optimization/9553: No induction variable elimination

optimization/9553: No induction variable elimination

[binsh00]

>Number:         9553
>Category:       optimization
>Synopsis:       No induction variable elimination
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Mon Feb 03 20:16:01 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     binsh00@hotmail.com
>Release:        gcc (GCC) 3.3 20030129 (prerelease)
>Organization:
>Environment:
Debian GNU/Linux (unstable)

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Installed/Config-files/Unpacked/Failed-config/Half-installed
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad)
||/ Name           Version        Description
+++-==============-==============-============================================
ii  gcc            3.2.2-0        The GNU C compiler.
>Description:
int a[20];
int b[20];
 
void
f()
{
    int i1, i2;
 
    for (i1 = 0, i2 = 0; i1 < 20; ++i1)
        a[i1] = b[i2++];
}

In the above code i2 equals i1 and can be eliminated.

% /usr/lib/gcc-snapshot/bin/gcc -Wall -O3 -S -o - bug.c

[...]
        xorl    %edx, %edx
        xorl    %ecx, %ecx
        .p2align 2,,3
.L6:
        movl    b(,%ecx,4), %eax
        movl    %eax, a(,%edx,4)
        incl    %edx
        incl    %ecx
        cmpl    $19, %edx
        jle     .L6
[...]
>How-To-Repeat:

>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:

Re: optimization/9553: No induction variable elimination

[ebotcazou]

Synopsis: No induction variable elimination

State-Changed-From-To: open->analyzed
State-Changed-By: ebotcazou
State-Changed-When: Tue Feb 18 08:14:53 2003
State-Changed-Why:
    Confirmed. GCC doesn't know how to prove that two (independent)
    basic induction variables are equal throughout the whole loop.
    I'm not sure whether that would be really useful in practice though.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9553