[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 (__builtin_expect (!!((intDptr[1] == 0x3ff0) &&
(intDptr[0] == 0)), 1)? 0 : (__assert_fail ("(intDptr[1] == 0x3ff0) &&
(intDptr[0] == 0)", "fptest.cpp", 12, __PRETTY_FUNCTION__), 0)));
(static_cast (__builtin_expect (!!(*intFptr == 0x3f80), 1) ? 0
: (__assert_fail ("*intFptr == 0x3f80", "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

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

[bobm75 at gmail dot com]

--- Additional Comments From bobm75 at gmail dot com  2005-03-02 00:33 
---

I read the documentation on -fstrict-aliasing and it makes sense to me why this
code breaks those rules. I just wonder why the compiler didn't warn about it,
since this seems to be a pretty straight forward case of aliasing happening.

Thanks for responding so quickly.

-- 


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

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

[bobm75 at gmail dot com]

--- Additional Comments From bobm75 at gmail dot com  2005-03-02 18:03 
---

Ok, thank you for your explanation. We have a huge codebase and we just
increased the optimization level from -O to -O2 (together with -march=pentium3),
that's when this came up. Since most of this code has been around for a long
time (and not written by me) there is no way to guarantee that I caught all
these cases. 

So I guess I'll just have to disable the strict aliasing option for now. Thanks
for all your help.



-- 


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

[Bug c++/19869] New: Allow to non-friend class to get access to function pointer

[bobm75 at gmail dot com]

Reading specs from /opt/gcc-3.3/lib/gcc-lib/i686-pc-linux-gnu/3.3.6/specs
Configured with: ../configure --prefix=/opt/gcc-3.3 --enable-languages=c,c++
Thread model: posix
gcc version 3.3.6 20050209 (prerelease)

The following compiles without errors on a vanilla build of gcc 3.3.5 and gcc
3.3.6. The 3.4 and 4.0 builds both correctly give an error while compiling this.

typedef void (*MyCall)(int j);
class P
{
private:
MyCall  myCall;
};
class J
{
public:
J(P* p) { p->myCall(5); }
};

-- 
   Summary: Allow to non-friend class to get access to function
pointer
   Product: gcc
   Version: 3.3.6
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
  GCC host triplet: i686-pc-linux
GCC target triplet: i686-pc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19869