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Restore cache.A before QR safety fallback after in-place LU corruption - #888
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ChrisRackauckas merged 2 commits intoFeb 19, 2026
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In-place LU factorizations (MKLLUFactorization, AppleAccelerateLU, etc.) corrupt cache.A via getrf!. When LU fails on a singular matrix and the DefaultLinearSolver safety fallback triggers, the QR fallback was operating on the corrupted matrix, producing incorrect results. Fix: Store a reference to the original prob.A in DefaultLinearSolverInit.A_backup (zero-cost since alias_A=false means cache.A is already a copy). Before the QR fallback, restore cache.A via copyto!. When alias_A=true (cache.A === prob.A), skip the fallback with a warning since there is no clean backup available. Fixes SciML#887 Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com> Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
- Replace @warn with @SciMLMessage(:default_lu_fallback) in the aliased fallback warning path for consistency with the rest of the codebase - Add regression test verifying QR fallback produces correct results on singular matrices and that prob.A is preserved as backup Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com> Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
cache.Aviagetrf!. When LU fails on a singular matrix andDefaultLinearSolver's safety fallback triggers QR, the QR operates on the corrupted matrix producing incorrect results.prob.AinDefaultLinearSolverInit.A_backup(zero-cost: whenalias_A=false(default),cache.Ais already a copy soprob.Ais untouched). Before QR fallback, restore viacopyto!(cache.A, backup).alias_A=true(cache.A === prob.A), skip the fallback with a clear warning since there is no clean backup.Fixes #887
Root Cause
MKLLUFactorization(and other in-place LU variants) call LAPACK'sgetrf!which overwritescache.Ain-place with the LU factors. When the matrix is singular,getrf!returns an error code andsolve!returnsReturnCode.Failure. TheDefaultLinearSolverthen falls back toQRFactorization(ColumnNorm()), butcache.Astill contains the corrupted LU factors rather than the original matrix. This causes the QR fallback to produce garbage results.This was discovered via Julia 1.11 vs 1.12 behavioral difference: MKL_jll loads on 1.11 (selecting
MKLLUFactorization) but not on 1.12 (selectingLUFactorizationwhich useslu(A, check=false)— non-destructive).Test plan
DefaultLinearSolvermatches directQRFactorization(ColumnNorm())resultprob.Ais not modified (serves as backup)A_backupreferencesprob.A(not the copy) withalias_A=falsealias_A=truecorrectly detected (cache.A === A_backup)🤖 Generated with Claude Code