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

[David Holmes]

On 20/03/2021 10:37 pm, Yasumasa Suenaga wrote:
> 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.

Thanks for confirming - I was going to make the same point about needing 
to check product build.

Now we need someone who can check clang output.

The rough benchmarking I did did not show any benefit to using the 
alloca on Linux. So I think we can safely say it can go on Linux.

The benchmarking on Windows, with alloca removed, did not show any 
degradation in performance. So I think we can asafely say it is okay to 
remove on Windows too.

The benchmarking on macOS show no degradation either, but until I know 
whether the alloca was being elided by clang that may not mean anything. 
If it is being elided we need to restore it and then check performance 
again. But I need to know the trick that worked on gcc will also work 
for clang.

David
-----

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