Optimize function call serializing

src/hyperlight_common/src/flatbuffer_wrappers/function_types.rs

@@ -181,7 +181,7 @@ impl TryFrom<Parameter<'_>> for ParameterValue {
                 ParameterValue::String(hlstring.value().unwrap_or_default().to_string())
             }),
             FbParameterValue::hlvecbytes => param.value_as_hlvecbytes().map(|hlvecbytes| {
-                ParameterValue::VecBytes(hlvecbytes.value().unwrap_or_default().iter().collect())
+                ParameterValue::VecBytes(hlvecbytes.value().unwrap_or_default().bytes().to_vec())
             }),
             other => {
                 bail!("Unexpected flatbuffer parameter value type: {:?}", other);

src/hyperlight_common/src/flatbuffer_wrappers/util.rs

@@ -18,6 +18,7 @@ use alloc::vec::Vec;
 
 use flatbuffers::FlatBufferBuilder;
 
+use crate::flatbuffer_wrappers::function_types::ParameterValue;
 use crate::flatbuffers::hyperlight::generated::{
     FunctionCallResult as FbFunctionCallResult, FunctionCallResultArgs as FbFunctionCallResultArgs,
     ReturnValue as FbReturnValue, hlbool as Fbhlbool, hlboolArgs as FbhlboolArgs,
@@ -169,3 +170,350 @@ impl FlatbufferSerializable for bool {
         }
     }
 }
+
+/// Estimates the required buffer capacity for encoding a FunctionCall with the given parameters.
+/// This helps avoid reallocation during FlatBuffer encoding when passing large slices and strings.
+///
+/// The function aims to be lightweight and fast and run in O(1) as long as the number of parameters is limited
+/// (which it is since hyperlight only currently supports up to 12).
+///
+/// Note: This estimates the capacity needed for the inner vec inside a FlatBufferBuilder. It does not
+/// necessarily match the size of the final encoded buffer. The estimation always rounds up to the
+/// nearest power of two to match FlatBufferBuilder's allocation strategy.
+///
+/// The estimations are numbers used are empirically derived based on the tests below and vaguely based
+/// on https://flatbuffers.dev/internals/ and https://github.com/dvidelabs/flatcc/blob/f064cefb2034d1e7407407ce32a6085c322212a7/doc/binary-format.md#flatbuffers-binary-format
+#[inline] // allow cross-crate inlining (for hyperlight-host calls)
+pub fn estimate_flatbuffer_capacity(function_name: &str, args: &[ParameterValue]) -> usize {
+    let mut estimated_capacity = 20;
+
+    // Function name overhead
+    estimated_capacity += function_name.len() + 12;
+
+    // Parameters vector overhead
+    estimated_capacity += 12 + args.len() * 6;
+
+    // Per-parameter overhead
+    for arg in args {
+        estimated_capacity += 16; // Base parameter structure
+        estimated_capacity += match arg {
+            ParameterValue::String(s) => s.len() + 20,
+            ParameterValue::VecBytes(v) => v.len() + 20,
+            ParameterValue::Int(_) | ParameterValue::UInt(_) => 16,
+            ParameterValue::Long(_) | ParameterValue::ULong(_) => 20,
+            ParameterValue::Float(_) => 16,
+            ParameterValue::Double(_) => 20,
+            ParameterValue::Bool(_) => 12,
+        };
+    }
+
+    // match how vec grows
+    estimated_capacity.next_power_of_two()
+}
+
+#[cfg(test)]
+mod tests {
+    use alloc::string::ToString;
+    use alloc::vec;
+    use alloc::vec::Vec;
+
+    use super::*;
+    use crate::flatbuffer_wrappers::function_call::{FunctionCall, FunctionCallType};
+    use crate::flatbuffer_wrappers::function_types::{ParameterValue, ReturnType};
+
+    /// Helper function to check that estimation is within reasonable bounds (±25%)
+    fn assert_estimation_accuracy(
+        function_name: &str,
+        args: Vec<ParameterValue>,
+        call_type: FunctionCallType,
+        return_type: ReturnType,
+    ) {
+        let estimated = estimate_flatbuffer_capacity(function_name, &args);
+
+        let fc = FunctionCall::new(
+            function_name.to_string(),
+            Some(args),
+            call_type.clone(),
+            return_type,
+        );
+        // Important that this FlatBufferBuilder is created with capacity 0 so it grows to its needed capacity
+        let mut builder = FlatBufferBuilder::new();
+        let _buffer = fc.encode(&mut builder);
+        let actual = builder.collapse().0.capacity();
+
+        let lower_bound = (actual as f64 * 0.75) as usize;
+        let upper_bound = (actual as f64 * 1.25) as usize;
+
+        assert!(
+            estimated >= lower_bound && estimated <= upper_bound,
+            "Estimation {} outside bounds [{}, {}] for actual size {} (function: {}, call_type: {:?}, return_type: {:?})",
+            estimated,
+            lower_bound,
+            upper_bound,
+            actual,
+            function_name,
+            call_type,
