Bug#935289: quick checks and possible patch

[Frédéric Bonnard]

Hi,
from my tests the test works with gcc-8 and fails now with gcc-9.
With gcc-9 and O1, the test passes and fails with O2.
I expanded the options (c++ -S) added by O2 compared to O1 in gcc-9 to compile
qd_test.cpp and got :

-falign-functions
-falign-jumps
-falign-labels
-falign-loops
-fcaller-saves
-fcode-hoisting
-fcrossjumping
-fcse-follow-jumps
-fdevirtualize
-fdevirtualize-speculatively
-fexpensive-optimizations
-fgcse
-fhoist-adjacent-loads
-findirect-inlining
-finline-small-functions
-fipa-bit-cp
-fipa-cp
-fipa-icf
-fipa-icf-functions
-fipa-icf-variables
-fipa-ra
-fipa-sra
-fipa-vrp
-fisolate-erroneous-paths-dereference
-flra-remat
-foptimize-sibling-calls
-foptimize-strlen
-fpartial-inlining
-fpeephole2
-freorder-functions
-frerun-cse-after-loop
-fschedule-insns
-fschedule-insns2
-fstore-merging
-fstrict-aliasing
-fthread-jumps
-ftree-pre
-ftree-switch-conversion
-ftree-tail-merge
-ftree-vrp

Then I recompiled qd_test.cpp with the original commands, adding
-fno-$OPT with $OPT being one of the above option (without -f) to check
which one had an impact and found out that adding either
-fno-expensive-optimizations or -fno-tree-pre made the test pass.
From g++-8 O2 to g++-9 O2 I could not determine any flag that made a
difference.

I don't know what is the relation between those flags and the actual
breakage though.
To avoid autoremoval of qd, I propose using one of this flag to workaround
ppc64el g++-9's issue and I'll try to ping the g++ ppc64el dev team to
help on that possible regression.

Here is the MR : https://salsa.debian.org/science-team/qd/merge_requests/1

F.

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