simd: withdraw mask_gather_chain_u32 (rejected design)

.claude/blackboard.md

@@ -1,3 +1,9 @@
+## 2026-10-12 — `mask_gather_chain_u32` withdrawn: rejected design, moved to backup
+
+- **Removed:** `mask_gather_chain_u32`, `mask_gather_chain_u32_under`, `ChainHop`, `ChainTest`, their tests and the parity case 0xD02/0xD03 (shipped in #358). The exact state is kept on branch `backup/session-011BMJ-2026-10-12`.
+- **Why it is wrong, not merely slow:** it treated a hop as a key read. Every anchor row walked every hop, each read's address being the value the previous read returned: scalar, dependent, proportional to rows × depth. The graph has no foreign keys; everything is Cartesian coordinates. A hop is a coordinate range on the sealed ordered address, and a chain of hops is the intersection of those ranges (`[max(lo), min(hi))`); no value is read as an address. lance-graph had already measured this before #358 was written: D-TELEPORT-0 (#1484) shows a gated gather accelerates work but does not eliminate it, while a range found at the crossing point eliminates it (COUNT `hi - lo`, ~28 ns). The crossword probe (D-PUZZLE-0) had also recorded the gather as "not needed: elimination is an AND within one space". Neither was read.
+- **No replacement here.** The rebuild uses what lance-graph's mask-risc and Quack already have (`Lowering::Range`, ternlog fusion over resident planes, `CountKeyRunsU32`, `fold_ranges`, `prefix_*`). A primitive is added to ndarray only if that rebuild measures a gap.
+
 ## 2026-10-11 — `cpu_guard` no longer kills library hosts; three OOB paths and the AMX rung closed; fleet co-research map
 
 - **Defect found and fixed (X1):** the pre-`main` hook from #348 runs in every artifact that links ndarray, a `cdylib` included. Measured: an `x86-64-v4` cdylib `dlopen`ed by a C host under `qemu -cpu Haswell` ended the host with 132. A JVM loading lance-graph-java's `.so`, or Python loading tesseract-ocr-python, would die the same way. The guard now exits only when its code lies in the main executable (ELF: object at `getauxval(AT_PHDR)`, FreeBSD `elf_aux_info`; macOS `dladdr` vs `_NSGetMachExecuteHeader()`; Windows `GetModuleHandleExW`); in a library it prints a `warning:` and returns. Every lookup failure answers "library".

crates/simd-masking-parity/src/lib.rs

@@ -41,14 +41,14 @@ use ndarray::simd::{
     ge_u8_to_mask, gt_i32_to_mask, gt_i32_to_mask_under, gt_u64_to_mask, gt_u8_to_mask, le_i32_to_mask,
     le_i32_to_mask_under, le_u64_to_mask, le_u8_to_mask, lt_i32_to_mask, lt_i32_to_mask_under, lt_u64_to_mask,
     lt_u8_to_mask, mask_all, mask_and, mask_and_assign, mask_andnot, mask_andnot_assign, mask_any,
-    mask_gather_chain_u32, mask_gather_chain_u32_under, mask_gather_u32, mask_gather_u32_under, mask_not,
+    mask_gather_u32, mask_gather_u32_under, mask_not,
     mask_not_assign, mask_or, mask_or_assign, mask_scatter_or_u32, mask_set_range, mask_shift_morton, mask_ternlog,
     mask_ternlog_any, mask_ternlog_assign, mask_ternlog_popcount, mask_xor, mask_xor_assign, masked_group_sum_i32,
     masked_group_sum_i32_via, masked_key_run_count_u32, masked_max_i32, masked_min_i32, masked_strided_group_sum,
     masked_sum_i32, masked_sum_wrapping_add_i32, ne_i32_to_mask, ne_i32_to_mask_under, ne_u32_to_mask,
     ne_u32_to_mask_under, ne_u64_to_mask, ne_u8_to_mask, ternary_match_strided16_to_mask,
     ternary_match_strided_to_mask, ternary_match_u32_to_mask, ternary_match_u32_to_mask_under,
-    ternary_match_u64_to_mask, ternary_match_u64_to_mask_under, ternlog, ChainHop, ChainTest, F64x8, I32x16, I8x16,
+    ternary_match_u64_to_mask, ternary_match_u64_to_mask_under, ternlog, F64x8, I32x16, I8x16,
     KeyRunCarry, MortonDir, U32x16, U64x8, U8x16,
 };
 
