revert: reject marginal numeric VM slot reuse

crates/fidc-core/src/numeric_expr_vm.rs

@@ -168,9 +168,10 @@ impl Program {
                 Instruction::Push(value) => scratch.stack.push(value),
                 Instruction::LoadVariable(index) => {
                     let index = usize::from(index);
-                    let value = if scratch.variable_generations[index] == scratch.generation {
+                    let cached = scratch.variables[index];
-                        scratch.variables[index]
+                    let value = match cached {
-                    } else {
+                        Some(value) => value,
+                        None => {
                             let expected_type = self.variable_types[index];
                             let value = resolve(index, &self.variables[index], expected_type)?;
                             if value.value_type() != expected_type {
@@ -181,27 +182,22 @@ impl Program {
                                     value.value_type()
                                 )));
                             }
-                        scratch.variables[index] = value;
+                            scratch.variables[index] = Some(value);
-                        scratch.variable_generations[index] = scratch.generation;
                             value
+                        }
                     };
                     scratch.stack.push(value);
                 }
                 Instruction::LoadLocal(index) => {
                     let index = usize::from(index);
-                    if scratch.local_generations[index] != scratch.generation {
+                    let value = scratch.locals[index].ok_or_else(|| {
-                        return Err(EvalError::new(format!(
+                        EvalError::new(format!("local slot {index} was not initialized"))
-                            "local slot {index} was not initialized"
+                    })?;
-                        )));
-                    }
-                    let value = scratch.locals[index];
                     scratch.stack.push(value);
                 }
                 Instruction::StoreLocal(index) => {
-                    let index = usize::from(index);
                     let value = pop(&mut scratch.stack)?;
-                    scratch.locals[index] = value;
+                    scratch.locals[usize::from(index)] = Some(value);
-                    scratch.local_generations[index] = scratch.generation;
                 }
                 Instruction::Unary(operator) => {
                     let value = pop(&mut scratch.stack)?;
@@ -264,11 +260,8 @@ impl Program {
 #[derive(Debug, Default)]
 pub(crate) struct Scratch {
     stack: Vec<Value>,
-    variables: Vec<Value>,
+    variables: Vec<Option<Value>>,
-    variable_generations: Vec<u32>,
+    locals: Vec<Option<Value>>,
-    locals: Vec<Value>,
-    local_generations: Vec<u32>,
-    generation: u32,
 }
 
 impl Scratch {
@@ -280,21 +273,10 @@ impl Scratch {
                 .len()
                 .saturating_sub(self.stack.capacity()),
         );
-        self.generation = self.generation.wrapping_add(1);
+        self.variables.clear();
-        if self.generation == 0 {
+        self.variables.resize(program.variables.len(), None);
-            self.variable_generations.fill(0);
+        self.locals.clear();
-            self.local_generations.fill(0);
+        self.locals.resize(program.local_count, None);
-            self.generation = 1;
-        }
-        if self.variables.len() < program.variables.len() {
-            self.variables
-                .resize(program.variables.len(), Value::Number(0.0));
-            self.variable_generations.resize(program.variables.len(), 0);
-        }
-        if self.locals.len() < program.local_count {
-            self.locals.resize(program.local_count, Value::Number(0.0));
-            self.local_generations.resize(program.local_count, 0);
-        }
     }
 }
 
@@ -1380,27 +1362,6 @@ mod tests {
         }
     }
 
-    #[test]
-    fn scratch_generation_rollover_invalidates_cached_slots() {
-        let program = compile("left + right", |_| Some(ValueType::Number)).expect("compile");
-        let mut scratch = Scratch::default();
-        let first = program
-            .evaluate(&mut scratch, |_index, name, _expected| {
-                Ok(Value::Number(if name == "left" { 1.0 } else { 2.0 }))
-            })
-            .expect("first evaluation");
-        assert_eq!(first, Value::Number(3.0));
-
-        scratch.generation = u32::MAX;
-        let second = program
-            .evaluate(&mut scratch, |_index, name, _expected| {
-                Ok(Value::Number(if name == "left" { 10.0 } else { 20.0 }))
-            })
-            .expect("evaluation after generation rollover");
-        assert_eq!(second, Value::Number(30.0));
-        assert_eq!(scratch.generation, 1);
-    }
-
     #[test]
     fn matches_rhai_for_representative_numeric_boolean_corpus() {
         let source = r#"

