https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115096
Bug ID: 115096
Summary: -fstrict-aliasing miscompilation or missing warning
Product: gcc
Version: 12.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: Philip.Taff at itron dot com
Target Milestone: ---
Created attachment 58209
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58209&action=edit
minimal test case
I'm not sure whether this code is a violation of strict-aliasing, in which case
a warning would be great, or it is valid, in which case it is being
miscompiled, removing some of the increments. According to Bug #29901 a similar
cast does break strict-aliasing rules, but the warning described in that bug is
not occurring.
Built with:
$ gcc -O1 -fstrict-aliasing -Wall -Wextra test.c
No compiler warnings or errors.
Executed with: ("ab." is the test data)
$ ./a.out ab.
GCC/system information:
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc-12
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/12/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
12.3.0-1ubuntu1~22.04' --with-bugurl=file:///usr/share/doc/gcc-12/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-12
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-12-ALHxjy/gcc-12-12.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-12-ALHxjy/gcc-12-12.3.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.3.0 (Ubuntu 12.3.0-1ubuntu1~22.04)
COLLECT_GCC_OPTIONS='-O1' '-fstrict-aliasing' '-Wall' '-Wextra' '-v'
'-save-temps' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
/usr/lib/gcc/x86_64-linux-gnu/12/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu
test.c -mtune=generic -march=x86-64 -Wall -Wextra -fstrict-aliasing -O1
-fpch-preprocess -fasynchronous-unwind-tables -fstack-protector-strong
-Wformat-security -fstack-clash-protection -fcf-protection -o a-test.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/12/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/12/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-O1' '-fstrict-aliasing' '-Wall' '-Wextra' '-v'
'-save-temps' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
/usr/lib/gcc/x86_64-linux-gnu/12/cc1 -fpreprocessed a-test.i -quiet -dumpdir a-
-dumpbase test.c -dumpbase-ext .c -mtune=generic -march=x86-64 -O1 -Wall
-Wextra -version -fstrict-aliasing -fasynchronous-unwind-tables
-fstack-protector-strong -Wformat-security -fstack-clash-protection
-fcf-protection -o a-test.s
GNU C17 (Ubuntu 12.3.0-1ubuntu1~22.04) version 12.3.0 (x86_64-linux-gnu)
compiled by GNU C version 12.3.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C17 (Ubuntu 12.3.0-1ubuntu1~22.04) version 12.3.0 (x86_64-linux-gnu)
compiled by GNU C version 12.3.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: d93534411cccf908c6f3926ba41cb359
COLLECT_GCC_OPTIONS='-O1' '-fstrict-aliasing' '-Wall' '-Wextra' '-v'
'-save-temps' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a-'
as -v --64 -o a-test.o a-test.s
GNU assembler version 2.38 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.38
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/12/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/12/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-O1' '-fstrict-aliasing' '-Wall' '-Wextra' '-v'
'-save-temps' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'
/usr/lib/gcc/x86_64-linux-gnu/12/collect2 -plugin
/usr/lib/gcc/x86_64-linux-gnu/12/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/12/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 -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie -z now -z relro
/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/Scrt1.o
/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/12/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/12
-L/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib -L/lib/x86_64-linux-gnu
-L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib
-L/usr/lib/gcc/x86_64-linux-gnu/12/../../.. 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-linux-gnu/12/crtendS.o
/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-O1' '-fstrict-aliasing' '-Wall' '-Wextra' '-v'
'-save-temps' '-mtune=generic' '-march=x86-64' '-dumpdir' 'a.'