+            return_type
+        );
+    }
+
+    #[test]
+    fn test_estimate_no_parameters() {
+        assert_estimation_accuracy(
+            "simple_function",
+            vec![],
+            FunctionCallType::Guest,
+            ReturnType::Void,
+        );
+    }
+
+    #[test]
+    fn test_estimate_single_int_parameter() {
+        assert_estimation_accuracy(
+            "add_one",
+            vec![ParameterValue::Int(42)],
+            FunctionCallType::Guest,
+            ReturnType::Int,
+        );
+    }
+
+    #[test]
+    fn test_estimate_multiple_scalar_parameters() {
+        assert_estimation_accuracy(
+            "calculate",
+            vec![
+                ParameterValue::Int(10),
+                ParameterValue::UInt(20),
+                ParameterValue::Long(30),
+                ParameterValue::ULong(40),
+                ParameterValue::Float(1.5),
+                ParameterValue::Double(2.5),
+                ParameterValue::Bool(true),
+            ],
+            FunctionCallType::Guest,
+            ReturnType::Double,
+        );
+    }
+
+    #[test]
+    fn test_estimate_string_parameters() {
+        assert_estimation_accuracy(
+            "process_strings",
+            vec![
+                ParameterValue::String("hello".to_string()),
+                ParameterValue::String("world".to_string()),
+                ParameterValue::String("this is a longer string for testing".to_string()),
+            ],
+            FunctionCallType::Host,
+            ReturnType::String,
+        );
+    }
+
+    #[test]
+    fn test_estimate_very_long_string() {
+        let long_string = "a".repeat(1000);
+        assert_estimation_accuracy(
+            "process_long_string",
+            vec![ParameterValue::String(long_string)],
+            FunctionCallType::Guest,
+            ReturnType::String,
+        );
+    }
+
+    #[test]
+    fn test_estimate_vector_parameters() {
+        assert_estimation_accuracy(
+            "process_vectors",
+            vec![
+                ParameterValue::VecBytes(vec![1, 2, 3, 4, 5]),
+                ParameterValue::VecBytes(vec![]),
+                ParameterValue::VecBytes(vec![0; 100]),
+            ],
+            FunctionCallType::Host,
+            ReturnType::VecBytes,
+        );
+    }
+
+    #[test]
+    fn test_estimate_mixed_parameters() {
+        assert_estimation_accuracy(
+            "complex_function",
+            vec![
+                ParameterValue::String("test".to_string()),
+                ParameterValue::Int(42),
+                ParameterValue::VecBytes(vec![1, 2, 3, 4, 5]),
+                ParameterValue::Bool(true),
+                ParameterValue::Double(553.14159),
+                ParameterValue::String("another string".to_string()),
+                ParameterValue::Long(9223372036854775807),
+            ],
+            FunctionCallType::Guest,
+            ReturnType::VecBytes,
+        );
+    }
+
+    #[test]
+    fn test_estimate_large_function_name() {
+        let long_name = "very_long_function_name_that_exceeds_normal_lengths_for_testing_purposes";
+        assert_estimation_accuracy(
+            long_name,
+            vec![ParameterValue::Int(1)],
+            FunctionCallType::Host,
+            ReturnType::Long,
+        );
+    }
+
+    #[test]
+    fn test_estimate_large_vector() {
+        let large_vector = vec![42u8; 10000];
+        assert_estimation_accuracy(
+            "process_large_data",
+            vec![ParameterValue::VecBytes(large_vector)],
+            FunctionCallType::Guest,
+            ReturnType::Bool,
+        );
+    }
+
+    #[test]
+    fn test_estimate_all_parameter_types() {
+        assert_estimation_accuracy(
+            "comprehensive_test",
+            vec![
+                ParameterValue::Int(i32::MIN),
+                ParameterValue::UInt(u32::MAX),
+                ParameterValue::Long(i64::MIN),
+                ParameterValue::ULong(u64::MAX),
+                ParameterValue::Float(f32::MIN),
+                ParameterValue::Double(f64::MAX),
+                ParameterValue::Bool(false),
+                ParameterValue::String("test string".to_string()),
+                ParameterValue::VecBytes(vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]),
+            ],
+            FunctionCallType::Host,
+            ReturnType::ULong,
+        );
+    }
+
+    #[test]
+    fn test_different_function_call_types() {
+        assert_estimation_accuracy(
+            "guest_function",
+            vec![ParameterValue::String("guest call".to_string())],
+            FunctionCallType::Guest,
+            ReturnType::String,
+        );
+
+        assert_estimation_accuracy(
+            "host_function",
