g++ version: g++ (Compiler-Explorer-Build-gcc--binutils-2.38) 12.1.0 Code link: https://godbolt.org/z/5MansKa5c Compile arguments: -std=c++11 -Wextra -Wall -O2 Code: int f1(); int f2(){ bool v2{v2}; // used uninitialized warning auto const & a = f1(); return a; } int f3(){ auto const & a = f1(); bool v3{v3}; // no warning in g++, got uninitialized warning in clang return a; }
When compile without -O2, g++ will generate "'v3' may be used uninitialized" warning. code link: https://godbolt.org/z/r3sY4srx9
The issue is that for f3() we'd emit a may-uninitialized warning because f1() could have thrown and thus v3{v3} is only executed conditionally. For f2() v2{v2} is always executed (when f2 is entered). The late diagnostic pass that would emit may-uninitialized warnings doesn't get to see the v3{v3} initialization because it was removed as dead code. When not optimizing we choose to diagnose may-uninit before eliminating v3{v3}. Confirmed. Not sure if we want to fix this. We eventually might use a different predicate than post-domination for whether a stmt is considered always executed (ignoring EH and abnormal control flow). Greedily following "fallthru" edges might work for this, also implementing the unreachable edge honoring the ??? comment says. I have a patch.
The master branch has been updated by Richard Biener <rguenth@gcc.gnu.org>: https://gcc.gnu.org/g:a1cd4d52d6ef90b977fb2d80c1cf17f3efa5b01d commit r13-2926-ga1cd4d52d6ef90b977fb2d80c1cf17f3efa5b01d Author: Richard Biener <rguenther@suse.de> Date: Fri Aug 19 15:11:14 2022 +0200 tree-optimization/105646 - re-interpret always executed in uninit diag The following fixes PR105646, not diagnosing int f1(); int f3(){ auto const & a = f1(); bool v3{v3}; return a; } with optimization because the early uninit diagnostic pass only diagnoses always executed cases. The patch does this by re-interpreting what always executed means and choosing to ignore exceptional and abnormal control flow for this. At the same time it improves things as suggested in a comment - when the value-numbering run done without optimizing figures there's a fallthru path, consider blocks on it as always executed. PR tree-optimization/105646 * tree-ssa-uninit.cc (warn_uninitialized_vars): Pre-compute the set of fallthru reachable blocks from function entry and use that to determine wlims.always_executed. * g++.dg/uninit-pr105646.C: New testcase.
Fixed on trunk.