https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107467
Bug ID: 107467
Summary: Miscompilation involing -Os , -flto and
-fno-strict-aliasing
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: translation
Assignee: unassigned at gcc dot gnu.org
Reporter: volker.weissmann at gmx dot de
Target Milestone: ---
Created attachment 53798
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53798&action=edit
Source Code
g++ -g Record.cpp -c -fPIC -Os -flto
g++ -g -shared Record.o -fno-strict-aliasing -o librecord.so
g++ main.cpp -L. -lrecord
LD_LIBRARY_PATH=$PWD ./a.out
This sometimes prints 1 and sometimes prints 0.
If I remove the -fno-strict-aliasing flag of the second g++ command, or add it
to the first g++ command, it always prints 1.
There is no violation of the strict-aliasing rule in Record.cpp, at least none
that I could spot.
If you are afraid of non-reproducibility, fear not, the generated assembly is
the same every time, and is obviously wrong:
The method Combined<OtherClass>::clashy() is missing a call to get_const
objdump librecord.so --disassemble=_ZN8CombinedI10OtherClassE6clashyEv.isra.0
000000000000113e <_ZN8CombinedI10OtherClassE6clashyEv.isra.0>:
113e: 48 83 ec 38 sub $0x38,%rsp
1142: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
1149: 00 00
114b: 48 89 44 24 28 mov %rax,0x28(%rsp)
1150: 31 c0 xor %eax,%eax
1152: 48 8d 74 24 18 lea 0x18(%rsp),%rsi
1157: 48 8d 7c 24 08 lea 0x8(%rsp),%rdi
115c: e8 b9 ff ff ff call 111a
<_Z13compare_pairsIP10OtherClassEbRKSt4pairIiT_ES6_>
1161: 48 8b 54 24 28 mov 0x28(%rsp),%rdx
1166: 64 48 2b 14 25 28 00 sub %fs:0x28,%rdx
116d: 00 00
116f: 74 05 je 1176
<_ZN8CombinedI10OtherClassE6clashyEv.isra.0+0x38>
1171: e8 ca fe ff ff call 1040 <__stack_chk_fail@plt>
1176: 48 83 c4 38 add $0x38,%rsp
117a: c3 ret
The almost identical Combined<SomeClass>::clashy() gets compiled correctly:
objdump librecord.so --disassemble=_ZN8CombinedI9SomeClassE6clashyEv.isra.0
000000000000117c <_ZN8CombinedI9SomeClassE6clashyEv.isra.0>:
117c: 48 83 ec 38 sub $0x38,%rsp
1180: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
1187: 00 00
1189: 48 89 44 24 28 mov %rax,0x28(%rsp)
118e: 31 c0 xor %eax,%eax
1190: 48 8d 74 24 18 lea 0x18(%rsp),%rsi
1195: 48 8d 7c 24 08 lea 0x8(%rsp),%rdi
119a: e8 97 ff ff ff call 1136
<_ZN8CombinedI9SomeClassE9get_constEv.constprop.0>
119f: 48 89 54 24 20 mov %rdx,0x20(%rsp)
11a4: 89 44 24 18 mov %eax,0x18(%rsp)
11a8: e8 89 ff ff ff call 1136
<_ZN8CombinedI9SomeClassE9get_constEv.constprop.0>
11ad: 48 89 54 24 10 mov %rdx,0x10(%rsp)
11b2: 89 44 24 08 mov %eax,0x8(%rsp)
11b6: e8 5f ff ff ff call 111a
<_Z13compare_pairsIP10OtherClassEbRKSt4pairIiT_ES6_>
11bb: 48 8b 54 24 28 mov 0x28(%rsp),%rdx
11c0: 64 48 2b 14 25 28 00 sub %fs:0x28,%rdx
11c7: 00 00
11c9: 74 05 je 11d0
<_ZN8CombinedI9SomeClassE6clashyEv.isra.0+0x54>
11cb: e8 70 fe ff ff call 1040 <__stack_chk_fail@plt>
11d0: 48 83 c4 38 add $0x38,%rsp
11d4: c3 ret
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (GCC)
$ cat main.cpp
void other_func();
int main() { other_func(); }
