[Bug c/16961] New: Poor x86-64 performance

["tomstdenis at iahu dot ca" <gcc-bugzilla at gcc dot gnu dot org>]

On the AMD64 using "-march=k8" we see really poor performance.  My beef is two 
issues. 
 
First off, 128-bit unsigned additions are emulated when addq/adcq will do the 
job just fine.  For example, 
 
typedef unsigned long      mp_word __attribute__ ((mode(TI))); 
mp_word a, b; 
void test(void) { a += b; } 
 
Produces via [-O3 -fomit-frame-pointer -march=k8] 
 
        movq    a(%rip), %r10 
        movq    b(%rip), %r8 
        xorl    %ecx, %ecx 
        movq    a+8(%rip), %rdi 
        movq    b+8(%rip), %r9 
        leaq    (%r10,%r8), %rax 
        leaq    (%rdi,%r9), %rsi 
        cmpq    %r10, %rax 
        movq    %rax, a(%rip) 
        setb    %cl 
        leaq    (%rcx,%rsi), %rdx 
        movq    %rdx, a+8(%rip) 
        ret 
 
Which is insane. 
 
The second beef is loop unrolling.  Somehow between the 32-bit cores and 
64-bit targets it was made WAY WORSE.   
 
In the old method loops could be handled with something like 
 
while (n&3) { do(); update_for_loop(); } 
while (n) { 
    do(); do(); do(); do(); 
    update_for_loop4x(); 
} 
 
Now I'm seeing  
 
top: goto off[n&7]; 
off_7:  do(); update_for_loop(); 
off_6:  do(); update_for_loop(); 
off_5:  do(); update_for_loop(); 
off_4:  do(); update_for_loop(); 
... 
if (n) goto top; 
 
In my case it's updating pointers in the "update_for_loop()" when it really 
doesn't have to. 
 
Tom

-- 
           Summary: Poor x86-64 performance
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tomstdenis at iahu dot ca
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: gcc version 3.4.1  (Gentoo Linux 3.4.1, ssp-3.4-2, pie-
                    8.7.6.3)
  GCC host triplet: Linux timmy 2.6.7-gentoo-r11 #1 Thu Aug 5 01:49:49 UTC
                    2004 x86_
GCC target triplet: gcc version 3.4.1  (Gentoo Linux 3.4.1, ssp-3.4-2, pie-
                    8.7.6.3)


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16961