Benchmark

The LEAF github repository contains two similar benchmarking programs, one using LEAF, the other configurable to use tl::expected or Boost Outcome, that simulate transporting error objects across 10 levels of stack frames, measuring the performance of the three libraries.

Links:

LEAF: https://boostorg.github.io/leaf
tl::expected: https://github.com/TartanLlama/expected
Boost Outcome V2: https://www.boost.org/doc/libs/release/libs/outcome/doc/html/index.html

Library design considerations

LEAF serves a similar purpose to other error handling libraries, but its design is very different. The benchmarks are comparing apples and oranges.

The main design difference is that when using LEAF, error objects are not communicated in return values. In case of a failure, the leaf::result<T> object transports only an int, the unique error ID.

Error objects skip the error neutral functions in the call stack and get moved directly to the the error handling scope that needs them. This mechanism does not depend on RVO or any other optimization: as soon as the program passes an error object to LEAF, it moves it to the correct error handling scope.

Other error handling libraries instead couple the static type of the return value of all error neutral functions with the error type an error reporting function may return. This approach suffers from the same problems as statically-enforced exception specifications:

It's difficult to use in polymorphic function calls, and
It impedes interoperability between the many different error types any non-trivial program must handle.

(The Boost Outcome library is also capable of avoiding such excessive coupling, by passing for the third P argument in the outcome<T, E, P> template a pointer that erases the exact static type of the object being transported. However, this would require a dynamic memory allocation).

Syntax

The most common check-only use case looks almost identically in LEAF and in Boost Outcome (tl::expected lacks a similar macro):

// Outcome
{
  BOOST_OUTCOME_TRY(v, f()); // Check for errors, forward failures to the caller
  // If control reaches here, v is the successful result (the call succeeded).
}

// LEAF
{
  BOOST_LEAF_AUTO(v, f()); // Check for errors, forward failures to the caller
  // If control reaches here, v is the successful result (the call succeeded).
}

When we want to handle failures, in Boost Outcome and in tl::expected, accessing the error object (which is always stored in the return value) is a simple continuation of the error check:

// Outcome, tl::expected
if( auto r = f() )
{
  auto v = r.value();
  // No error, use v
}
else
{ // Error!
  switch( r.error() )
  {
  error_enum::error1:
    /* handle error_enum::error1 */
    break;

  error_enum::error2:
    /* handle error_enum::error2 */
    break;

  default:
    /* handle any other failure */
  }
}

When using LEAF, we must explicitly state our intention to handle errors, not just check for failures:

// LEAF
leaf::try_handle_all

  []() -> leaf::result<T>
  {
    BOOST_LEAF_AUTO(v, f());
    // No error, use v
  },

  []( leaf::match<error_enum, error_enum::error1> )
  {
    /* handle error_enum::error1 */
  },

  []( leaf::match<error_enum, error_enum::error2> )
  {
    /* handle error_enum::error2 */
  },

  []
  {
    /* handle any other failure */
  } );

The use of try_handle_all reserves storage on the stack for the error object types being handled (in this case, error_enum). If the failure is either error_enum::error1 or error_enum::error2, the matching error handling lambda is invoked.

Code generation considerations

Benchmarking C++ programs is tricky, because we want to prevent the compiler from optimizing out things it shouldn't normally be able to optimize in a real program, yet we don't want to interfere with "legitimate" optimizations.

The primary approach we use to prevent the compiler from optimizing everything out to nothing is to base all computations on a call to std::rand().

When benchmarking error handling, it makes sense to measure the time it takes to return a result or error across multiple stack frames. This calls for disabling inlining.

The technique used to disable inlining in this benchmark is to mark functions with __attribute__((noinline)). This is imperfect, because optimizers can still peek into the body of the function and optimize things out, as is seen in this example:

__attribute__((noinline)) int val() {return 42;}

int main()
{
  return val();
}

Which on clang 9 outputs:

val():
        mov     eax, 42
          ret
main:
        mov     eax, 42
        ret

It does not appear that anything like this is occurring in our case, but it is still a possibility.

NOTES:

The benchmarks are compiled with exception handling disabled.

LEAF is able to work with external result<> types. The benchmark uses leaf::result<T>.

