Raising: drop accesses through null buffers - #3010
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Staging helpers (mfem's Read/Write) return null for empty buffers, so a captured device pointer reaches the kernel as select(size > 0, ptr, null) and the raising fails on the select. When the pointer's value is only ever consumed as the address of a memory access, the null arm can only fault, so the select collapses to the real pointer. Pointers whose value is observed directly (compared, cast to int, stored as data, passed to a call) are left alone. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_016zErYp7upmqr4NHfhod9UD
# Conflicts: # src/enzyme_ad/jax/Passes/AffineToStableHLORaising.cpp
An empty optional buffer arrives as a null base pointer - directly, behind offset arithmetic in a select arm, or as a mixed-typed view of the null. Every access through it sits on a path that can only fault: loads read as zero, stores vanish, and the null never has to become a kernel argument. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_016zErYp7upmqr4NHfhod9UD
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Stacks on #3006 and includes #2992's null-select fold (will rebase when those merge).
mfem's staging helpers return null for empty buffers, so a captured device pointer can arrive as a null base pointer — directly, hidden behind offset arithmetic in a select arm (
select(p, gep(buf, i), gep(null, i))), or as mixed-typed views of the null (an emptyAhat_iiviewed as both?xi32pivots and?xf64data). Every access through it sits on a path that can only fault: loads read as zero, stores vanish, and the null never has to become a kernel argument.Part of #2968 (hybridization/derefmat triage).
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https://claude.ai/code/session_016zErYp7upmqr4NHfhod9UD