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Thu Jul 26 22:39:28 JST 2018
Fork#38417: Memory load reordering optimization (-O2) bug
Open Date: 2018-07-26 22:39
Last Update: 2018-07-26 22:39
URL for this Ticket:
https://osdn.net//projects/mingw/ticket/38417
RSS feed for this Ticket:
https://osdn.net/ticket/ticket_rss.php?group_id=3917&tid=38417
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Last Changes/Comment on this Ticket:
2018-07-26 22:39 Updated by: nik-ponomarev
* New Ticket "Memory load reordering optimization (-O2) bug" created
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Ticket Status:
Reporter: nik-ponomarev
Owner: (None)
Type: Issues
Status: Open
Priority: 5 - Medium
MileStone: (None)
Component: (None)
Severity: 5 - Medium
Resolution: None
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Ticket details:
DescriptionThe bug is related to some internal code structure, so tried to reproduce the situation as close as possible.
mingw with -O2 optimization flag loads memset-ed value on the first load of atomic (just struct name) value read and initialized value (right value) at the second load call.
Fixes:
* Removing volatile keyword
* Changing int32_t to unsigned long
* Other reduction of code complexity
What is wrong with this code? Is it a bug? GCC7 does not have this issue.
Reproducer #include <new> #include <iostream> #include <cstring> struct atomic { unsigned long my_value; public: atomic() = default ; atomic(unsigned long value) : my_value(value) {} unsigned long load() { int32_t* v = reinterpret_cast<int32_t*>(&my_value); return (volatile int32_t&)(*v); } }; //-------------------------------------------------------------------------------------------------------- struct TestStruct { atomic storage; TestStruct(){ memset(static_cast<void*>(&storage),-1,sizeof(storage)); } void operator()(unsigned long i){ atomic* ptr2 = new (&storage) atomic(i); unsigned long val1 = ptr2->load(); std::cout << "Expected:" << i << " Value first: " << val1 << std::endl; unsigned long val2 = ptr2->load(); std::cout << "Expected:" << i << " Value second: " << val2 << std::endl; } }; int main () { TestStruct()(0L); //just long also should fail; return 0; }Output:
Expected:0 Value first: 4294967295
Expected:0 Value second: 0
EnvironmentWindows Server 2016
gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=d:/mingwcompilers/mingw_15.4_gcc_7.3.0/bin/../libexec/gcc/x86_64-w64-mingw32/7.3.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../src/configure --enable-languages=c,c++ --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --disable-multilib --prefix=/c/temp/gcc/dest --with-sysroot=/c/temp/gcc/dest --disable-libstdcxx-pch --disable-libstdcxx-verbose --disable-nls --disable-shared --disable-win32-registry --with-tune=haswell --enable-threads=posix --enable-libgomp
Thread model: posix
gcc version 7.3.0 (GCC)
ld -v
GNU ld (GNU Binutils) 2.29.1
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URL for this Ticket:
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