Show me the code!

The following source:

leaf::result<int> f();

leaf::result<int> g()
{
  BOOST_LEAF_AUTO(x, f());
  return x+1;
}

Generates this code on clang (Godbolt):

g():                                  # @g()
        push    rbx
        sub     rsp, 32
        mov     rbx, rdi
        lea     rdi, [rsp + 8]
        call    f()
        mov     eax, dword ptr [rsp + 24]
        mov     ecx, eax
        and     ecx, 3
        cmp     ecx, 3
        jne     .LBB0_1
        mov     eax, dword ptr [rsp + 8]
        add     eax, 1
        mov     dword ptr [rbx], eax
        mov     eax, 3
        jmp     .LBB0_3
.LBB0_1:
        cmp     ecx, 2
        jne     .LBB0_3
        mov     rax, qword ptr [rsp + 8]
        mov     qword ptr [rbx], rax
        mov     rax, qword ptr [rsp + 16]
        mov     qword ptr [rbx + 8], rax
        mov     eax, 2
.LBB0_3:
        mov     dword ptr [rbx + 16], eax
        mov     rax, rbx
        add     rsp, 32
        pop     rbx
        ret

Description:

The happy path can be recognized by the add eax, 1 instruction generated for x + 1.

.LBB0_3: Regular failure; the returned result<T> object holds only the int discriminant.

.LBB0_1: Failure; the returned result<T> holds the int discriminant and a std::shared_ptr<leaf::polymorphic_context> (used to hold error objects transported from another thread).

Note that f is undefined, hence the call instruction. Predictably, if we provide a trivial definition for f:

leaf::result<int> f()
{
  return 42;
}

leaf::result<int> g()
{
  BOOST_LEAF_AUTO(x, f());
  return x+1;
}

We get:

g():                                  # @g()
          mov     rax, rdi
          mov     dword ptr [rdi], 43
          mov     dword ptr [rdi + 16], 3
          ret

With a less trivial definition of f:

leaf::result<int> f()
{
  if( rand()%2 )
    return 42;
  else
    return leaf::new_error();
}

leaf::result<int> g()
{
  BOOST_LEAF_AUTO(x, f());
  return x+1;
}

We get (Godbolt):

g():                                  # @g()
        push    rbx
        mov     rbx, rdi
        call    rand
        test    al, 1
        jne     .LBB1_2
        mov     eax, 4
        lock            xadd    dword ptr [rip + boost::leaf::leaf_detail::id_factory<void>::counter], eax
        add     eax, 4
        mov     dword ptr fs:[boost::leaf::leaf_detail::id_factory<void>::current_id@TPOFF], eax
        and     eax, -4
        or      eax, 1
        mov     dword ptr [rbx + 16], eax
        mov     rax, rbx
        pop     rbx
        ret
.LBB1_2:
        mov     dword ptr [rbx], 43
        mov     eax, 3
        mov     dword ptr [rbx + 16], eax
        mov     rax, rbx
        pop     rbx
        ret

Above, the call to f() is inlined:

.LBB1_2: Success
The atomic add is from the initial error reporting machinery in LEAF, generating a unique error ID for the error being reported.

Benchmark matrix dimensions

The benchmark matrix has 2 dimensions:

Error object type:

a. The error object transported in case of a failure is of type e_error_code, which is a simple enum.

b. The error object transported in case of a failure is of type struct e_system_error { e_error_code value; std::string what; }.

c. The error object transported in case of a failure is of type e_heavy_payload, a struct of size 4096.
Error rate: 2%, 98%

Now, transporting a large error object might seem unusual, but this is only because it is impractical to return a large object as the return value in case of an error. LEAF has two features that make communicating any, even large error objects, practical:

The return type of error neutral functions is not coupled with the error object types that may be reported. This means that in case of a failure, any function can easily contribute any error information it has available.
LEAF will only bother with transporting a given error object if an active error handling scope needs it. This means that library functions can and should contribute any and all relevant information when reporting a failure, because if the program doesn't need it, it will simply be discarded.

