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

[Yasumasa Suenaga]

On Sat, 20 Mar 2021 10:20:50 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>>> 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.

@magicus I checked assembly code of release build, it is similar to fastdebug build. The stack (RSP) is expanded.
(I confirmed it with Visual Studio 16.9.2 because I received update notification before your reply...)
  // 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();
00007FF80F4E10ED  mov         eax,dword ptr [_initial_pid (07FF80F9D8164h)]  
00007FF80F4E10F3  test        eax,eax  
00007FF80F4E10F5  jne         thread_native_entry+3Dh (07FF80F4E10FDh)  
00007FF80F4E10F7  call        qword ptr [__imp__getpid (07FF80F6EF748h)]  
  _alloca(((pid ^ counter++) & 7) * 128);
00007FF80F4E10FD  mov         ecx,dword ptr [counter (07FF80F9D8300h)]  
00007FF80F4E1103  mov         edx,ecx  
00007FF80F4E1105  xor         edx,eax  
00007FF80F4E1107  inc         ecx  
00007FF80F4E1109  mov         dword ptr [counter (07FF80F9D8300h)],ecx  
00007FF80F4E110F  and         edx,7  
00007FF80F4E1112  shl         edx,7  
00007FF80F4E1115  mov         eax,edx  
00007FF80F4E1117  lea         rcx,[rdx+0Fh]  
00007FF80F4E111B  cmp         rcx,rax  
00007FF80F4E111E  ja          thread_native_entry+6Ah (07FF80F4E112Ah)  
00007FF80F4E1120  mov         rcx,0FFFFFFFFFFFFFF0h  
00007FF80F4E112A  and         rcx,0FFFFFFFFFFFFFFF0h  
00007FF80F4E112E  mov         rax,rcx  
00007FF80F4E1131  call        __chkstk (07FF80F6ED370h)  
00007FF80F4E1136  sub         rsp,rcx

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