other/9359: odd performance behavior on Athlon XP1800+

other/9359: odd performance behavior on Athlon XP1800+

[lda]

>Number:         9359
>Category:       other
>Synopsis:       odd performance behavior on Athlon XP1800+
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Fri Jan 17 14:16:00 PST 2003
>Closed-Date:
>Last-Modified:
>Originator:     Larry Auton
>Release:        gcc-2.95.3
>Organization:
>Environment:
GNU/Linux on Athlon XP1800+
>Description:
unexpected performance difference with no obvious explanation.
take a look at the attached program.  on an Athlon XP1800+,
the run-time varies by a factor of four with a single change
that should make no difference at all.

the code runs the same speed on Intel machines regardless of the change.

I diffed the assembly language code and found only a single line difference:

$ diff fa sl
28c28
<       movl $0,-8(%ebp)
---
>       nop
>How-To-Repeat:
: eagle 632$; cc t.c ; time a.out

real    0m2.306s
user    0m2.250s
sys     0m0.050s
: eagle 633$; cc -DFAST t.c ; time a.out

real    0m0.671s
user    0m0.620s
sys     0m0.060s
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: application/octet-stream; name="t.c"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="t.c"

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