[Bug middle-end/16541] New: code quality issue for bit manipulations

["dann at godzilla dot ics dot uci dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

The function below is from crafty, from the current version of crafty, not the
one in SPEC2000, but they are functionally identical. 

extern unsigned char first_one[65536];

int FirstOne(unsigned long long arg1)
{
  if (arg1 >> 48)
    return (first_one[arg1 >> 48]);
  if ((arg1 >> 32) & 65535)
    return (first_one[(arg1 >> 32) & 65535] + 16);
  if ((arg1 >> 16) & 65535)
    return (first_one[(arg1 >> 16) & 65535] + 32);
  return (first_one[arg1 & 65535] + 48);
}

The code mainline generates for this with -O2 on x86 is: 

        pushl   %ebp
        xorl    %edx, %edx
        movl    %esp, %ebp
        subl    $12, %esp
        movl    %ebx, (%esp)
        movl    %edx, %ecx
        movl    %esi, 4(%esp)
        movl    %edi, 8(%esp)
        movl    12(%ebp), %edi
        movl    8(%ebp), %esi
        movl    %edi, %eax
        shrl    $16, %eax
        orl     %eax, %ecx
        je      .L2
        movzbl  first_one(%eax), %eax
        movl    (%esp), %ebx
        movl    4(%esp), %esi
        movl    8(%esp), %edi
        movl    %ebp, %esp
        popl    %ebp
        ret
        .p2align 4,,7
.L2:
        movl    %edi, %eax
        xorl    %ebx, %ebx
        movzwl  %ax,%ecx
        movl    %ebx, %eax
        orl     %ecx, %eax
        je      .L5
        movzbl  first_one(%ecx), %eax
        movl    (%esp), %ebx
        movl    4(%esp), %esi
        movl    8(%esp), %edi
        movl    %ebp, %esp
        popl    %ebp
        addl    $16, %eax
        ret
        .p2align 4,,7
.L5:
        movl    %esi, %eax
        xorl    %ebx, %ebx
        shrdl   $16, %edi, %eax
        movzwl  %ax,%ecx
        movl    %ebx, %eax
        orl     %ecx, %eax
        je      .L7
        movzbl  first_one(%ecx), %eax
        movl    (%esp), %ebx
        movl    4(%esp), %esi
        movl    8(%esp), %edi
        movl    %ebp, %esp
        popl    %ebp
        addl    $32, %eax
        ret
        .p2align 4,,7
.L7:
        movl    %esi, %eax
        movl    (%esp), %ebx
        andl    $65535, %eax
        movl    4(%esp), %esi
        movzbl  first_one(%eax), %eax
        movl    8(%esp), %edi
        movl    %ebp, %esp
        popl    %ebp
        addl    $48, %eax
        ret

The code generated on SPARC for this function can be seen in PR16532

Intel's compiler  generates much nicer code: 

# parameter 1: 4 + %esp
..B1.1:                         # Preds ..B1.0
        movl      8(%esp), %edx                                 #3.5
        movl      %edx, %ecx                                    #5.15
        shrl      $16, %ecx                                     #5.15
        xorl      %eax, %eax                                    #5.3
        orl       %ecx, %eax                                    #5.3
        jne       ..B1.7        # Prob 28%                      #5.3
                                # LOE edx ecx ebx ebp esi edi
..B1.2:                         # Preds ..B1.1
        movzwl    %dx, %edx                                     #7.22
        xorl      %eax, %eax                                    #7.3
        orl       %edx, %eax                                    #7.3
        jne       ..B1.6        # Prob 28%                      #7.3
                                # LOE edx ebx ebp esi edi
..B1.3:                         # Preds ..B1.2
        movl      4(%esp), %edx                                 #9.16
        shrl      $16, %edx                                     #9.16
        xorl      %eax, %eax                                    #9.3
        orl       %edx, %eax                                    #9.3
        je        ..B1.5        # Prob 50%                      #9.3
                                # LOE edx ebx ebp esi edi
..B1.4:                         # Preds ..B1.3
        movzbl    first_one(%edx), %eax                         #10.13
        addl      $32, %eax                                     #10.47
        ret                                                     #10.47
                                # LOE
..B1.5:                         # Preds ..B1.3
        movl      4(%esp), %edx                                 #11.28
        movzwl    %dx, %ecx                                     #11.28
        movzbl    first_one(%ecx), %eax                         #11.11
        addl      $48, %eax                                     #11.37
        ret                                                     #11.37
                                # LOE
..B1.6:                         # Preds ..B1.2
        movzbl    first_one(%edx), %eax                         #8.13
        addl      $16, %eax                                     #8.47
        ret                                                     #8.47
                                # LOE
..B1.7:                         # Preds ..B1.1
        movzbl    first_one(%ecx), %eax                         #6.13
        ret                                                     #6.13

-- 
           Summary: code quality issue for bit manipulations
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dann at godzilla dot ics dot uci dot edu
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16541