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

["us15 at os dot inf dot tu-dresden dot de" <gcc-bugzilla at gcc dot gnu dot org>]

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