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* [Bug optimization/14504] New: Missed Bit Twiddling Optimization
@ 2004-03-09 17:17 stl at caltech dot edu
  2004-03-09 17:35 ` [Bug optimization/14504] " pinskia at gcc dot gnu dot org
                   ` (8 more replies)
  0 siblings, 9 replies; 10+ messages in thread
From: stl at caltech dot edu @ 2004-03-09 17:17 UTC (permalink / raw)
  To: gcc-bugs

g++ 3.3.2 on i686-pc-linux-gnu with -s -O3 -fomit-frame-pointer compiles:

unsigned long cond_mask(bool flag, unsigned long mask, unsigned long target) {
    return flag ? target | mask : target & ~mask;
}

into:

cmpb    $0, 4(%esp)
movl    8(%esp), %eax
movl    12(%esp), %edx
je      .L2
orl     %edx, %eax
ret
.L2:
notl    %eax
andl    %edx, %eax
ret

This appears to be a straightforward translation of the code into assembly. 
With the same options, this:

unsigned long cond_mask(bool flag, unsigned long mask, unsigned long target) {
    return (mask | target ^ 0xFFFFFFFFUL + flag) ^ 0xFFFFFFFFUL + flag;
}

is compiled into:

movzbl  4(%esp), %edx
movl    12(%esp), %eax
movl    8(%esp), %ecx
decl    %edx
xorl    %edx, %eax
orl     %ecx, %eax
xorl    %edx, %eax
ret

Again this appears to be a straightforward translation of the code into 
assembly (instead of flag + 0xFFFFFFFFUL, it uses static_cast<unsigned long>
(flag) - 1, which is the same thing).

However, the rewritten code lacks a branch yet does the exact same thing.

g++ ought to be able to perform this reasonably simple transformation on its 
own - if, indeed, the transformation is desirable (which I think it is).

NB:
I do not know assembly, so I may have deleted important information from g++'s 
output. I don't think I have, however.

This case was suggested by ryanm@microsoft.com, who said "this is something I 
will never expect a compiler to be able to optimize for me.  ^_^". I wrote the 
transformed code; perhaps there is a cleverer way to transform it which I am 
not aware of.

-- 
           Summary: Missed Bit Twiddling Optimization
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: stl at caltech dot edu
                CC: gcc-bugs at gcc dot gnu dot org,ryanm at microsoft dot
                    com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14504


^ permalink raw reply	[flat|nested] 10+ messages in thread
end of thread, other threads:[~2004-03-11 22:29 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2004-03-09 17:17 [Bug optimization/14504] New: Missed Bit Twiddling Optimization stl at caltech dot edu
2004-03-09 17:35 ` [Bug optimization/14504] " pinskia at gcc dot gnu dot org
2004-03-09 18:01 ` [Bug optimization/14504] New: " Falk Hueffner
2004-03-11  0:09 ` [Bug optimization/14504] " kazu at cs dot umass dot edu
2004-03-11 21:33 ` stl at caltech dot edu
2004-03-11 21:35 ` kazu at cs dot umass dot edu
2004-03-11 21:41 ` stl at caltech dot edu
2004-03-11 22:10 ` falk at debian dot org
2004-03-11 22:21 ` stl at caltech dot edu
2004-03-11 22:29 ` kazu at cs dot umass dot edu

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