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Ok, so your nice Nim program crashes under various hard-to-reproduce conditions. What can you do about that?

Test with different GCs

--gc:boehm
--gc:refc
--gc:markAndSweep

Note that if your program does not crash with a different GC, it doesn't imply you found a GC bug! It's just a weak indicator.

Test under different OSes

Linux
MacOS X
Windows

Test under different CPUs

i386 (32bit)
amd64 (64bit)
arm

Test different compiler options

-d:release vs debug mode is the obvious choice, but the Nim GC, allocator and standard library have many more checks you can should enable:

-d:useSysAssert enables assertions in the system.nim, especially in Nim's allocator. -d:useGcAssert enables assertions in Nim's GC. -d:nimBurnFree overwrite deallocated memory with 0xff bytes so that "access after free" triggers a segfault.

Edit lib/system/mmdisp.nim and gc.nim

Even more debugging options can be enabled by editing lib/system/mmdisp.nim. Most of these have no --define equivalent, unfortunately.

One problem with corruptions is their non-deterministic nature, in particular heap and stack addresses change from run to run. Define -d:corruption to enable "cell IDs", so that every "cell" (that is every ref/string/seq) gets a unique ID. It's often interesting to see if the corrupted cell has the same ID from run to run or if it differs. If it differs the bug is non-deterministic. Within the GC writeCell can be used to output offending cells.