[Bug rtl-optimization/50339] New: suboptimal register allocation for abs(__int128_t)

[wouter.vermaelen at scarlet dot be]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50339

             Bug #: 50339
           Summary: suboptimal register allocation for abs(__int128_t)
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: wouter.vermaelen@scarlet.be


This function:

__int128_t abs128(__int128_t a)
{
    return (a >= 0) ? a : -a;
}

Currently generates the following code (with -O3):
(linux x86_64, g++-4.7.0, SVN revision 178692)

   49 89 f9                mov    %rdi,%r9
   48 89 f7                mov    %rsi,%rdi
   49 89 f2                mov    %rsi,%r10
   48 c1 ff 3f             sar    $0x3f,%rdi
   48 89 f8                mov    %rdi,%rax
   48 89 fa                mov    %rdi,%rdx
   4c 31 c8                xor    %r9,%rax
   4c 31 d2                xor    %r10,%rdx
   48 29 f8                sub    %rdi,%rax
   48 19 fa                sbb    %rdi,%rdx
   c3                      retq   

But the following has 2 'mov' instructions less:

   48 89 f8                mov    %rdi,%rax
   48 89 f2                mov    %rsi,%rdx
   48 89 d1                mov    %rdx,%rcx
   48 c1 f9 3f             sar    $0x3f,%rcx
   48 31 c8                xor    %rcx,%rax
   48 31 ca                xor    %rcx,%rdx
   48 29 c8                sub    %rcx,%rax
   48 19 ca                sbb    %rcx,%rdx
   c3                      retq