https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99623
Bug ID: 99623
Summary: Code behaves differently at -O2 optimization
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: sebastiano.vigna at unimi dot it
Target Milestone: ---
Created attachment 50406
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50406&action=edit
Output of --save-temps
The included code compares two variants of Ben Pfaff's GNU LibAVL library. One
implementation is the original one, and it stores pointers to uint64_t values,
comparing them with a user-supplied function. The second implementation is a
mild variation storing the uint64_t values directly into the map, and comparing
them directly using the language operators. The two implementations should be
equivalent--the code builds the same RB tree with the same values and compares
the results after each insertion.
The two trees are identical:
- with clang, any level optimization, Mac OS X, Linux
- with gcc 10.2.1, no optimization or -O1, Mac OS X, Linux
The two trees diverge after 5 insertions:
- with gcc 10.2.1, -O2, Mac OS X, Linux
We have tried all possible diagnostics, including the sanitizer, but any
perturbation to the code makes the two trees identical. The problem seems to
lie in myprob_probe()--even adding a printf(""); anywhere between lines 207-231
makes the test work.
Note that this test is reported using a compare() function for trees that might
be bugged, but the original bug was that when deleting the tree the library
would give a segmentation fault. We tracked down the problem to a divergence in
the construction of the tree.
To replicate the bug, try
gcc -O1 bug.c
./a.out
gcc -O2 bug.c
./a.out
