empty loop elimination (lack of) ?

[Andrew Pinski]

I compiled you test with the following options:
-O3 -W -Wall -fssa-dce -fssa-ccp -fregmove -fcprop-registers 
-ffunction-cse -fbranch-count-reg -fsched-interblock -frename-registers 
-freorder-blocks -fschedule-insns2 -march=pentiumpro 
-foptimize-register-move -fpeephole2 -frerun-cse-after-loop -fident 
-fmerge-all-constants -fmerge-constants -fmessage-length=0 -fmem-report 
-ftime-report -fthread-jumps -freduce-all-givs 
-fguess-branch-probability -frerun-loop-opt -finline 
-fexpensive-optimizations -foptimize-sibling-calls -fomit-frame-pointer 
-save-temps -fmove-all-movables -fprefetch-loop-arrays 
-funroll-all-loops -mmmx -msse --param max-gcse-passes=10 --param 
max-gcse-memory=100 --param max-inline-insns=10000 
-Wdisabled-optimization

I did not see the results you got.
I even don't get a List<int>::isValid() const

I get the following asm:

main:
.LFB1:
         pushl   %ebp
.LCFI0:
         movl    %esp, %ebp
.LCFI1:
         subl    $56, %esp
.LCFI2:
         andl    $-16, %esp
         leal    -40(%ebp), %edx
         movl    %edx, (%esp)
         call    List<int>::List()
         movl    $.LC0, (%esp)
         movl    $1, 4(%esp)
         call    printf
         movl    %ebp, %esp
         xorl    %eax, %eax
         popl    %ebp
         ret



But if I compiled with just -O3 I get that results.
So one of the options I used just fixed that problem, I don't know which 
one.
Thanks,
Andrew Pinski

On Monday, January 28, 2002, at 04:03 , Cyrille Chepelov wrote:

> Hi,
>
>   while testing some code on Borland C++, and as a matter of 
> cross-checking,
> I've found that there is an opportunty for optimisation that g++ misses
> (bcc32 also, but that is off-topic).
>
> Consider the attached loop-destroy.cpp file, in particular the 
> compilation
> of List<int>::isValid(). gcc (2.95.3 and 3.0.3 on ix86 give me the 
> following
> results (with -O3):
>
> _isValid__Ct4List1Zi:
>         pushl %ebp
>         xorl %eax,%eax
>         movl %esp,%ebp
>         .align 4
> L15:
>         incl %eax
>         cmpl $3,%eax
>         jbe L15
>         movl %ebp,%esp
>         movb $1,%al
>         popl %ebp
>         ret
>
> ... including an empty, useless stunt with %eax, which is finally 
> replaced
> with the contents of the last movb. (There is also some games with %esp 
> and
> %ebp, which don't look totally useful, but 3.0.3 looks better than 
> 2.95.3
> with that respect).
>
> (in case you wonder, bcc32 makes even more stupid things).
>
> Compiling with -O3 -funroll-loops gives this:
> _isValid__Ct4List1Zi:
>         pushl %ebp
>         movb $1,%al
>         movl %esp,%ebp
>         movl %ebp,%esp
>         popl %ebp
>         ret
>
> .. somewhat better, still some stuff which probably ought to be 
> optimised.
>
> As a point of comparison, VC++ 6.0 is completely inlining this stuff 
> (with
> fairly default optimisations) into a single (true) constant. Just as 
> what
> g++ does when the original function is simply a "return true;"
>
> 2.95.3 and 3.0.3 give the same results, give or take a movl %ebp,%esp.
>
> 1) How hard would it be to notice that the loop is empty and produces no
> useful results (without unrolling it), and then to remove it ?
>
> 2) How hard would it be to re-do an inlining pass once the loop
> unrolling/empty loop removal pass has been done ?
>
> 3) How hard would it be to omit stack frames whenever there is no
> dereference in a tail function ? Wouldn't that be a sensible default ?
>
> I'd be more than happy to give that a try, if someone gently shows me 
> where
> to fiddle with (no warranties, though).
>
> 	-- Cyrille
>
> For comparison's sake, there is a tarball at
> http://www.chepelov.org/cyrille/loop-destroy.tar.gz, which contains this
> .cpp file, and assembly outputs of a few compilers.
> --
> Grumpf.
>
>