[Refactor] Simplify index sign state handling in LegalizeNegativeIndex (#1354)

This commit refines the logic for determining the sign state of indices in the LegalizeNegativeIndex transformation. It prioritizes vector patterns, specifically Ramp and Broadcast nodes, to avoid compile-time lane queries. The handling of scalar indices is also streamlined, ensuring clearer diagnostics when non-negativity cannot be proven. These changes enhance the robustness and clarity of index handling in the transformation pass.

src/transform/legalize_negative_index.cc

@@ -44,52 +44,28 @@ private:
       PrimExpr simplified = analyzer_.Simplify(indices[i]);
       IndexSignState state = IndexSignState::kUnknown;
 
-      // Handle scalar indices with the standard analyzer
-      if (simplified.dtype().lanes() == 1) {
-        if (analyzer_.CanProve(simplified >= 0))
+      // Handle vector patterns first to avoid querying lanes() on
+      // scalable vectors (which is not allowed at compile-time).
+      if (const auto *ramp = simplified.as<RampNode>()) {
+        // For scalable vectors, we cannot rely on a constant lane count.
+        // Use sufficient (but not necessary) conditions:
+        // - If base >= 0 and stride >= 0, all lanes are non-negative.
+        // - If base < 0 and stride <= 0, all lanes are negative.
+        bool base_nonneg = analyzer_.CanProve(ramp->base >= 0);
+        bool base_neg = analyzer_.CanProve(ramp->base < 0);
+        bool stride_nonneg = analyzer_.CanProve(ramp->stride >= 0);
+        bool stride_nonpos = analyzer_.CanProve(ramp->stride <= 0);
+
+        if (base_nonneg && stride_nonneg) {
           state = IndexSignState::kNonNegative;
-        else if (analyzer_.CanProve(simplified < 0))
+        } else if (base_neg && stride_nonpos) {
           state = IndexSignState::kNegative;
-        else
+        } else {
           DLOG(WARNING)
               << "LegalizeNegativeIndex: cannot prove non-negative index "
               << simplified << " for buffer " << buffer_name << " (axis " << i
               << ", index " + indices[i]->Script() + ").";
         }
-      // Vector indices: try to reason about non-negativity/negativity
-      // Common patterns are Ramp(base, stride, lanes) and Broadcast(value,
-      // lanes).
-      else if (const auto *ramp = simplified.as<RampNode>()) {
-        // Compute a safe lower/upper bound for the vector lanes
-        // lower_bound = base_min + min(0, stride_min) * (lanes - 1)
-        // upper_bound = base_max + max(0, stride_max) * (lanes - 1)
-        auto base_bound = analyzer_.const_int_bound(ramp->base);
-        auto stride_bound = analyzer_.const_int_bound(ramp->stride);
-        int lanes = *as_const_int(ramp->lanes);
-
-        int64_t base_min = base_bound->min_value;
-        int64_t base_max = base_bound->max_value;
-        int64_t s_min = stride_bound->min_value;
-        int64_t s_max = stride_bound->max_value;
-
-        // Guard against overflow is not strictly necessary here because
-        // bounds may be +/-inf represented by sentinel values.
-        int64_t lower = base_min;
-        if (s_min < 0)
-          lower += s_min * (lanes - 1);
-        int64_t upper = base_max;
-        if (s_max > 0)
-          upper += s_max * (lanes - 1);
-
-        if (lower >= 0)
-          state = IndexSignState::kNonNegative;
-        else if (upper < 0)
-          state = IndexSignState::kNegative;
-        else
-          DLOG(WARNING)
-              << "LegalizeNegativeIndex: cannot prove non-negative index "
-              << simplified << " for buffer " << buffer_name << " (axis " << i
-              << ", index " + indices[i]->Script() + ").";
       } else if (const auto *broadcast = simplified.as<BroadcastNode>()) {
         auto v = analyzer_.Simplify(broadcast->value);
         if (analyzer_.CanProve(v >= 0))
@@ -109,6 +85,20 @@ private:
                 << simplified << " for buffer " << buffer_name << " (axis " << i
                 << ", index " + indices[i]->Script() + ").";
         }
+      } else {
+        // Assume scalar (or non-Ramp/Broadcast) index; avoid querying lanes().
+        // Fall back to scalar reasoning. If this expression is actually a
+        // vector-but-not-Ramp/Broadcast, treat as unknown to be safe.
+        // Try to prove scalar first; if proof fails, leave as unknown.
+        if (analyzer_.CanProve(simplified >= 0))
+          state = IndexSignState::kNonNegative;
+        else if (analyzer_.CanProve(simplified < 0))
+          state = IndexSignState::kNegative;
+        else
+          DLOG(WARNING)
+              << "LegalizeNegativeIndex: cannot prove non-negative index "
+              << simplified << " for buffer " << buffer_name << " (axis " << i
+              << ", index " + indices[i]->Script() + ").";
       }
       states.push_back(state);
     }