RFR (S): 8245833: crash_with_sigfpe uses pthread_kill(SIGFPE) on macOS

[gerard ziemski]

On 5/29/20 11:52 AM, gerard ziemski wrote:
> hi David,
>
> Thank you for the review.
>
> On 5/28/20 7:03 PM, David Holmes wrote:
>> Hi Gerard,
>>
>> On 29/05/2020 3:34 am, gerard ziemski wrote:
>>> hi all,
>>>
>>> Please review this small and simple fix, that implements 
>>> crash_with_sigfpe() in a way that causes an actual crash on macOS, 
>>> so it doesn't need to fallback that uses pthread_kill()
>>>
>>> bug link at https://bugs.openjdk.java.net/browse/JDK-8245833
>>> webrev at http://cr.openjdk.java.net/~gziemski/8245833_rev1
>>> passes Mach5 hs_tier1,2,3,4,5
>>
>> Fix looks fine.
>>
>> So presumably this old code:
>>
>>    volatile int x = 0;
>>    volatile int y = 1/x;
>>
>> is actually elided by the compiler when we build for macOS?
>
> It's not exactly elided, since the compiler still generates assembly 
> for that code, but I noticed that while normally the compiler would 
> complain about the unused "y", in this case it does not, so it 
> probably optimizes it without actually performing the division by 
> zero, due to some compiler flag we are using (I don't know which one 
> makes the difference here), i.e.:
>
>   volatile int x = 0;
>   volatile int y = 1/x;
>
>   xorl  %eax, %eax
>   .loc  37 1751 16              ## 
> open/src/hotspot/share/utilities/vmError.cpp:1751:16
>   movl  %eax, -88(%rbp)
>   .loc  37 1752 22              ## 
> open/src/hotspot/share/utilities/vmError.cpp:1752:22
>   movl  -88(%rbp), %ecx
>   .loc  37 1752 21 is_stmt 0    ## 
> open/src/hotspot/share/utilities/vmError.cpp:1752:21
>   leal  1(%rcx), %edx
>   cmpl  $3, %edx
>   cmovael  %eax, %ecx
>   .loc  37 1752 16              ## 
> open/src/hotspot/share/utilities/vmError.cpp:1752:16
>   movl  %ecx, -152(%rbp)
>
> I don't see division instruction here, however for:
>
>  sigfpe_int = sigfpe_int/sigfpe_int;
>
>   .loc  37 1751 16              ## 
> open/src/hotspot/share/utilities/vmError.cpp:1751:16
>   movl  _sigfpe_int(%rip), %eax
>   .loc  37 1751 26 is_stmt 0    ## 
> open/src/hotspot/share/utilities/vmError.cpp:1751:26
>   cltd
>   idivl  _sigfpe_int(%rip)
>   .loc  37 1751 14              ## 
> open/src/hotspot/share/utilities/vmError.cpp:1751:14
>   movl  %eax, _sigfpe_int(%rip)
>
> we see the "idivl" instruction in the assembly.
>
> For reference, a simple C test case with standard compiler flags 
> produces:
>
>  volatile int x = 0;
>  volatile int y = 1/x;
>
>  .loc  1 439 16                ## hello/main.cpp:439:16
>  movl  $0, -20(%rbp)
>  .loc  1 440 22                ## hello/main.cpp:440:22
>  movl  -20(%rbp), %ecx
>  .loc  1 440 21 is_stmt 0      ## hello/main.cpp:440:21
>  movl  $1, %edx
>  movl  %eax, -28(%rbp)         ## 4-byte Spill
>  movl  %edx, %eax
>  cltd
>  idivl  %ecx
>  .loc  1 440 16                ## hello/main.cpp:440:16
>  movl  %eax, -24(%rbp)
>  .loc  1 441 3 is_stmt 1       ## hello/main.cpp:441:3
>
> which also has the "idivl" instruction and also crashes, so it must be 
> one of our compiler flags that optimizes the unused variable?

It must be more than optimizing an unused variable, because even when I 
do use the "y" (print its vale out - it's 0) the code still will not 
crash. Some other optimization is at play here...


cheers