RFR: 8331854: ubsan: copy.hpp:218:10: runtime error: addition of unsigned offset to 0x7fc2b4024518 overflowed to 0x7fc2b4024510

[Dean Long]

On Tue, 4 Jun 2024 21:15:07 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> When building with ubsan, we see a number of overflows at this code location :
>> 
>> /jdk/src/hotspot/share/utilities/copy.hpp:218:10: runtime error: addition of unsigned offset to 0x7fc2b4024518 overflowed to 0x7fc2b4024510
>>     #0 0x10b70896d in Copy::conjoint_words_to_higher(HeapWordImpl* const*, HeapWordImpl**, unsigned long) copy.hpp:218
>>     #1 0x10c4f78f1 in Node_Array::insert(unsigned int, Node*) node.cpp:2783
>>     #2 0x10b8a1386 in Block::insert_node(Node*, unsigned int) block.hpp:134
>>     #3 0x10c556630 in PhaseOutput::fill_buffer(C2_MacroAssembler*, unsigned int*) output.cpp:1792
>>     #4 0x10c552f6b in PhaseOutput::Output() output.cpp:367
>>     #5 0x10b9ba859 in Compile::Code_Gen() compile.cpp:3035
>>     #6 0x10b9b7cb1 in Compile::Compile(ciEnv*, ciMethod*, int, Options, DirectiveSet*) compile.cpp:896
>>     #7 0x10b859912 in C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*) c2compiler.cpp:142
>>     #8 0x10b9dd4f1 in CompileBroker::invoke_compiler_on_method(CompileTask*) compileBroker.cpp:2305
>>     #9 0x10b9dc345 in CompileBroker::compiler_thread_loop() compileBroker.cpp:1963
>>     #10 0x10bfd5ebf in JavaThread::thread_main_inner() javaThread.cpp:760
>>     #11 0x10bfd5b62 in JavaThread::run() javaThread.cpp:745
>>     #12 0x10c9310d6 in Thread::call_run() thread.cpp:221
>>     #13 0x10c53ece4 in thread_native_entry(Thread*) os_bsd.cpp:598
>
> I wonder if using for-loop works here. (Since #iterations is known, for-loop seems more natural.)
> 
> 
>   for (size_t i = 0; i < count; ++i) {
>     to[count - 1 - i] = from[count - 1 - i];
>   }

@albertnetymk, that rewrite seems fine, but at least to me it's less obvious what it does, and I wonder if the C++ compiler generates equivalent code.

If we are going to change the loop, here's another alternative:

    from += count;
    to   += count;
    while (count-- > 0) {
      *--to = *--from;
}

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PR Comment: https://git.openjdk.org/jdk/pull/19541#issuecomment-2148819381