Avoid some GCC 10.X warnings in HotSpot
Koichi Sakata
sakatakui at oss.nttdata.com
Fri Jun 19 01:59:09 UTC 2020
Hi David,
> This is in relation to the hotspot part as these issues need to be
> handled separately. I have filed:
>
> https://bugs.openjdk.java.net/browse/JDK-8247818
Thank you, David.I have something to ask you.
Should I send only the other part of the patch (i.e. NetworkInterface.c
and k_standard.c) to core-lib ML again? I've sent the whole one to
core-lib before.
> I'm not really clear on the warning here but this is an area where we
> trick the compiler somewhat. The _body[] is declared with a size of 2,
> but when we allocate Symbols we allocate sufficient memory for _body to
> contain the entire symbol.
>
> That said I'm struggling to see how we allocate the additional space
> needed for the _hash_and_refcount and _length fields ???
I was thinking exactly the same thing. I've learned a lot from Ioi's
explanation.
> The check for length==0 introduces more overhead than just always
> setting _body[0]=0, so there is no need to add it.
I understood that clearly. Thank you for teaching me.
Thanks,
Koichi
On 2020/06/18 10:56, David Holmes wrote:
> Hi Koichi,
>
> This is in relation to the hotspot part as these issues need to be
> handled separately. I have filed:
>
> https://bugs.openjdk.java.net/browse/JDK-8247818
>
> On 18/06/2020 8:46 am, Koichi Sakata wrote:
>> Hi all,
>>
>> I tried to build OpenJDK fastdebug with GCC 10.1 on Ubuntu 18.04, but
>> I saw some compiler warnings as follows:
>>
>> In file included from
>> /home/jyukutyo/code/jdk/src/hotspot/share/classfile/systemDictionary.hpp:31,
>>
>> from
>> /home/jyukutyo/code/jdk/src/hotspot/share/classfile/javaClasses.hpp:28,
>> from
>> /home/jyukutyo/code/jdk/src/hotspot/share/precompiled/precompiled.hpp:35:
>> In member function 'void Symbol::byte_at_put(int, u1)',
>> inlined from 'Symbol::Symbol(const u1*, int, int)' at
>> /home/jyukutyo/code/jdk/src/hotspot/share/oops/symbol.cpp:55:16:
>> /home/jyukutyo/code/jdk/src/hotspot/share/oops/symbol.hpp:130:18:
>> error: writing 1 byte into a region of size 0
>> [-Werror=stringop-overflow=]
>> 130 | _body[index] = value;
>> | ~~~~~~~~~~~~~^~~~~~~
>
> I'm not really clear on the warning here but this is an area where we
> trick the compiler somewhat. The _body[] is declared with a size of 2,
> but when we allocate Symbols we allocate sufficient memory for _body to
> contain the entire symbol.
>
> That said I'm struggling to see how we allocate the additional space
> needed for the _hash_and_refcount and _length fields ???
>
>> I can resolve them with the following patch. I believe it fixes those
>> potential bugs, so I'd like to contribute it.
>> (Our company has signed OCA.)
>>
>> Thanks,
>> Koichi
>>
>> ===== PATCH =====
>> diff -r 20d92fe3ac52 src/hotspot/share/oops/symbol.cpp
>> --- a/src/hotspot/share/oops/symbol.cpp Tue Jun 16 03:16:41 2020 +0000
>> +++ b/src/hotspot/share/oops/symbol.cpp Thu Jun 18 07:08:50 2020 +0900
>> @@ -50,9 +50,10 @@
>> Symbol::Symbol(const u1* name, int length, int refcount) {
>> _hash_and_refcount = pack_hash_and_refcount((short)os::random(),
>> refcount);
>> _length = length;
>> - _body[0] = 0; // in case length == 0
>> - for (int i = 0; i < length; i++) {
>> - byte_at_put(i, name[i]);
>> + if (length == 0) {
>> + _body[0] = 0;
>
> The check for length==0 introduces more overhead than just always
> setting _body[0]=0, so there is no need to add it.
>
>> + } else {
>> + memcpy(_body, name, length);
>> }
>> }
>
> So you are replacing byte_at_put with a memcpy call. On the surface that
> seems reasonable, but I have to wonder why we were using the loop in the
> first place. It may just be historical or it may relate to an alignment
> issue, or something else. Hopefully someone else (e.g. Coleen :) ) can
> shed more light here.
>
> Thanks,
> David
> -----
>
>> diff -r 20d92fe3ac52 src/hotspot/share/oops/symbol.hpp
>> --- a/src/hotspot/share/oops/symbol.hpp Tue Jun 16 03:16:41 2020 +0000
>> +++ b/src/hotspot/share/oops/symbol.hpp Thu Jun 18 07:08:50 2020 +0900
>> @@ -125,11 +125,6 @@
>> return (int)heap_word_size(byte_size(length));
>> }
>>
>> - void byte_at_put(int index, u1 value) {
>> - assert(index >=0 && index < length(), "symbol index overflow");
>> - _body[index] = value;
>> - }
>> -
>> Symbol(const u1* name, int length, int refcount);
>> void* operator new(size_t size, int len) throw();
>> void* operator new(size_t size, int len, Arena* arena) throw();
>
>
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