@@ -1591,79 +1591,6 @@ fn check_gather_scatter_group() -> Result<(), u32> {
             return Err(0xD01);
         }
 
-        // ── mask_gather_chain_u32[_under] ────────────────────────────────
-        // Two hops: index -> mid (live + one test, keyed onward through
-        // `mid_next`) -> far (live + one negated test). The reference walks
-        // the chain row by row from the same lanes; out-of-range keys at
-        // both depths exercise the "false by contract" arm.
-        let mid_rows = 30usize;
-        let far_rows = 20usize;
-        let mid_live: Vec<u64> = (0..words_for(mid_rows)).map(|_| rng.next()).collect();
-        let far_live: Vec<u64> = (0..words_for(far_rows)).map(|_| rng.next()).collect();
-        let mid_lane: Vec<u32> = (0..mid_rows).map(|_| (rng.next() % 3) as u32).collect();
-        let far_lane: Vec<u32> = (0..far_rows).map(|_| (rng.next() % 3) as u32).collect();
-        let mid_next: Vec<u32> = (0..mid_rows)
-            .map(|j| {
-                if j % 7 == 0 {
-                    (far_rows + 2) as u32
-                } else {
-                    (rng.next() % far_rows as u64) as u32
-                }
-            })
-            .collect();
-        let cindex: Vec<u32> = (0..n)
-            .map(|i| {
-                if i % 5 == 0 {
-                    (mid_rows + 1 + i) as u32
-                } else {
-                    (rng.next() % mid_rows as u64) as u32
-                }
-            })
-            .collect();
-        let mid_tests = [ChainTest {
-            lane: &mid_lane,
-            value: 1,
-            negate: false,
-        }];
-        let far_tests = [ChainTest {
-            lane: &far_lane,
-            value: 2,
-            negate: true,
-        }];
-        let hops = [
-            ChainHop {
-                live: &mid_live,
-                rows: mid_rows,
-                tests: &mid_tests,
-                next: Some(&mid_next),
-            },
-            ChainHop {
-                live: &far_live,
-                rows: far_rows,
-                tests: &far_tests,
-                next: None,
-            },
-        ];
-        let bit = |w: &[u64], k: usize| (w[k / 64] >> (k % 64)) & 1 == 1;
-        let holds = |i: usize| {
-            let a = cindex[i] as usize;
-            if a >= mid_rows || !bit(&mid_live, a) || mid_lane[a] != 1 {
-                return false;
-            }
-            let b = mid_next[a] as usize;
-            b < far_rows && bit(&far_live, b) && far_lane[b] != 2
-        };
-        let mut out = vec![u64::MAX; out_len];
-        mask_gather_chain_u32(&cindex, &hops, &mut out);
-        if out != reference_mask(n, out_len, holds) {
-            return Err(0xD02);
-        }
-        let mut out = vec![u64::MAX; out_len];
-        mask_gather_chain_u32_under(&cindex, &gate, &hops, &mut out);
-        if out != reference_mask(n, out_len, |i| bit(&gate, i) && holds(i)) {
-            return Err(0xD03);
-        }
-
         // ── mask_scatter_or_u32 ──────────────────────────────────────────
         // `out2` starts explicitly zeroed (the caller's job now, not the
         // primitive's): the accumulate contract means a dirty prefill would

src/simd.rs

@@ -868,8 +868,6 @@ pub use crate::simd_masking_ops::{
     mask_andnot,
     mask_andnot_assign,
     mask_any,
-    mask_gather_chain_u32,
-    mask_gather_chain_u32_under,
     mask_gather_u32,
     mask_gather_u32_under,
     mask_not,
@@ -933,8 +931,6 @@ pub use crate::simd_masking_ops::{
     widen_bounded_cross_power_sums,
     widen_bounded_power_sums,
     BoundedFoldError,
-    ChainHop,
-    ChainTest,
     CrossPowerSums,
     KeyRunCarry,
     MortonDir,