Re: [analyzer PATCH] Restore bootstrap with g++ 4.8.

[Jonathan Wakely <jwakely@redhat.com>]

On 07/06/24 19:40 +0100, Roger Sayle wrote:
>
>This patch restores bootstrap when using g++ 4.8 as a host compiler.
>Returning a std::unique_ptr requires a std::move on C++ compilers
>(pre-C++17) that don't guarantee copy elision/return value optimization.

It doesn't though.  The C++17 guaranteed copy elision rules are not
relevant here.  This is about lookup for the constructor used in the
return statement, and whether that lookup considers the variable to be
an lvalue or an rvalue.  C++11 already says this is valid:

i#include <memory>

std::unique_ptr<int> f()
{
     std::unique_ptr<int> m;
     return m;
}

See C++11 12.8 [class.copy] p31:

   This elision of copy/move operations, called copy elision, is
   permitted in the following circumstances (which may be combined to
   eliminate multiple copies):

   - in a return statement in a function with a class return type, when
   the expression is the name of a non-volatile automatic object (other
   than a function or catch-clause parameter) with the same cv-
   unqualified type as the function return type, the copy/move
   operation can be omitted by constructing the automatic object
   directly into the function’s return value

and then p32:

   When the criteria for elision of a copy operation are met or would
   be met save for the fact that the source object is a function
   parameter, and the object to be copied is designated by an lvalue,
   overload resolution to select the constructor for the copy is first
   performed as if the object were designated by an rvalue.

The constructor isn't required to be elided in C++11, but the compiler
is required to use a move constructor instead of a copy constructor,
if a move constructor is available. So you don't need to use std::move
on the return value.

And the code above compiles fine with gcc 4.8.5 on CentOS 7 (and even
with 4.7.1).

So the std::move calls you've added are redundant, and will cause
-Wredundant-move warnings.

What's the error you were seeing?


>Bootstrapped on x86_64-pc-linux-gnu using both gcc 4.8.5 (system) and
>gcc 10.2.1 (using "scl enable devetoolset-10") as host compilers.
>Ok for mainline?
>
>
>2024-06-07  Roger Sayle  <roger@nextmovesoftware.com>
>
>gcc/analyzer/ChangeLog
>        * constraint-manager.cc (equiv_class::make_dump_widget): Use
>        std::move to return a std::unique_ptr.
>        (bounded_ranges_constraint::make_dump_widget): Likewise.
>        (constraint_manager::make_dump_widget): Likewise.
>        * program_state.cc (sm_state_map::make_dump_widget): Likewise.
>        (program_state::make_dump_widget): Likewise.
>        * region-model.cc (region_to_value_map::make_dump_widget): Likewise.
>        (region_model::make_dump_widget): Likewise.
>        * region.cc (region::make_dump_widget): Likewise.
>        * store.cc (binding_cluster::make_dump_widget): Likewise.
>        (store::make_dump_widget): Likewise.
>        * svalue.cc (svalue::make_dump_widget): Likewise.
>
>Thanks in advance,
>Roger
>--
>

