[Bug target/58416] New: Incorrect x87-based union copying code

["stichnot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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 âargcâ [-Wunused-parameter]
nantest1.cpp:21:5: warning: unused parameter âargvâ [-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