https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112289
--- Comment #4 from xiao xiang <Sean.XiaoXiang at outlook dot com> ---
--------------- description ----------------
some store instructions will be lost under certain conditions.
1、 GCC version >= 12.1
2、 Optimization enable at ‘-Os’ (actually is -fstore-merging)
3、 Code like the sample below
We also had some tests and the results (based on GCC version 13.2.0)
Fails with -Os
Passes with -Os -fno-store-merging
Passes with -Os -fno-ipa-modref
Passes with -Os -fno-ssa-phiopt
Passes with -Os -fno-tree-dse
Passes with -Os -fno-tree-pta
Passes with -O1
Fails with -O1 -fstore-merging
---------------Sample code merge.c ----------------
#include <stdio.h>
typedef union {
struct {
int bit0:1;
int bit1:1;
int bit2:1;
};
int data;
} BITS;
void init(BITS *val, int flag)
{
val->bit1 = 1;
val->bit2 = 1;
if (flag > 1) {
val->bit2 = 0;
}
return;
}
int main(int argc, char **argv)
{
(void)argv;
BITS value;
value.data = 0;
value.bit0 = 1; // lost instrument
init(&value, argc);
printf("%x\n", value);
return 0;
}
--------------- command line ----------------
--------------- compile option ----------------
--------------- expected and actual behavior ----------------
$ gcc -Os merge.c
$ ./a.exe
6 <--------- expect 7, but bit0 not set
--------------- system ----------------
windows 10 with mingw64
Linux x86_64 tested the same
--------------- gcc version ----------------
$ gcc -v
Using built-in specs.
COLLECT_GCC=D:\mingw64\bin\gcc.exe
COLLECT_LTO_WRAPPER=D:/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/13.2.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../../../src/gcc-13.2.0/configure --host=x86_64-w64-mingw32
--build=x86_64-w64-mingw32 --target=x86_64-w64-mingw32
--prefix=/mingw64--with-sysroot=/c/buildroot/x86_64-1320-posix-seh-msvcrt-rt_v11-rev0/mingw64
--enable-host-shared --disable-multilib --enable-languages=c,c++,fortran,lto
--enable-libstdcxx-time=yes --enable-threads=posix --enable-libgomp
--enable-libatomic --enable-lto --enable-graphite --enable-checking=release
--enable-fully-dynamic-string --enable-version-specific-runtime-libs
--enable-libstdcxx-filesystem-ts=yes --disable-libssp --disable-libstdcxx-pch
--disable-libstdcxx-debug --enable-bootstrap --disable-rpath
--disable-win32-registry --disable-nls --disable-werror --disable-symvers
--with-gnu-as --with-gnu-ld --with-arch=nocona --with-tune=core2
--with-libiconv --with-system-zlib
--with-gmp=/c/buildroot/prerequisites/x86_64-w64-mingw32-static
--with-mpfr=/c/buildroot/prerequisites/x86_64-w64-mingw32-static
--with-mpc=/c/buildroot/prerequisites/x86_64-w64-mingw32-static
--with-isl=/c/buildroot/prerequisites/x86_64-w64-mingw32-static
--with-pkgversion='x86_64-posix-seh-rev0, Built by MinGW-Builds project'
--with-bugurl=https://github.com/niXman/mingw-builds CFLAGS='-O2 -pipe
-fno-ident
-I/c/buildroot/x86_64-1320-posix-seh-msvcrt-rt_v11-rev0/mingw64/opt/include
-I/c/buildroot/prerequisites/x86_64-zlib-static/include
-I/c/buildroot/prerequisites/x86_64-w64-mingw32-static/include' CXXFLAGS='-O2
-pipe -fno-ident
-I/c/buildroot/x86_64-1320-posix-seh-msvcrt-rt_v11-rev0/mingw64/opt/include
-I/c/buildroot/prerequisites/x86_64-zlib-static/include
-I/c/buildroot/prerequisites/x86_64-w64-mingw32-static/include' CPPFLAGS='
-I/c/buildroot/x86_64-1320-posix-seh-msvcrt-rt_v11-rev0/mingw64/opt/include
-I/c/buildroot/prerequisites/x86_64-zlib-static/include
-I/c/buildroot/prerequisites/x86_64-w64-mingw32-static/include' LDFLAGS='-pipe
-fno-ident
-L/c/buildroot/x86_64-1320-posix-seh-msvcrt-rt_v11-rev0/mingw64/opt/lib
-L/c/buildroot/prerequisites/x86_64-zlib-static/lib
-L/c/buildroot/prerequisites/x86_64-w64-mingw32-static/lib '
LD_FOR_TARGET=/c/buildroot/x86_64-1320-posix-seh-msvcrt-rt_v11-rev0/mingw64/bin/ld.exe
--with-boot-ldflags=' -Wl,--disable-dynamicbase -static-libstdc++
-static-libgcc'
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 13.2.0 (x86_64-posix-seh-rev0, Built by MinGW-Builds project)