>diff --git a/gcc/analyzer/constraint-manager.cc b/gcc/analyzer/constraint-manager.cc
>index 707385d..883f33b 100644
>--- a/gcc/analyzer/constraint-manager.cc
>+++ b/gcc/analyzer/constraint-manager.cc
>@@ -1176,7 +1176,7 @@ equiv_class::make_dump_widget (const text_art::dump_widget_info &dwi,
>       ec_widget->add_child (tree_widget::make (dwi, &pp));
>     }
>
>-  return ec_widget;
>+  return std::move (ec_widget);
> }
>
> /* Generate a hash value for this equiv_class.
>@@ -1500,7 +1500,7 @@ make_dump_widget (const text_art::dump_widget_info &dwi) const
>     (tree_widget::from_fmt (dwi, nullptr,
> 			    "ec%i bounded ranges", m_ec_id.as_int ()));
>   m_ranges->add_to_dump_widget (*brc_widget.get (), dwi);
>-  return brc_widget;
>+  return std::move (brc_widget);
> }
>
> bool
>@@ -1853,7 +1853,7 @@ constraint_manager::make_dump_widget (const text_art::dump_widget_info &dwi) con
>   if (cm_widget->get_num_children () == 0)
>     return nullptr;
>
>-  return cm_widget;
>+  return std::move (cm_widget);
> }
>
> /* Attempt to add the constraint LHS OP RHS to this constraint_manager.
>diff --git a/gcc/analyzer/program-state.cc b/gcc/analyzer/program-state.cc
>index dc2d4bd..efaf569 100644
>--- a/gcc/analyzer/program-state.cc
>+++ b/gcc/analyzer/program-state.cc
>@@ -382,7 +382,7 @@ sm_state_map::make_dump_widget (const text_art::dump_widget_info &dwi,
>       state_widget->add_child (tree_widget::make (dwi, pp));
>     }
>
>-  return state_widget;
>+  return std::move (state_widget);
> }
>
> /* Return true if no states have been set within this map
>@@ -1247,7 +1247,7 @@ program_state::make_dump_widget (const text_art::dump_widget_info &dwi) const
> 	state_widget->add_child (smap->make_dump_widget (dwi, m_region_model));
>   }
>
>-  return state_widget;
>+  return std::move (state_widget);
> }
>
> /* Update this program_state to reflect a top-level call to FUN.
>diff --git a/gcc/analyzer/region-model.cc b/gcc/analyzer/region-model.cc
>index d142d85..4fbc970 100644
>--- a/gcc/analyzer/region-model.cc
>+++ b/gcc/analyzer/region-model.cc
>@@ -288,7 +288,7 @@ make_dump_widget (const text_art::dump_widget_info &dwi) const
>       sval->dump_to_pp (pp, true);
>       w->add_child (text_art::tree_widget::make (dwi, pp));
>     }
>-  return w;
>+  return std::move (w);
> }
>
> /* Attempt to merge THIS with OTHER, writing the result
>@@ -556,7 +556,7 @@ region_model::make_dump_widget (const text_art::dump_widget_info &dwi) const
> 			       m_mgr->get_store_manager ()));
>   model_widget->add_child (m_constraints->make_dump_widget (dwi));
>   model_widget->add_child (m_dynamic_extents.make_dump_widget (dwi));
>-  return model_widget;
>+  return std::move (model_widget);
> }
>
> /* Assert that this object is valid.  */
>diff --git a/gcc/analyzer/region.cc b/gcc/analyzer/region.cc
>index 71bae97..050feb6 100644
>--- a/gcc/analyzer/region.cc
>+++ b/gcc/analyzer/region.cc
>@@ -1119,7 +1119,7 @@ region::make_dump_widget (const text_art::dump_widget_info &dwi,
>   if (m_parent)
>     w->add_child (m_parent->make_dump_widget (dwi, "parent"));
>
>-  return w;
>+  return std::move (w);
> }
>
> void
>diff --git a/gcc/analyzer/store.cc b/gcc/analyzer/store.cc
>index d14cfa3..b20bc29 100644
>--- a/gcc/analyzer/store.cc
>+++ b/gcc/analyzer/store.cc
>@@ -1489,7 +1489,7 @@ binding_cluster::make_dump_widget (const text_art::dump_widget_info &dwi,
>
>       m_map.add_to_tree_widget (*cluster_widget, dwi);
>
>-      return cluster_widget;
>+      return std::move (cluster_widget);
>     }
> }
>
>@@ -2766,7 +2766,7 @@ store::make_dump_widget (const text_art::dump_widget_info &dwi,
>       store_widget->add_child (std::move (parent_reg_widget));
>     }
>
>-  return store_widget;
>+  return std::move (store_widget);
> }
>
> /* Get any svalue bound to REG, or NULL.  */
>diff --git a/gcc/analyzer/svalue.cc b/gcc/analyzer/svalue.cc
>index f1fd21e..b67780a 100644
>--- a/gcc/analyzer/svalue.cc
>+++ b/gcc/analyzer/svalue.cc
>@@ -252,7 +252,7 @@ svalue::make_dump_widget (const text_art::dump_widget_info &dwi,
>
>   add_dump_widget_children (*w, dwi);
>
>-  return w;
>+  return std::move (w);
> }
>
> /* If this svalue is a constant_svalue, return the underlying tree constant.