Kind of undefined behavior (or bug?) with GCC 10.1

[Roland Denis]

Hi,

I encounter a strange bug while compiling a project with GCC 10.1 and 
-O3 that looks like an undefined behavior but I don't found where. 
Result is correct with previous version of GCC, with LLVM & Intel.

I have attached a snippet (also on godbolt 
https://gcc.godbolt.org/z/EC4WYZ) that I tried to minimize but remaining 
lines seems to be important to reproduce the erroneous result. 
Uncommenting any of the indicated lines solve the issue.

To explain the code a bit: from a 3D Point with integer coordinates, I 
convert it into another coordinate system (using uSpel that returns a 
Cell). I use a dimension iterator q (DirIterator, that doesn't look like 
an iterator anymore) and the uIncident function to visit neighbors of 
the cell. From the point (0, 0, 0), I should get the cell c=(1, 1, 1) 
and the incident cell f1=(0, 1, 1). I additionally copy f1 into f2 (to 
increase the weirdness of the bug).

Here are the result with and without -O3 flag:

$ g++ bug_minimal.cpp && ./a.out
q.dir = 0 ; f1 = 0 1 1  ; f2 = 0 1 1

$ g++ -O3 bug_minimal.cpp && ./a.out
q.dir = 0 ; f1 = 1 1 1  ; f2 = 0 1 1

The bug is in the first coordinate of f1 that should be 0.

As I explain above, slightly modifying the code implies the right 
result, e.g.:
- declaring default copy or move constructor for Point
- using std::array in place of Point
- using Point in place of Cell
- passing Cell by reference instead of copy in uIncident
- constructing Cell without uSpel
- specifying the incidence direction by hand

Am I missing something important or does that look like a GCC bug ?

Regards,

PS: result is correct with trunk version of GCC in godbolt, does that 
mean it is an already solved bug?

-- 
Roland DENIS


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