optimization/3078: First time through loop produces different result

[spitzak at d2 dot com]

>Number:         3078
>Category:       optimization
>Synopsis:       First time through loop produces different result
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Jun 07 13:16:02 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Bill Spitzak
>Release:        gcc version 2.96 20000731 (Red Hat Linux 7.0)
>Organization:
>Environment:
/usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
>Description:
// This code is designed to make a lookup table that uses the top 16
// bits of a floating point number to index it. The error is that the
// first time through the loop the floating point number is calculated
// wrong. Compare the output when compiled with no optimization and the
// output when compiled with -O2 or better.

// In our actual program we got a significantly different value. This version
// seems to only differ in the low 16 bits.
>How-To-Repeat:
Compile above program with and without -O2 and run diff on the output.
>Fix:
I fixed our code by artificially running the loop one extra
time for an index of -1 and ignoring that result. It appears
to produce the correct output on all passes except the first.

Any recommendations on how to construct such a lookup table
without relying on aliasing would be appreciated.

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