Fix Tridiagonal cache mutation with parameterized DirectLdiv!{cache} #856
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…ciML#825) The issue: DirectLdiv! for Tridiagonal matrices calls ldiv!(u, A, b) which performs in-place LU factorization, mutating cache.A and breaking subsequent solves with the same cache. The fix preserves DirectLdiv! for performance while preventing cache.A mutation: - Add init_cacheval for DirectLdiv! with Tridiagonal/SymTridiagonal that allocates a copy of the matrix during init (one-time allocation) - Add specialized solve! methods that copy cache.A values to the cached workspace before calling ldiv! (non-allocating copyto!) - The original cache.A is never touched, preserving it for subsequent solves This gives the best of both worlds: - Fast DirectLdiv! performance (direct ldiv! without lu factorization overhead) - Non-destructive behavior (cache.A is preserved) - Minimal allocations during solve! (same 48 bytes as LUFactorization) Added regression test that verifies: - Default algorithm for Tridiagonal is DirectLdiv! on Julia 1.11+ - cache.A is not mutated after solve! - Multiple solves with same cache give correct answers - Minimal allocations during solve! 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
This allows explicit control over whether DirectLdiv! caches a copy
of the matrix during init. DirectLdiv!{true} caches, DirectLdiv!{false}
does not. Default is Val(true) for backwards compatibility.
- DirectLdiv!() → DirectLdiv!{true} (default, caches)
- DirectLdiv!(Val(true)) → DirectLdiv!{true} (explicit caching)
- DirectLdiv!(Val(false)) → DirectLdiv!{false} (no caching, may mutate A)
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
ChrisRackauckas
approved these changes
Dec 15, 2025
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Summary
solve!onLinearCachewithTridiagonalmatrix now correctly preservescache.Awithout mutationVal{cache}type parameter toDirectLdiv!to control whether a matrix copy is cached:DirectLdiv!()→DirectLdiv!{true}(default, caches for Tridiagonal/SymTridiagonal)DirectLdiv!(Val(true))→ explicit cachingDirectLdiv!(Val(false))→ no caching (may mutate A)TridiagonalandSymTridiagonal,ldiv!performs in-place LU factorization which mutates the matrix. The caching variant allocates a workspace duringinitand uses non-allocatingcopyto!duringsolve!Implementation
DirectLdiv!struct to have a{cache}type parameterinit_cachevalmethods forDirectLdiv!{true}withTridiagonal/SymTridiagonalthat allocate a copysolve!methods that copy values into the workspace then callldiv!on the copycache.Ais not mutated and multiple solves work correctlyTest plan
DirectLdiv!()defaults toDirectLdiv!{true}Tridiagonalmatrix is not mutated aftersolve!solve!on same cache work correctlySymTridiagonalalso works correctly🤖 Generated with Claude Code