[Bug c++/20269] New: Optimizer problem with aliasing floating point variable

[bobm75 at gmail dot com]

gcc -v output:
Reading specs from /opt/gcc-3.4/lib/gcc/i686-pc-linux-gnu/3.4.3/specs
Configured with: ./configure --prefix=/opt/gcc-3.4 --enable-languages=c,c++
Thread model: posix
gcc version 3.4.3

See following preprocessed output:

extern "C" {
extern void __assert_fail (__const char *__assertion, __const char *__file,
            unsigned int __line, __const char *__function)
throw () __attribute__ ((__noreturn__));
}

static void DoTest()
{
        double doubleOne = 1.0;
        float floatOne = 1.0F;

        unsigned int *intDptr = (unsigned int *) &doubleOne;
        unsigned int *intFptr = (unsigned int *) &floatOne;

        (static_cast<void> (__builtin_expect (!!((intDptr[1] == 0x3ff00000) &&
(intDptr[0] == 0)), 1)? 0 : (__assert_fail ("(intDptr[1] == 0x3ff00000) &&
(intDptr[0] == 0)", "fptest.cpp", 12, __PRETTY_FUNCTION__), 0)));
        (static_cast<void> (__builtin_expect (!!(*intFptr == 0x3f800000), 1) ? 0
: (__assert_fail ("*intFptr == 0x3f800000", "fptest.cpp", 13,
__PRETTY_FUNCTION__), 0)));
}

int main()
{
        DoTest();
}

This compiles fine up to -O1, but with -O2 the test fails. When looking at the
generated assembler it seems to me that the compiler first issues the compare
before putting a value in it. Generated assembler:

gcc -O2 -S -o fptest.S fptest.cpp
main:
.LFB3:
        pushl   %ebp
.LCFI0:
        fld1
        movl    %esp, %ebp
.LCFI1:
        subl    $24, %esp
.LCFI2:
        andl    $-16, %esp
        subl    $16, %esp
        cmpl    $1072693248, -4(%ebp)
        fstpl   -8(%ebp)
        jne     .L4
        movl    -8(%ebp), %eax
        testl   %eax, %eax
        jne     .L4
        leave
        xorl    %eax, %eax
        ret
        .p2align 4,,7
.L4:
        movl    $.LC2, (%esp)
        movl    $_ZZ6DoTestvE19__PRETTY_FUNCTION__, %ecx
        movl    $12, %edx
        movl    %ecx, 12(%esp)
        movl    $.LC1, %eax
        movl    %edx, 8(%esp)
        movl    %eax, 4(%esp)
        call    __assert_fail

Specifying volatile for all the local variables fixes it.

-- 
           Summary: Optimizer problem with aliasing floating point variable
           Product: gcc
           Version: 3.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bobm75 at gmail dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20269