possible issue with aggressive optimisations in cernlib

Asked by schwicke on 2018-11-28

Hi, all,

within the data preservation activities for one of the old LEP experiments which relies on CERNLIB (32bit) I stumbled over an issue which may be relevant for the building CERNLIB for Ubuntu (or any other distro) with gfortran > 4.4.

While recompiling packlib (in 32bit_ and in particular hbook I get a new warning
"gfortran iteration 2 invokes undefined behavior [-Waggressive-loop-optimizations]"
(I used (Ubuntu 7.3.0-27ubuntu1~18.04)
With CERNLIB compiled like that, I found that in more complex programs at least hbook cannot fill historgrams which have been booked before, claiming they do not exist which points to a zebra issue. Simple hbook based programs apparently still worked fine.

After adding -fno-aggressive-loop-optimizations to the compiler options when compiling cernlib this issue seems to have gone.
So this looks to me as if the aggressive loop optimization feature you get when compiling with -O2 breaks ZEBRA in some cases at least.

So take this as a heads-up. If somebody can reproduce this it may be worth to turn this into a bug.

Best,
Ulrich

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2018-11-28
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