From 8a8ea86baa9181b24706cbd930da51c21f83bd3a Mon Sep 17 00:00:00 2001
From: Axel Huebl <axel.huebl@plasma.ninja>
Date: Tue, 24 Sep 2024 16:29:37 -0700
Subject: [PATCH] `Named` Mixing Class: 40 -> 8 Bytes

Make smaller, because we will always copy the whole
class to GPU, even though we do not access the name
member on it.
---
 src/particles/elements/mixin/named.H | 62 ++++++++++++++++++++++++++--
 1 file changed, 58 insertions(+), 4 deletions(-)

diff --git a/src/particles/elements/mixin/named.H b/src/particles/elements/mixin/named.H
index 5d75028e9..e801792e2 100644
--- a/src/particles/elements/mixin/named.H
+++ b/src/particles/elements/mixin/named.H
@@ -13,8 +13,11 @@
 #include "particles/ImpactXParticleContainer.H"
 
 #include <AMReX_Extension.H>
+#include <AMReX_GpuQualifiers.H>
 
+#include <cstring>
 #include <optional>
+#include <stdexcept>
 #include <string>
 
 
@@ -28,13 +31,54 @@ namespace impactx::elements
          *
          * @param name a user defined and not necessarily unique name of the element
          */
+        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
         Named (
             std::optional<std::string> name
         )
-        : m_name(name)
         {
+            if (name.has_value()) {
+                m_name = new char[name->size() + 1];
+                std::strcpy(m_name, name->c_str());
+            }
         }
 
+        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        ~Named () {
+            AMREX_IF_ON_HOST((
+                delete[] m_name;
+                m_name = nullptr;
+            ))
+        }
+
+        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        Named (Named const & other) {
+            AMREX_IF_ON_HOST((
+                if (other.has_name()) {
+                    m_name = new char[std::strlen(other.m_name) + 1];
+                    std::strcpy(m_name, other.m_name);
+                }
+            ))
+        }
+
+        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        Named& operator=(Named const & other) {
+            AMREX_IF_ON_HOST((
+                if (&other != this) {
+                    if (other.has_name()) {
+                        m_name = new char[std::strlen(other.m_name) + 1];
+                        std::strcpy(m_name, other.m_name);
+                    }
+                }
+            ))
+            return *this;
+        }
+
+        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        Named (Named&&) noexcept = default;
+
+        AMREX_GPU_HOST_DEVICE AMREX_FORCE_INLINE
+        Named& operator=(Named && other) noexcept = default;
+
         /** Return the user provided name of the element
          *
          * @return user defined and not necessarily unique name
@@ -42,7 +86,10 @@ namespace impactx::elements
         AMREX_FORCE_INLINE
         std::string name () const
         {
-            return m_name.value();
+            if (!has_name()) {
+                throw std::runtime_error("Name not set on element!");
+            }
+            return std::string(m_name);
         }
 
         /** Return true if the user provided a name for this element
@@ -52,10 +99,17 @@ namespace impactx::elements
         AMREX_FORCE_INLINE
         bool has_name () const
         {
-            return m_name.has_value();
+            return m_name != nullptr;
         }
 
-        std::optional<std::string> m_name; //! a user defined and not necessarily unique name of the element
+        // Implementation note:
+        // This is used as a mixin class in elements that are copied to GPU. GPU compilers copy
+        // a whole element by its sizeof(T).
+        // To save on this copy operation at kernel start, this is implemented
+        // as a simple C pointer (8 bytes), contrary to a std::string (32 bytes) or
+        // a std::optional<std::string> (40 bytes). m_name points to host-side memory and
+        // must not be dereferenced (used) on GPU.
+        char * m_name = nullptr; //! a user defined and not necessarily unique name of the element
     };
 
 } // namespace impactx::elements