RFR: 8263718: unused-result warning happens at os_linux.cpp
Yasumasa Suenaga
ysuenaga at openjdk.java.net
Fri Mar 19 08:50:40 UTC 2021
On Fri, 19 Mar 2021 06:09:16 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:
>>> > BTW this is not the first time we have had this issue with gcc making a
>>> > function deprecated and casting to void not fixing it. Unfortunately I
>>> > can't remember enough of the previous case's details to actually look it
>>> > up and see what we resolved to do. :(
>>>
>>> I found [JDK-6879689](https://bugs.openjdk.java.net/browse/JDK-6879689), and it fixed unused-return as following:
>>>
>>> https://github.com/openjdk/jdk/blob/9b5a9b61899cf649104c0ff70e14549f64a89561/src/hotspot/share/adlc/archDesc.cpp#L1082-L1083
>>>
>>> I prefer it rather than `(void)!`.
>>
>> Does that work in release builds too?
>
> 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)
-------------
PR: https://git.openjdk.java.net/jdk/pull/3042
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