From b21ff00f9bc03e5b87f259bc25e45594ab245a34 Mon Sep 17 00:00:00 2001
From: Jonathan Poelen <jonathan.poelen@gmail.com>
Date: Mon, 19 Jun 2023 22:51:22 +0200
Subject: [PATCH] optimize mp_transform_if (extract f from the struct to
 benefit from memoization)

compiler |       gcc-12      |      clang-15
before   | 0:00.13s - 60432K | 0:00.16s - 100956K
after    | 0:00.11s - 47300K | 0:00.16s - 100484K

```cpp
using namespace boost::mp11;

template<class T> using p = mp_bool<T::value & 1>;
template<class T> using f = mp_size_t<T::value + 1>;
template<class I> using test = mp_transform_if<p, f, mp_iota<I>>;

using r1 = mp_transform<test, mp_iota_c<50>>;
```
---
 include/boost/mp11/algorithm.hpp | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index 07a8ffd..0c8b4b2 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -172,7 +172,7 @@ template<template<class...> class F, template<class...> class L1, class... T1, t
 namespace detail
 {
 
-template<template<class...> class P, template<class...> class F, class... L> struct mp_transform_if_impl
+template<template<class...> class P, template<class...> class F> struct mp_transform_if_impl_f
 {
     // the stupid quote-unquote dance avoids "pack expansion used as argument for non-pack parameter of alias template"
 
@@ -189,8 +189,11 @@ template<template<class...> class P, template<class...> class F, class... L> str
     template<class... U> using _f = mp_eval_if_q<mp_not<mp_invoke_q<Qp, U...>>, mp_first<mp_list<U...>>, Qf, U...>;
 
 #endif
+};
 
-    using type = mp_transform<_f, L...>;
+template<template<class...> class P, template<class...> class F, class... L> struct mp_transform_if_impl
+{
+    using type = mp_transform<mp_transform_if_impl_f<P, F>::template _f, L...>;
 };
 
 } // namespace detail