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

[Magnus Ihse Bursie]

On Sat, 20 Mar 2021 03:03:09 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>>> It would be good to confirm that the alloca is also being elided with
>>> clang and VS.
>> 
>> I debug'ed fastdebug JDK on Visual Studio, it seems to work `alloca()`:
>> 
>>   // Try to randomize the cache line index of hot stack frames.
>>   // This helps when threads of the same stack traces evict each other's
>>   // cache lines. The threads can be either from the same JVM instance, or
>>   // from different JVM instances. The benefit is especially true for
>>   // processors with hyperthreading technology.
>>   static int counter = 0;
>>   int pid = os::current_process_id();
>> 00007FFFDEBDCFF0  add         byte ptr [rbx-7A78FEF3h],cl  
>>   _alloca(((pid ^ counter++) & 7) * 128);
>> 00007FFFDEBDCFF6  add         byte ptr [rbx-2FCC2Fh],cl  
>> 00007FFFDEBDCFFC  ror         dword ptr [rcx-7A790AF3h],0  
>> 00007FFFDEBDD003  and         edx,7  
>> 00007FFFDEBDD006  shl         edx,7  
>> 00007FFFDEBDD009  mov         eax,edx  
>> 00007FFFDEBDD00B  lea         rcx,[rdx+0Fh]  
>> 00007FFFDEBDD00F  cmp         rcx,rax  
>> 00007FFFDEBDD012  ja          thread_native_entry+6Eh (07FFFDEBDD01Eh)  
>> 00007FFFDEBDD014  mov         rcx,0FFFFFFFFFFFFFF0h  
>> 00007FFFDEBDD01E  and         rcx,0FFFFFFFFFFFFFFF0h  
>> 00007FFFDEBDD022  mov         rax,rcx  
>> 00007FFFDEBDD025  call        __chkstk (07FFFDEEFF430h)  
>> 00007FFFDEBDD02A  sub         rsp,rcx  
>> 
>> `alloca()` is also in os_bsd.cpp and os_aix.cpp, but I cannot check them because I do not have them.
>
>> I debug'ed fastdebug JDK on Visual Studio, it seems to work `alloca()`:
> 
> I used cl.exe from Visual Studio 2019 (16.9.1). The generated code might be different by compiler version of course.

@YaSuenag The code generated by debug builds is often significantly different from release builds. You might want to check the latter instead to figure out if this has any effect on what we actually ship.

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