Source code

deep_stack_leaf.cpp

deep_stack_other.cpp

Godbolt

Godbolt has built-in support for Boost (Outcome/LEAF), but tl::expected both provide a single header, which makes it very easy to use them online as well. To see the generated code for the benchmark program, you can copy and paste the following into Godbolt:

leaf::result<T> (godbolt)

#include "https://raw.githubusercontent.com/boostorg/leaf/master/benchmark/deep_stack_leaf.cpp"

tl::expected<T, E> (godbolt)

#include "https://raw.githubusercontent.com/TartanLlama/expected/master/include/tl/expected.hpp"
#include "https://raw.githubusercontent.com/boostorg/leaf/master/benchmark/deep_stack_other.cpp"

outcome::result<T, E> (godbolt)

#define BENCHMARK_WHAT 1
#include "https://raw.githubusercontent.com/boostorg/leaf/master/benchmark/deep_stack_other.cpp"

Build options

To build both versions of the benchmark program, the compilers are invoked using the following command line options:

-std=c++17: Required by other libraries (LEAF only requires C++11);
-fno-exceptions: Disable exception handling;
-O3: Maximum optimizations;
-DNDEBUG: Disable asserts.

In addition, the LEAF version is compiled with:

-DBOOST_LEAF_CFG_DIAGNOSTICS=0: Disable diagnostic information for error objects not recognized by the program. This is a debugging feature, see Configuration Macros.

Results

Below is the output the benchmark programs running on an old MacBook Pro. The tables show the elapsed time for 10,000,000 iterations of returning a result across 10 stack frames, depending on the error type and the rate of failures. In addition, the programs generate a benchmark.csv file in the current working directory.

gcc 9.2.0:

leaf::result<T>:

Error type	2% (μs)	98% (μs)
e_error_code	594965	545882
e_system_error	614688	1203154
e_heavy_payload	736701	7397756

tl::expected<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	921410	820757
e_system_error	670191	5593513
e_heavy_payload	1331724	31560432

outcome::result<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	1080512	773206
e_system_error	577403	1201693
e_heavy_payload	13222387	32104693

outcome::outcome<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	832916	1170731
e_system_error	947298	2330392
e_heavy_payload	13342292	33837583

clang 11.0.0:

leaf::result<T>:

Error type	2% (μs)	98% (μs)
e_error_code	570847	493538
e_system_error	592685	982799
e_heavy_payload	713966	5144523

tl::expected<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	461639	312849
e_system_error	620479	3534689
e_heavy_payload	1037434	16078669

outcome::result<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	431219	446854
e_system_error	589456	1712739
e_heavy_payload	12387405	16216894

outcome::outcome<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	711412	1477505
e_system_error	835691	2374919
e_heavy_payload	13289404	29785353

msvc 19.24.28314:

leaf::result<T>:

Error type	2% (μs)	98% (μs)
e_error_code	1205327	1449117
e_system_error	1290277	2332414
e_heavy_payload	1503103	13682308

tl::expected<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	938839	867296
e_system_error	1455627	8943881
e_heavy_payload	2637494	49212901

outcome::result<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	935331	1202475
e_system_error	1228944	2269680
e_heavy_payload	15239084	55618460

outcome::outcome<T, E>:

Error type	2% (μs)	98% (μs)
e_error_code	1472035	2529057
e_system_error	1997971	4004965
e_heavy_payload	16027423	64572924

Charts

The charts below are generated from the results from the previous section, converted from elapsed time in microseconds to millions of calls per second.

gcc 9.2.0:

clang 11.0.0:

msvc 19.24.28314:

Thanks

Thanks for the valuable feedback: Peter Dimov, Glen Fernandes, Sorin Fetche, Niall Douglas, Ben Craig, Vinnie Falco, Jason Dictos

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Benchmark

Library design considerations

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Show me the code!

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Results

gcc 9.2.0:

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Charts

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msvc 19.24.28314:

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Benchmark

Library design considerations

Syntax

Code generation considerations

Show me the code!

Benchmark matrix dimensions

Source code

Godbolt

Build options

Results

gcc 9.2.0:

clang 11.0.0:

msvc 19.24.28314:

Charts

gcc 9.2.0:

clang 11.0.0:

msvc 19.24.28314:

Thanks