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

[Yasumasa Suenaga]

On Fri, 19 Mar 2021 08:47:39 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> I checked `alloca()` call on Fedora 33 x86_64 and Alpine 3.13 x86_64. Both of them seem to elide `alloca()`.
>> (It is fastdebug build - slow debug might be different, but production build might be same)
>> 
>> 659       thread->record_stack_base_and_size();
>>    0x00007ffff7154d44 <+20>:    call   0x7ffff75b3a20 <_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       alloca(((pid ^ counter++) & 7) * 128);
>>    0x00007ffff7154d51 <+33>:    addl   $0x1,0xc1db10(%rip)        # 0x7ffff7d72868 <_ZZL19thread_native_entryP6ThreadE7counter>
>> 
>> 669
>> 670       thread->initialize_thread_current();
>> => 0x00007ffff7154d4e <+30>:    mov    %r13,%rdi
>>    0x00007ffff7154d58 <+40>:    call   0x7ffff75b3800 <_ZN6Thread25initialize_thread_currentEv>
>> 
>>> I prefer it rather than (void)!.
>>>
>>> Does that work in release builds too?
>> 
>> It will not work as David said :) we need to use `(void)!` if we should left it.
>
>> + 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.

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