+            vec![ParameterValue::String("host call".to_string())],
+            FunctionCallType::Host,
+            ReturnType::String,
+        );
+    }
+
+    #[test]
+    fn test_different_return_types() {
+        let args = vec![
+            ParameterValue::Int(42),
+            ParameterValue::String("test".to_string()),
+        ];
+
+        let void_est = estimate_flatbuffer_capacity("test_void", &args);
+        let int_est = estimate_flatbuffer_capacity("test_int", &args);
+        let string_est = estimate_flatbuffer_capacity("test_string", &args);
+
+        assert!((void_est as i32 - int_est as i32).abs() < 10);
+        assert!((int_est as i32 - string_est as i32).abs() < 10);
+
+        assert_estimation_accuracy(
+            "test_void",
+            args.clone(),
+            FunctionCallType::Guest,
+            ReturnType::Void,
+        );
+        assert_estimation_accuracy(
+            "test_int",
+            args.clone(),
+            FunctionCallType::Guest,
+            ReturnType::Int,
+        );
+        assert_estimation_accuracy(
+            "test_string",
+            args,
+            FunctionCallType::Guest,
+            ReturnType::String,
+        );
+    }
+
+    #[test]
+    fn test_estimate_many_large_vectors_and_strings() {
+        assert_estimation_accuracy(
+            "process_bulk_data",
+            vec![
+                ParameterValue::String("Large string data: ".to_string() + &"x".repeat(2000)),
+                ParameterValue::VecBytes(vec![1u8; 5000]),
+                ParameterValue::String(
+                    "Another large string with lots of content ".to_string() + &"y".repeat(3000),
+                ),
+                ParameterValue::VecBytes(vec![255u8; 7500]),
+                ParameterValue::String(
+                    "Third massive string parameter ".to_string() + &"z".repeat(1500),
+                ),
+                ParameterValue::VecBytes(vec![128u8; 10000]),
+                ParameterValue::Int(42),
+                ParameterValue::String("Final large string ".to_string() + &"a".repeat(4000)),
+                ParameterValue::VecBytes(vec![64u8; 2500]),
+                ParameterValue::Bool(true),
+            ],
+            FunctionCallType::Host,
+            ReturnType::VecBytes,
+        );
+    }
+
+    #[test]
+    fn test_estimate_twenty_parameters() {
+        assert_estimation_accuracy(
+            "function_with_many_parameters",
+            vec![
+                ParameterValue::Int(1),
+                ParameterValue::String("param2".to_string()),
+                ParameterValue::Bool(true),
+                ParameterValue::Float(3213.14),
+                ParameterValue::VecBytes(vec![1, 2, 3]),
+                ParameterValue::Long(1000000),
+                ParameterValue::Double(322.718),
+                ParameterValue::UInt(42),
+                ParameterValue::String("param9".to_string()),
+                ParameterValue::Bool(false),
+                ParameterValue::ULong(9999999999),
+                ParameterValue::VecBytes(vec![4, 5, 6, 7, 8]),
+                ParameterValue::Int(-100),
+                ParameterValue::Float(1.414),
+                ParameterValue::String("param15".to_string()),
+                ParameterValue::Double(1.732),
+                ParameterValue::Bool(true),
+                ParameterValue::VecBytes(vec![9, 10]),
+                ParameterValue::Long(-5000000),
+                ParameterValue::UInt(12345),
+            ],
+            FunctionCallType::Guest,
+            ReturnType::Int,
+        );
+    }
+
+    #[test]
+    fn test_estimate_megabyte_parameters() {
+        assert_estimation_accuracy(
+            "process_megabyte_data",
+            vec![
+                ParameterValue::String("MB String 1: ".to_string() + &"x".repeat(1_048_576)), // 1MB string
+                ParameterValue::VecBytes(vec![42u8; 2_097_152]), // 2MB vector
+                ParameterValue::String("MB String 2: ".to_string() + &"y".repeat(1_572_864)), // 1.5MB string
+                ParameterValue::VecBytes(vec![128u8; 3_145_728]), // 3MB vector
+                ParameterValue::String("MB String 3: ".to_string() + &"z".repeat(2_097_152)), // 2MB string
+            ],
+            FunctionCallType::Host,
+            ReturnType::VecBytes,
+        );
+    }
+}

src/hyperlight_guest/Cargo.toml

@@ -16,6 +16,7 @@ anyhow = { version = "1.0.99", default-features = false }
 serde_json = { version = "1.0", default-features = false, features = ["alloc"] }
 hyperlight-common = { workspace = true }
 hyperlight-guest-tracing = { workspace = true, default-features = false }
+flatbuffers = { version= "25.2.10", default-features = false }
 
 [features]
 default = []