Re: optimization/9319: slower code generated for simple loop on linux

[Amnon Cohen <amnon_cohen at yahoo dot com>]

Hi ebotcazou,
Thanks for getting back to me so fast.
I have done a few more tests, and it seems 
that this is a regression
on all platforms  (not just intel) introduced by
gcc3.1
Note that the while loop in the example is equivalent
to
for (i=0; i<n; i++) s+=i;

The for loop generates faster code.

Another problem, if we compile with
-funroll-all-loops,
the loop does not get unrolled on gcc3.2.1, whereas
it does on gcc3.0


Here is the information you asked for.
For the newer compiler which shows the regression

>gcc321 -v
Reading specs from
/usr/local/lib/gcc-lib/i686-pc-linux-gnu/3.2.1/specs
Configured with: ./configure 
Thread model: posix
gcc version 3.2.1

Wheras the old version
>gcc3 -v
Reading specs from
/usr/local/lib/gcc-lib/i686-pc-linux-gnu/3.0.3/specs
Configured with: ./configure 
Thread model: single
gcc version 3.0.3

I have attached the assembler files 
(produced by gcc -S loop3.c)

The performance is significantly reduced on intel
linux and on Solaris (I have not sent solaris asm
files, let me know if you need them). So I do 
not think that this is architecture related

Please let me know if you need any more info!

- Amnon

--- ebotcazou@gcc.gnu.org wrote:
> Synopsis: slower code generated for simple loop on
> linux
> 
> Responsible-Changed-From-To: unassigned->ebotcazou
> Responsible-Changed-By: ebotcazou
> Responsible-Changed-When: Thu Jan 16 03:58:11 2003
> Responsible-Changed-Why:
>     Investigating.
> State-Changed-From-To: open->feedback
> State-Changed-By: ebotcazou
> State-Changed-When: Thu Jan 16 03:58:11 2003
> State-Changed-Why:
>     Could you provide us with more details ? What
> does gcc -v output for each compiler ? Could you
> post the assembly code generated for the loop by
> each compiler (gcc -O3 -S) ? On what hardware did
> you experience the performance hit ?
> 
> http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9319

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Attachment: loop3.linux.gcc3.s
Description: loop3.linux.gcc3.s

Attachment: loop3.linux.gcc321.s
Description: loop3.linux.gcc321.s