Hi,
I'm having an issue with callgrind, and unfortunately I didn't find any
further information in the docs:
The instruction count is not what I expect and differs from single stepping
the binary.
I'm on x86_64-linux-gnu
I created a simple hello world example:
global _start
section .text
_start:
mov rax, 1 ; write(
mov rdi, 1 ; STDOUT_FILENO,
mov rsi, msg ; "Hello, world!\n",
mov rdx, msglen ; sizeof("Hello, world!\n")
syscall ; );
mov rax, 60 ; exit(
mov rdi, 0 ; EXIT_SUCCESS
syscall ; );
section .rodata
msg: db "Hello, world!", 10
msglen: equ $ - msg
Which has 8 instructions:
$ objdump -d hello
hello: file format elf64-x86-64
Disassembly of section .text:
0000000000401000 <_start>:
401000: b8 01 00 00 00 mov $0x1,%eax
401005: bf 01 00 00 00 mov $0x1,%edi
40100a: 48 be 00 20 40 00 00 movabs $0x402000,%rsi
401011: 00 00 00
401014: ba 0e 00 00 00 mov $0xe,%edx
401019: 0f 05 syscall
40101b: b8 3c 00 00 00 mov $0x3c,%eax
401020: bf 00 00 00 00 mov $0x0,%edi
401025: 0f 05 syscall
Running "valgrind --tool=callgrind ./hello" gives me only 5 instructions:
Hello, world!
==443229==
==443229== Events : Ir
==443229== Collected : 5
==443229==
==443229== I refs: 5
When I create a dump "valgrind --tool=callgrind --dump-instr=yes
--simulate-cache=yes --collect-jumps=yes ./hello"
and check the machie code out using KCachegrind:
# Ir Hex Assembly Instructions Source Position
40 1000 █ 20.00 b8 01 00 00 00 mov $0x1, %eax (unknown)
40 1005 █ 20.00 bf 01 00 00 00 mov $0x1, %edi (unknown)
40 100A █ 20.00 48 be 00 20 40 00 00 movabs $0x402000, %rsi
(unknown)
40 1011 20.00 00 00 00
40 1014 █ 20.00 ba 0e 00 00 00 mov $0xe, %edx (unknown)
40 1019 █ 20.00 0f 05 syscall (unknown)
40 101B b8 3c 00 00 00 mov $0x3c, %eax
40 1020 bf 00 00 00 00 mov $0x0, %edi
So it seems it doesn't capture the last syscall and doesn't count the last
two mov instructions.
I also checked with c files:
#include <stdio.h>
int main()
{
printf("Hello, world!\n");
return 0;
}
where valgrind gives me 139,482:
Hello, world!
==444986==
==444986== Events : Ir
==444986== Collected : 139482
==444986==
==444986== I refs: 139,482
whilst when I use fork, execv,
ptrace(PTRACE_TRACEME, 0, 0, 0)
ptrace(PTRACE_SINGLESTEP, pid, 0, 0)
to single step the binary, and count the steps, I get 126,042.
Am I missing something, or is callgrind counting something else than steps?
How can I make these numbers line up?
KR
Pepper
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