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

[Yasumasa Suenaga]

On Fri, 19 Mar 2021 09:04:12 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>>> + static void* volatile _stack_pad = alloca(((pid ^ counter++) & 7) * 128);
>>> + if (_stack_pad != 0) {
>>> + ((char*)_stack_pad)[0] = 1;
>>> + }
>> 
>> I guess `_stack_pad` will be overwritten in each `threaad_native_entry()` call, so it might be elided.
>> I modified the code as following, it seems to work - we cannot see `alloca()`, however the stack is expanded.
>> 
>> diff --git a/src/hotspot/os/linux/os_linux.cpp b/src/hotspot/os/linux/os_linux.cpp
>> index 5af63befb58..bdb2dc89615 100644
>> --- a/src/hotspot/os/linux/os_linux.cpp
>> +++ b/src/hotspot/os/linux/os_linux.cpp
>> @@ -665,7 +665,8 @@ static void *thread_native_entry(Thread *thread) {
>>    // processors with hyperthreading technology.
>>    static int counter = 0;
>>    int pid = os::current_process_id();
>> -  alloca(((pid ^ counter++) & 7) * 128);
>> +  void *ptr = alloca(((pid ^ counter++) & 7) * 128);
>> +  ((char *)ptr)[0] = 1;
>> 
>>    thread->initialize_thread_current();
>> 
>> 659       thread->record_stack_base_and_size();
>>    0x00007ffff7154d53 <+35>:    call   0x7ffff75b3a80 <_ZN6Thread26record_stack_base_and_sizeEv>
>> 
>> 660
>> 661       // Try to randomize the cache line index of hot stack frames.
>> 662       // This helps when threads of the same stack traces evict each other's
>> 663       // cache lines. The threads can be either from the same JVM instance, or
>> 664       // from different JVM instances. The benefit is especially true for
>> 665       // processors with hyperthreading technology.
>> 666       static int counter = 0;
>> 
>> 667       int pid = os::current_process_id();
>> 
>> 668       void *ptr = alloca(((pid ^ counter++) & 7) * 128);
>>    0x00007ffff7154d63 <+51>:    mov    0xc1daff(%rip),%eax        # 0x7ffff7d72868 <_ZZL19thread_native_entryP6ThreadE7counter>
>>    0x00007ffff7154d69 <+57>:    lea    0x1(%rax),%edx
>>    0x00007ffff7154d6c <+60>:    xor    %r8d,%eax
>>    0x00007ffff7154d6f <+63>:    shl    $0x7,%rax
>>    0x00007ffff7154d73 <+67>:    mov    %edx,0xc1daef(%rip)        # 0x7ffff7d72868 <_ZZL19thread_native_entryP6ThreadE7counter>
>>    0x00007ffff7154d79 <+73>:    and    $0x380,%eax
>>    0x00007ffff7154d7e <+78>:    add    $0x17,%rax
>>    0x00007ffff7154d82 <+82>:    and    $0x7f0,%eax
>>    0x00007ffff7154d87 <+87>:    sub    %rax,%rsp
>>    0x00007ffff7154d8a <+90>:    lea    0xf(%rsp),%rax
>>    0x00007ffff7154d8f <+95>:    and    $0xfffffffffffffff0,%rax
>> 
>> 669       ((char *)ptr)[0] = 1;
>>    0x00007ffff7154d93 <+99>:    movb   $0x1,(%rax)
>
>> > 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.

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