[Bug c++/33349] New: Redundant zero-extension of registers

[Bug c++/33349] New: Redundant zero-extension of registers

[us15 at os dot inf dot tu-dresden dot de]

template <typename T>
__attribute__((always_inline))
static inline void out (unsigned port, T val)
{
    asm volatile ("out %0, %w1" : : "a" (val), "Nd" (port));
}

void func (unsigned bits, unsigned val)
{
    switch (bits) {
        case 8:
            out (0x80, static_cast<unsigned char>(val));
            return;                                     
        case 16:
            out (0x80, static_cast<unsigned short>(val));
            return;                                      
        case 32:
            out (0x80, static_cast<unsigned long>(val));
            return;                                     
    }
}

Above test program compiled with:
gcc -Os -fomit-frame-pointer -c x.cc
produces the following object code:

00000000 <func(unsigned int, unsigned int)>:
   0:   8b 54 24 04             mov    0x4(%esp),%edx
   4:   8b 44 24 08             mov    0x8(%esp),%eax
   8:   83 fa 10                cmp    $0x10,%edx
   b:   74 10                   je     1d <func(unsigned int, unsigned
int)+0x1d>
   d:   83 fa 20                cmp    $0x20,%edx
  10:   74 12                   je     24 <func(unsigned int, unsigned
int)+0x24>
  12:   83 fa 08                cmp    $0x8,%edx
  15:   75 0f                   jne    26 <func(unsigned int, unsigned
int)+0x26>
  17:   0f b6 c0                movzbl %al,%eax
  1a:   e6 80                   out    %al,$0x80
  1c:   c3                      ret    
  1d:   0f b7 c0                movzwl %ax,%eax
  20:   66 e7 80                out    %ax,$0x80
  23:   c3                      ret    
  24:   e7 80                   out    %eax,$0x80
  26:   c3                      ret    

gcc unnecessarily zero-extends %eax before the out operations.

gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc-svn/configure --prefix=/usr --enable-shared
--enable-threads=posix --enable-__cxa_atexit --enable-languages=c,c++
--disable-checking --with-gnu-ld --verbose
Thread model: posix
gcc version 4.3.0 20070903 (experimental) (GCC)


-- 
           Summary: Redundant zero-extension of registers
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: us15 at os dot inf dot tu-dresden dot de
  GCC host triplet: 686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33349

[Bug middle-end/33349] Redundant zero-extension of registers

[steven at gcc dot gnu dot org]

------- Comment #1 from steven at gcc dot gnu dot org  2009-02-06 21:45 -------
This would be fixed if someone would fix the Sign Extension Elimination pass
(yes, it also handles zero extensions).  But that pass is probably broken
beyond repair at this point, and likely needs a rewrite instead of just a fix.


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-02-06 21:45:16
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33349