gcc-14 bugs, pile 1

July 12, 2023

Around beginning of the May gcc-14 development got opened for major changes to be merged into master branch. 2 months have passed since and I collected a “round” number of bugs: 16. Today is also a day when I managed to build my whole system by gcc-master without encountering any obvious bugs.

Summary

Here is the full list of bugs I encountered in chronological order:

ipa/109643: ICE on pkg-config
ipa/109652: ICE on valgrind
tree-optimization/109732: wrong code on json test suite.
c++/109755: imprecise warning location.
tree-optimization/110067: wrong code on pixman test suite.
middle-end/110228: wrong code on llvm JIT test suite.
tree-optimization/110271: ICE on pycryptodome
target/110274: wrong code on highway test suite.
debug/110308: ICE on audiofile.
tree-optimization/110332: ICE on llvm.
middle-end/110443: ICE on a52dec.
middle-end/110515: wrong code on llvm DWARF test suite.
c++/110523: ICE on json.
c++/110580: rejects valid on nix code.
c++/110598: wrong code for llvm AMDGCN test suite.
tree-optimization/110601: ICE on systemd.

Let’s look at a few histograms. Looking at the manifestation of the bug:

ICE: 8
wrong-code: 6
other: 2

Wrong code is almost as much as compiler crashes. Some of wrong codes took me a while to extract from the real project. Once again robust test suites make it so much easier to extract an example for upstream reporting.

Looking at the subsystems:

tree-optimization: 5
c++: 4
middle-end: 3
ipa: 2
debug: 1
target: 1

Half the bugs happened in generic optimization phase (tree-optimization and middle-end). A few bugs in c++ frontend (mainly in tempalte instantiation code). And only one failure was target-specific. It should be obvious by now that I did no test gcc on anything besides x86_64 :).

Tip of the day

When chasing wrong code generation down to a particular source file I found the #pragma GCC optimize(1) injection hack to be useful:

# flip first halg to -O1
$ git grep -LF 'pragma GCC optimize' -- '**.cpp' | head -n $NUMBER |
    xargs sed '1i #pragma GCC optimize(1)' -i

# flip second half to -O1
$ git grep -LF 'pragma GCC optimize' -- '**.cpp' | tail -n +$(($NUMBER+1)) |
    xargs sed '1i #pragma GCC optimize(1)' -i

That way I flip $NUMBER files down to -O1 optimization level (assuming it is enough to inhibit the bug from happening) and bisect it down to a single file without -O1 injected.

ccache makes pragma addition and removal very cheap. You effectively need to compile project only twice: with default options and with -O1 pragma override. The rest is a bit of linking.

Parting words

Running gcc-master is still a lot of fun. I keep finding ~2 new bugs per week on a regular basis. I did not get to fix any of them: in almost all cases maintainers figured out the fix way before I got closer to the culprit code in gcc. Maybe one day I’ll get there first :)

llvm and json remain to be the hardest stress tests for gcc.

Have fun!