[Bug target/34653] New: unnecessary REX prefix

["dean at arctic dot org" <gcc-bugzilla at gcc dot gnu dot org>]

for this code:

extern unsigned long table[];

unsigned long foo(unsigned char *p) {
  unsigned long long tag = *p;
  return table[tag >> 4];
}

gcc generates:

0000000000000000 <foo>:
   0:   0f b6 07                movzbl (%rdi),%eax
   3:   48 c1 e8 04             shr    $0x4,%rax
   7:   48 8b 04 c5 00 00 00    mov    0x0(,%rax,8),%rax
   e:   00
                        b: R_X86_64_32S table
   f:   c3                      retq

that "shr $0x4,%rax" would be better as "shr $0x4,%eax" because it produces the
same result (due to dominating movzbl) and it's one byte shorter which favours
both space and the narrow decoder on the core2.

thanks
-dean

/home/odo/gcc/bin/gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc/configure --prefix=/home/odo/gcc --disable-multilib
--disable-biarch x86_64-unknown-linux-gnu --enable-languages=c
Thread model: posix
gcc version 4.3.0 20071128 (experimental) (GCC)


-- 
           Summary: unnecessary REX prefix
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dean at arctic dot org
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34653