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Some compilers (for example clang-15) optimize bpf_testmod_loop_test and remove the loop: gcc version (gdb) disassemble bpf_testmod_loop_test Dump of assembler code for function bpf_testmod_loop_test: 0x0000000000000570 <+0>: callq 0x575 <bpf_testmod_loop_test+5> 0x0000000000000575 <+5>: xor %eax,%eax 0x0000000000000577 <+7>: test %edi,%edi 0x0000000000000579 <+9>: jle 0x587 <bpf_testmod_loop_test+23> 0x000000000000057b <+11>: xor %edx,%edx 0x000000000000057d <+13>: add %edx,%eax 0x000000000000057f <+15>: add $0x1,%edx 0x0000000000000582 <+18>: cmp %edx,%edi 0x0000000000000584 <+20>: jne 0x57d <bpf_testmod_loop_test+13> 0x0000000000000586 <+22>: retq 0x0000000000000587 <+23>: retq clang-15 version (gdb) disassemble bpf_testmod_loop_test Dump of assembler code for function bpf_testmod_loop_test: 0x0000000000000450 <+0>: nopl 0x0(%rax,%rax,1) 0x0000000000000455 <+5>: test %edi,%edi 0x0000000000000457 <+7>: jle 0x46b <bpf_testmod_loop_test+27> 0x0000000000000459 <+9>: lea -0x1(%rdi),%eax 0x000000000000045c <+12>: lea -0x2(%rdi),%ecx 0x000000000000045f <+15>: imul %rax,%rcx 0x0000000000000463 <+19>: shr %rcx 0x0000000000000466 <+22>: lea -0x1(%rdi,%rcx,1),%eax 0x000000000000046a <+26>: retq 0x000000000000046b <+27>: xor %eax,%eax 0x000000000000046d <+29>: retq Note: The jne instruction is removed in clang-15 version. Force the compile to keep the loop by making sum volatile. Signed-off-by: Song Liu <song@kernel.org> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20230412210423.900851-4-song@kernel.org
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.