RFR: 8346193: CrashGCForDumpingJavaThread do not trigger expected crash build with clang17 [v4]

[Kim Barrett]

On Tue, 31 Dec 2024 10:41:23 GMT, SendaoYan <syan at openjdk.org> wrote:

>> Hi all,
>> Function `frame::oops_do_internal` in src/hotspot/share/runtime/frame.cpp assign value to a nullptr `char *t` and intended to cause jvm crash. But after the assignment the nullptr do not use anymore, so clang17 consider the `char *t` initialization and assignment is "dead code". This PR add `volatile` modifier to `char *t`, to make avoid clang do the "dead code" elimination. Risk is low.
>> 
>> Here is the example explain the "dead code" elimination.
>> 
>> 1. Without volatile modifier, clang will delete the "dead code" and cause no more Segmentation fault error by -O1.
>> 
>> 
>>> cat demo.c 
>> int main() { char *t = 0; *t = 'c'; return 0; }
>>> clang -O0 demo.c && ./a.out ; echo $?
>> Segmentation fault (core dumped)
>> 139
>>> clang -O1 demo.c && ./a.out ; echo $?
>> 0
>> 
>> 
>> 2. With volatile modifier, clang do not delete the "dead code" again and and the expected Segmentation fault occur by -O1.
>> 
>>> cat demo.c 
>> int main() { volatile char *t = 0; *t = 'c'; return 0; }
>>> clang -O0 demo.c && ./a.out ; echo $?
>> Segmentation fault (core dumped)
>> 139
>>> clang -O1 demo.c && ./a.out ; echo $?
>> Segmentation fault (core dumped)
>> 139
>
> SendaoYan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   use guarantee(!CrashGCForDumpingJavaThread, "") to generate jvm crash

src/hotspot/share/runtime/frame.cpp line 1166:

> 1164:   // simulate GC crash here to dump java thread in error report
> 1165:   if (CrashGCForDumpingJavaThread) {
> 1166:     guarantee(!CrashGCForDumpingJavaThread, "");

The surrounding `if` is no longer needed.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22757#discussion_r1900342411