Fix rasterizer test segfaults in conda-forge builds

momentum/rasterizer/fwd.h

@@ -23,6 +23,19 @@ using IntrinsicsModel = IntrinsicsModelT<float>;
 constexpr size_t kSimdPacketSize = drjit::DefaultSize;
 constexpr size_t kSimdAlignment = 4 * kSimdPacketSize;
 
+// Compile-time sanity check: Ensure SIMD packet size is reasonable (typically 4, 8, 16, or 32)
+// This helps catch misconfigured builds where DefaultSize might be incorrect
+static_assert(
+    kSimdPacketSize >= 1 && kSimdPacketSize <= 64,
+    "kSimdPacketSize must be between 1 and 64. If this assertion fails, "
+    "drjit::DefaultSize may be misconfigured in your build environment.");
+
+// Additional check: packet size should be a power of 2 for most SIMD implementations
+static_assert(
+    (kSimdPacketSize & (kSimdPacketSize - 1)) == 0,
+    "kSimdPacketSize should be a power of 2 (1, 2, 4, 8, 16, 32, 64). "
+    "If this fails, check your drjit configuration.");
+
 using IntP = drjit::Packet<int32_t, kSimdPacketSize>;
 using UintP = drjit::Packet<uint32_t, kSimdPacketSize>;
 using Uint8P = drjit::Packet<uint8_t, kSimdPacketSize>;

momentum/rasterizer/tensor.h

@@ -9,6 +9,7 @@
 
 #include <mdspan/mdspan.hpp>
 #include <momentum/common/aligned.h>
+#include <momentum/common/exception.h>
 #include <momentum/rasterizer/fwd.h>
 #include <algorithm>
 #include <array>
@@ -40,7 +41,31 @@ class Tensor {
   explicit Tensor(const extents_t& extents, const T& defaultValue = T{}) : _extents(extents) {
     auto size = calculateSize(extents);
     if (size > 0) {
-      _data.resize(size, defaultValue);
+      // Sanity check: if size is unreasonably large (e.g. > 1GB for floats/ints),
+      // throw a more descriptive error than std::bad_alloc
+      constexpr index_t kMaxReasonableElements =
+          256 * 1024 * 1024; // 256M elements (~1GB for floats)
+      MT_THROW_IF(
+          size > kMaxReasonableElements,
+          "Tensor allocation size is unreasonably large: {} elements. "
+          "This may indicate a configuration error with SIMD packet size. "
+          "Expected extents: [{}]",
+          size,
+          formatExtents(extents));
+      try {
+        _data.resize(size, defaultValue);
+      } catch (const std::bad_alloc& e) {
+        MT_THROW(
+            "Failed to allocate tensor of size {} elements ({} bytes). "
+            "Extents: [{}]. "
+            "This may indicate misconfigured SIMD packet size (kSimdPacketSize={}). "
+            "Original error: {}",
+            size,
+            size * sizeof(T),
+            formatExtents(extents),
+            kSimdPacketSize,
+            e.what());
+      }
     }
   }
 
@@ -131,6 +156,17 @@ class Tensor {
     }
     return size;
   }
+
+  static std::string formatExtents(const extents_t& extents) {
+    std::string result;
+    for (size_t i = 0; i < Rank; ++i) {
+      if (i > 0) {
+        result += ", ";
+      }
+      result += std::to_string(extents[i]);
+    }
+    return result;
+  }
 };
 
 using Tensor2f = Tensor<float, 2>;

momentum/test/rasterizer/test_software_rasterizer.cpp

@@ -196,6 +196,12 @@ TEST(SoftwareRasterizer, OneQuad) {
   const int width = 20;
   const int height = 10;
 
+  // Add diagnostic information to help debug allocation issues
+  // This can help identify when SIMD packet size is misconfigured
+  SCOPED_TRACE(
+      "SIMD Configuration: kSimdPacketSize=" + std::to_string(kSimdPacketSize) +
+      ", kSimdAlignment=" + std::to_string(kSimdAlignment));
+
   // Create OpenCV intrinsics with no distortion
   OpenCVDistortionParametersT<float> distortionParams; // All zeros by default
   auto intrinsics = std::make_shared<OpenCVIntrinsicsModel>(