https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116768
Bug ID: 116768
Summary: Strict aliasing breaks autovectorization with -O3
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: imachug at gmail dot com
Target Milestone: ---
This returns 0 (wrong) with strict aliasing enabled and 1 (correct) with strict
aliasing disabled. Looks like a bug to me (no casts, sanitizers are silent, the
example is a minimization of an std::bitset-based reproducer). -O3 -mavx is
required to trigger the bug.
I believe this is a bug in TBAA, because defining Parent to Child or replacing
`y_child->` with `y->child.` fixes the miscompilation. A quick check with
Godbolt shows the code is reduced to 'return 0' by the last tree pass, so I'm
tentatively labeling this tree-optimization.
This can be reproduced starting with 11.2 up to trunk.
https://godbolt.org/z/1v16bPdfv
```
typedef struct {
unsigned long words[2];
} Child;
typedef struct {
Child child;
} Parent;
Parent my_or(Parent x, const Parent *y) {
const Child *y_child = &y->child;
for (int i = 0; i < 2; i++) {
x.child.words[i] |= y_child->words[i];
}
return x;
}
int main() {
Parent bs[4];
__builtin_memset(bs, 0, sizeof(bs));
bs[0].child.words[0] = 1;
for (int i = 1; i <= 3; i++) {
bs[i] = my_or(bs[i], &bs[i - 1]);
}
return bs[2].child.words[0];
}
```
Here's -v for my local compiler if you find it useful.
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust
--enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://gitlab.archlinux.org/archlinux/packaging/packages/gcc/-/issues
--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 14.2.1 20240805 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx' '-mtune=generic'
'-march=x86-64' '-dumpdir' 'a-'
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/cc1 -E -quiet -v test.c -mavx
-mtune=generic -march=x86-64 -O3 -fpch-preprocess -o a-test.i
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/include
/usr/local/include
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx' '-mtune=generic'
'-march=x86-64' '-dumpdir' 'a-'
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/cc1 -fpreprocessed a-test.i -quiet
-dumpdir a- -dumpbase test.c -dumpbase-ext .c -mavx -mtune=generic
-march=x86-64 -O3 -version -o a-test.s
GNU C17 (GCC) version 14.2.1 20240805 (x86_64-pc-linux-gnu)
compiled by GNU C version 14.2.1 20240805, GMP version 6.3.0, MPFR
version 4.2.1, MPC version 1.3.1, isl version isl-0.26-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: faa3163d33b78b77071c76eebeab3034
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx' '-mtune=generic'
'-march=x86-64' '-dumpdir' 'a-'
as -v --64 -o a-test.o a-test.s
GNU assembler version 2.43.0 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.43.0
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx' '-mtune=generic'
'-march=x86-64' '-dumpdir' 'a.'
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/collect2 -plugin
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/lto-wrapper
-plugin-opt=-fresolution=a.res -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --build-id
--eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../lib/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../lib/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1
-L/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../.. a-test.o
-lgcc --push-state --as-needed -lgcc_s --pop-state -lc -lgcc --push-state
--as-needed -lgcc_s --pop-state
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../lib/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O3' '-mavx' '-mtune=generic'
'-march=x86-64' '-dumpdir' 'a.'
