http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416
Bug ID: 58416 Summary: Incorrect x87-based union copying code Product: gcc Version: 4.6.3 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: stichnot at google dot com Created attachment 30819 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30819&action=edit .ii file generated from nantest1.cpp The program below produces incorrect code on x86-32 with gcc 4.6.3. (It was also found to fail in 4.7 but succeed in 4.8.) The problem happens when: 1. The union is initialized via its 64-bit double field 2. The union is then updated via its 64-bit integer field 3. The union is copied to another variable When this happens, the 64-bit copy in step 3 is done via the x87 fldl/fstpl instructions. If the 64-bit integer value in step 2 happens to be an sNaN pattern, the fldl instruction transforms it to a qNaN pattern and the fstpl instruction writes the wrong value. If step 1 is omitted, the x87 instructions are not used and the code is correct. This suggests that the optimization of using x87 for 64-bit transfers is enabled when a use of the double field is found, whereas it should probably only happen if a *live* use of the double field is found. // Compile with g++ -O3 -m32 // -O[123] all exhibit the problem. #include <assert.h> #include <stdlib.h> #include <stdio.h> union MyClass { MyClass() : DoubleVal(0.0) {} // this ctor causes failure //MyClass() : IntVal(0) {} // this ctor works correctly int64_t IntVal; double DoubleVal; }; __attribute__((noinline)) void create(MyClass &dest, int64_t Val) { MyClass MCOp; MCOp.IntVal = Val; dest = MCOp; } int main(int argc, char **argv) { MyClass copy; int64_t magic = 0xfff0000000000001; // happens to be sNaN create(copy, magic); // copy is changed to qNaN printf("magic=%llx\ncopy= %llx\n", magic, copy.IntVal); assert(magic == copy.IntVal); return 0; } $ g++ -v -save-temps -Wall -Wextra -fno-strict-aliasing -fwrapv -O3 -m32 nantest1.cpp Using built-in specs. COLLECT_GCC=/usr/bin/g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing' '-fwrapv' '-O3' '-m32' '-shared-libgcc' '-mtune=generic' '-march=i686' /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1plus -E -quiet -v -imultilib 32 -imultiarch i386-linux-gnu -D_GNU_SOURCE nantest1.cpp -m32 -mtune=generic -march=i686 -Wall -Wextra -fno-strict-aliasing -fwrapv -O3 -fpch-preprocess -fstack-protector -o nantest1.ii ignoring nonexistent directory "/usr/local/include/i386-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../x86_64-linux-gnu/include" ignoring nonexistent directory "/usr/include/i386-linux-gnu" #include "..." search starts here: #include <...> search starts here: /usr/include/c++/4.6 /usr/include/c++/4.6/x86_64-linux-gnu/32 /usr/include/c++/4.6/backward /usr/lib/gcc/x86_64-linux-gnu/4.6/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing' '-fwrapv' '-O3' '-m32' '-shared-libgcc' '-mtune=generic' '-march=i686' /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1plus -fpreprocessed nantest1.ii -quiet -dumpbase nantest1.cpp -m32 -mtune=generic -march=i686 -auxbase nantest1 -O3 -Wall -Wextra -version -fno-strict-aliasing -fwrapv -fstack-protector -o nantest1.s GNU C++ (Ubuntu/Linaro 4.6.3-1ubuntu5) version 4.6.3 (x86_64-linux-gnu) compiled by GNU C version 4.6.3, GMP version 5.0.2, MPFR version 3.1.0-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 GNU C++ (Ubuntu/Linaro 4.6.3-1ubuntu5) version 4.6.3 (x86_64-linux-gnu) compiled by GNU C version 4.6.3, GMP version 5.0.2, MPFR version 3.1.0-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 65b5171ac1bd7b3f07dbea6bdb24be3d nantest1.cpp:21:5: warning: unused parameter ‘argc’ [-Wunused-parameter] nantest1.cpp:21:5: warning: unused parameter ‘argv’ [-Wunused-parameter] COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing' '-fwrapv' '-O3' '-m32' '-shared-libgcc' '-mtune=generic' '-march=i686' as --32 -o nantest1.o nantest1.s COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/32/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../i386-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib32/:/lib/i386-linux-gnu/:/lib/../lib32/:/usr/lib/i386-linux-gnu/:/usr/lib/../lib32/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../i386-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../:/lib/i386-linux-gnu/:/lib/:/usr/lib/i386-linux-gnu/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-fno-strict-aliasing' '-fwrapv' '-O3' '-m32' '-shared-libgcc' '-mtune=generic' '-march=i686' /usr/lib/gcc/x86_64-linux-gnu/4.6/collect2 --sysroot=/ --build-id --no-add-needed --as-needed --eh-frame-hdr -m elf_i386 --hash-style=gnu -dynamic-linker /lib/ld-linux.so.2 -z relro /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib32/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib32/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.6/32/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.6/32 -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../i386-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib32 -L/lib/i386-linux-gnu -L/lib/../lib32 -L/usr/lib/i386-linux-gnu -L/usr/lib/../lib32 -L/usr/lib/gcc/x86_64-linux-gnu/4.6 -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../i386-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../.. -L/lib/i386-linux-gnu -L/usr/lib/i386-linux-gnu nantest1.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/4.6/32/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib32/crtn.o