http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51759
Bug #: 51759
Summary: miscompile writes past end of bitfield
Classification: Unclassified
Product: gcc
Version: 4.5.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: nob...@dreamwidth.org
Compiling the attached preprocessed file with this:
g++-4.5 -Os -fPIC -g -pedantic -Wno-long-long -fno-exceptions -o Type2.cpp.o -c
Type2.ii
Results in writing 32 bits to a 24-bit bitfield, overwriting the first byte of
the next member variable.
These two members of class Type are (on x86_64) at offset 0x8:
TypeID ID : 8;
unsigned SubclassData : 24;
When setSubclassData() isn't inlined, it's called (from StructType::setBody()
and PointerType's constructor) with the address of 'SubclassData' in %rdi...:
0x00007ffff76d684f <+71>: lea 0x9(%rdi),%r12
0x00007ffff76d6853 <+75>: or $0x1,%esi
0x00007ffff76d6856 <+78>: mov %r12,%rdi
0x00007ffff76d6859 <+81>: callq 0x7ffff76d6774
<llvm::Type::setSubclassData(unsigned int)>
...but then, setSubclassData writes more than 24 bits to that address:
0x00007ffff76d6774 <+0>: mov %esi,%eax
0x00007ffff76d6776 <+2>: sub $0x8,%rsp
0x00007ffff76d677a <+6>: and $0xffffff,%eax
0x00007ffff76d677f <+11>: cmp %esi,%eax
0x00007ffff76d6781 <+13>: mov %eax,(%rdi) # corruption
