── ↓ ja   40056c <foo(int, int)+0x26>
>  13.37 │4005│0:   mov  -0x18(%rbp),%eax
>        │4005│3:   add  %eax,-0x14(%rbp)
>        │4005│6:   addl $0x1,-0x4(%rbp)
>   0.11 │4005│a: ↑ jmp  400557 <foo(int, int)+0x11>
>        │4005└─→   mov  -0x14(%rbp),%eax

So, before your patch, this is what I am seeing:

  [acme@...e c]$ cat cpp_args_annotate.c
  int
  foo(int a, int b)
  {
     for (unsigned i = 0; i < 1000000000; i++)
       a += b;
     return a;
  }
  
  int main()
  {
     foo (3, 4);
     return 0;
  }
  [acme@...e c]$ gcc --version |& head -1
  gcc (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9)
  [acme@...e c]$ gcc -g cpp_args_annotate.c -o cpp_args_annotate
  [acme@...e c]$ perf record ./cpp_args_annotate
  [ perf record: Woken up 2 times to write data ]
  [ perf record: Captured and wrote 0.275 MB perf.data (7188 samples) ]
  [acme@...e c]$ perf annotate --stdio2 foo
  Samples: 7K of event 'cycles:u', 4000 Hz, Event count (approx.): 7468429289, [percent: local period]
  foo() /home/acme/c/cpp_args_annotate
  Percent
              0000000000401106 <foo>:
              foo():
              int
              foo(int a, int b)
              {
                push %rbp
                mov  %rsp,%rbp
                mov  %edi,-0x14(%rbp)
                mov  %esi,-0x18(%rbp)
              for (unsigned i = 0; i < 1000000000; i++)
                movl $0x0,-0x4(%rbp)
              ↓ jmp  1d
              a += b;
   13.45  13:   mov  -0x18(%rbp),%eax
                add  %eax,-0x14(%rbp)
              for (unsigned i = 0; i < 1000000000; i++)
                addl $0x1,-0x4(%rbp)
    0.09  1d:   cmpl $0x3b9ac9ff,-0x4(%rbp)
   86.46      ↑ jbe  13
              return a;
                mov  -0x14(%rbp),%eax
              }
                pop  %rbp
              ← retq
  [acme@...e c]$


Ok, now I see:

[acme@...e c]$ g++ -g cpp_args_annotate.c -o cpp_args_annotate
[acme@...e c]$ perf record ./cpp_args_annotate
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.268 MB perf.data (6976 samples) ]
[acme@...e c]$ perf annotate --stdio2 foo
Samples: 6K of event 'cycles:u', 4000 Hz, Event count (approx.): 7380681761, [percent: local period]
foo() /home/acme/c/cpp_args_annotate
Percent
            0000000000401106 <foo(int, int)>:
            foo(int, int):
            int
            foo(int a, int b)
            {
              push %rbp
              mov  %rsp,%rbp
              mov  %edi,-0x14(%rbp)
              mov  %esi,-0x18(%rbp)
            for (unsigned i = 0; i < 1000000000; i++)
              movl $0x0,-0x4(%rbp)
              cmpl $0x3b9ac9ff,-0x4(%rbp)
 86.53      → ja   40112c <foo(int, int)+0x26>
            a += b;
 13.32        mov  -0x18(%rbp),%eax
  0.00        add  %eax,-0x14(%rbp)
            for (unsigned i = 0; i < 1000000000; i++)
              addl $0x1,-0x4(%rbp)
  0.15      → jmp  401117 <foo(int, int)+0x11>
            return a;
              mov  -0x14(%rbp),%eax
            }
              pop  %rbp
            ← retq
[acme@...e c]$
  
Ok, continuing the test...

- Arnaldo