RFR: 8263718: unused-result warning happens at os_linux.cpp

[Thomas Stuefe]

On Fri, 19 Mar 2021 12:51:07 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>>> > I modified the code as following, it seems to work - we cannot see `alloca()`, however the stack is expanded.
>>> 
>>> Sorry but I'm not seeing where the stack actually gets expanded?
>> 
>> 0x00007ffff7154d87 <+87>:    sub    %rax,%rsp
>> 
>> I guess `%rax` seems to contain the result of `((pid ^ counter++) & 7) * 128`, then `alloca()` is replaced to `sub` for `%RSP`.
>> I saw the warning for this issue as `void* __builtin_alloca(long unsigned int)`. It might be it. We can just expand `%RSP` if we want to allocate buffer on the stack.
>
> I objdump'ed libjvm.so in JDK 16 Linux x64 from jdk.java.net , it also does not seem to expand the stack:
> 
> 0000000000bd8500 <thread_native_entry(Thread*)>:
>   bd8500:       55                      push   %rbp
>   bd8501:       48 89 e5                mov    %rsp,%rbp
>   bd8504:       41 56                   push   %r14
>   bd8506:       41 55                   push   %r13
>   bd8508:       49 89 fd                mov    %rdi,%r13
>   bd850b:       41 54                   push   %r12
>   bd850d:       53                      push   %rbx
>   bd850e:       e8 ad 1e 1a 00          callq  d7a3c0 <Thread::record_stack_base_and_size()>
>   bd8513:       e8 08 27 66 ff          callq  23ac20 <getpid at plt>
>   bd8518:       4c 89 ef                mov    %r13,%rdi
>   bd851b:       83 05 e6 a3 64 00 01    addl   $0x1,0x64a3e6(%rip)        # 1222908 <thread_native_entry(Thread*)::counter>
>   bd8522:       e8 39 1e 1a 00          callq  d7a360 <Thread::initialize_thread_current()>
>   bd8527:       49 8b 9d 70 02 00 00    mov    0x270(%r13),%rbx
>   bd852e:       31 c0                   xor    %eax,%eax
> 
> Result from `getpid()` will be stored into `%RAX`, however it is not used until `xor` at bd852e.
> And also I could not find out both `alloca()` call and manipulating `%RSP` at here.

Writing to the *end* of the allocated area may do the trick.

..Thomas

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PR: https://git.openjdk.java.net/jdk/pull/3042