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optimization/8126: Floating point computation far slower in 3.2 than in 2.95


>Number:         8126
>Category:       optimization
>Synopsis:       Floating point computation far slower in 3.2 than in 2.95
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Wed Oct 02 07:56:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Olivier Lauffenburger
>Release:        gcc version 3.2
>Organization:
>Environment:
Cygwin 1.3.12-2 / Windows 2000 SP3 / Pentium III 800MHz
>Description:
The enclosed file test.ii computes one hundred million vector products.
Compiled with gcc 2.95, it takes 2770 ms. Compiled with gcc 3.2, it takes 4470 ms.
A examination of the assembly code generated by gcc 3.2 shows that the stack is used instead of the floating point registers.
The code generated by gcc 3.2 with optimization -O3 looks like the one generated by gcc 2.95 with optimization -O1.
>How-To-Repeat:
g++ -O3 -ffast-math -fomit-frame-pointer test.ii -o optim.exe
>Fix:
With -funroll-loops, the code is better (but still slower than gcc 2.95)
>Release-Note:
>Audit-Trail:
>Unformatted:
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