docs/evidence/numeric-vm-binding-generation-rejection-20260906.json

@@ -0,0 +1,74 @@
+{
+  "schemaVersion": "fidc-numeric-vm-binding-generation-rejection/v1",
+  "measuredAt": "2026-09-06T05:36:00+08:00",
+  "host": "192.168.31.177",
+  "baseline": {
+    "engineCommit": "5f08978",
+    "primaryFiveYearHotMedianEngineSeconds": 2.629,
+    "primaryFiveYearTotalReturn": 0.9219861819172002,
+    "primaryFiveYearTradeCount": 26088,
+    "primaryCanonicalSha256": "b42fea66237d06eadb24f6b8c9e2760e7319fe3699f315b99e01f433ef2aa234",
+    "primaryResultStoreSha256": "9ccf0c0fc6f5d72e974381ad4cd09a80241d7de99f2649f2b01736f7c80dc2c7",
+    "profileInstructions": 16600366059,
+    "profileBranches": 2592214949
+  },
+  "compileTimeIdentifierBinding": {
+    "engineCommit": "2135a5b",
+    "implementation": "map every numeric VM identifier to a typed runtime enum during expression plan compilation",
+    "primaryFiveYearHotEngineSeconds": [2.608, 2.624, 2.651, 2.619, 2.781],
+    "primaryFiveYearHotMedianEngineSeconds": 2.624,
+    "observedMedianImprovementPercent": 0.190186,
+    "profileInstructions": 16360894880,
+    "instructionReductionPercent": 1.442566,
+    "secondaryFiveYearResultConsistent": true,
+    "secondaryFiveYearPerformanceExcluded": true,
+    "secondaryFiveYearExclusionReason": "the symbolic campaign entered a high-memory-bandwidth phase between the primary and secondary batches",
+    "netCodeLinesAdded": 562,
+    "retained": false
+  },
+  "generationStampedScratchSlots": {
+    "engineCommit": "5122c73",
+    "implementation": "invalidate numeric VM variable and local slots with a generation counter instead of clearing Option arrays for each evaluation",
+    "localReleaseBenchmark": {
+      "baselineVmNanosecondsPerEvaluation": 86.992,
+      "candidateSamples": [83.284, 94.166, 86.275, 84.138, 85.453],
+      "candidateMedianNanosecondsPerEvaluation": 85.453,
+      "componentImprovementPercent": 1.769136,
+      "comparisonStrength": "weak because the baseline contains one sample"
+    },
+    "primaryFiveYearHotEngineSeconds": [2.634, 2.636, 2.608, 2.642, 2.710],
+    "primaryFiveYearHotMedianEngineSeconds": 2.636,
+    "observedMedianRegressionPercent": 0.266261,
+    "profileInstructions": 16521146700,
+    "instructionReductionPercent": 0.477215,
+    "netCodeLinesAdded": 39,
+    "retained": false
+  },
+  "hostLoad": {
+    "symbolicWorkersObserved": 3,
+    "symbolicWorkerCpuPercentApproximate": [720, 718, 698],
+    "symbolicWorkerRssBytesApproximate": [54479982592, 53353455616, 53941170176],
+    "wallTimeComparisonsAcrossPhasesExcluded": true
+  },
+  "testGate": {
+    "workspacePassedBeforeFirstCandidateRejection": 539,
+    "workspacePassedForGenerationCandidate": 540,
+    "failed": 0,
+    "ignoredManualBenchmarks": 8
+  },
+  "remoteArtifacts": [
+    "/srv/fidc/canonical/run/fidc-private/evidence/goal-primary-five-year-20260906.json",
+    "/srv/fidc/canonical/run/fidc-private/evidence/identifier-binding-primary-five-year-20260906.json",
+    "/srv/fidc/canonical/run/fidc-private/evidence/identifier-binding-secondary-five-year-20260906.json",
+    "/srv/fidc/canonical/run/fidc-private/evidence/identifier-binding-candidate-perf-stat-20260906.csv",
+    "/srv/fidc/canonical/run/fidc-private/evidence/identifier-binding-rollback-primary-five-year-20260906.json",
+    "/srv/fidc/canonical/run/fidc-private/evidence/identifier-binding-rollback-perf-stat-20260906.csv",
+    "/srv/fidc/canonical/run/fidc-private/evidence/vm-generation-primary-five-year-20260906.json",
+    "/srv/fidc/canonical/run/fidc-private/evidence/vm-generation-candidate-perf-stat-20260906.csv"
+  ],
+  "decision": {
+    "status": "rejected_and_removed",
+    "reason": "both candidates preserved exact business results but failed to produce a material, stable end-to-end improvement; the typed binding added disproportionate code and the generation slots slightly regressed the five-year median",
+    "nextTarget": "profile and specialize the numeric VM instruction dispatch or runtime helper execution without changing expression, PIT, or lazy short-circuit semantics"
+  }
+}