Add some more documentation to SIMD/no-SIMD testing.

.github/workflows/ci.yml

@@ -67,11 +67,12 @@ jobs:
       run: cargo doc --verbose
     - name: Run SIMD/no-SIMD tests
       run: |
+        # See crosstest/main.rs for more info on the test
         cd crosstest
-        # Create some no-simd test files
+        # Create a no-simd test file in /tmp
         cargo run --verbose --features no_simd -- write
         cargo clean
-        # Create some simd test files and test the no-simd files
+        # Create a simd-enabled test file and test the no-simd file
         cargo run --verbose -- write read
         cargo clean
         # Test the simd-enabled files we generated in the last step

crosstest/src/main.rs

@@ -1,5 +1,20 @@
 // This test makes sure that a hash table generated with SIMD support
-// can be loaded on a platform without SIMD support.
+// can be loaded on a platform without SIMD support and vice versa.
+//
+// It works this way:
+//
+// The executable has two options: "write" and "read". When "write" is
+// enabled, it will write a hashtable to /tmp. When "read" is enabled,
+// it will try to read a hashtable from /tmp and verify that it contains
+// the expected data.
+//
+// We compile the executable two times, once with and once without SIMD
+// support. With both versions we generate a hashtable in /tmp and then
+// try to load and verify that hashtable with the respective other
+// version of the executable, that is, the SIMD-version will try to load
+// the non-SIMD version and vice versa.
+//
+// See .github/workflows/ci.yml for how this is used.
 
 struct FxConfig;
 

src/memory_layout.rs

@@ -62,8 +62,11 @@ impl Header {
 
         if raw_bytes.len() != bytes_needed::<C>(self.slot_count()) {
             return Err(Error(format!(
-                "Provided allocation has wrong size for slot count {}",
-                self.slot_count()
+                "Provided allocation has wrong size for slot count {}. \
+                 The allocation's size is {} but the expected size is {}.",
+                self.slot_count(),
+                raw_bytes.len(),
+                bytes_needed::<C>(self.slot_count()),
             )));
         }