[flang] Add & use a better visit() (take 2)

Adds flang/include/flang/Common/log2-visit.h, which defines
a Fortran::common::visit() template function that is a drop-in
replacement for std::visit().  Modifies most use sites in
the front-end and runtime to use common::visit().

The C++ standard mandates that std::visit() have O(1) execution
time, which forces implementations to build dispatch tables.
This new common::visit() is O(log2 N) in the number of alternatives
in a variant<>, but that N tends to be small and so this change
produces a fairly significant improvement in compiler build
memory requirements, a 5-10% improvement in compiler build time,
and a small improvement in compiler execution time.

Building with -DFLANG_USE_STD_VISIT causes common::visit()
to be an alias for std::visit().

Calls to common::visit() with multiple variant arguments
are referred to std::visit(), pending further work.

This change is enabled only for GCC builds with GCC >= 9;
an earlier attempt (D122441) ran into bugs in some versions of
clang and was reverted rather than simply disabled; and it is
not well tested with MSVC. In non-GCC and older GCC builds,
common::visit() is simply an alias for std::visit().

1 files changed, 15 insertions, 15 deletions

diff --git a/flang/lib/Semantics/program-tree.cpp b/flang/lib/Semantics/program-tree.cpp
index 9d76cfa..e96a6c4 100644
--- a/flang/lib/Semantics/program-tree.cpp
+++ b/flang/lib/Semantics/program-tree.cpp
@@ -88,7 +88,7 @@ static ProgramTree BuildSubprogramTree(const parser::Name &name, const T &x) {
   if (subps) {
     for (const auto &subp :
         std::get<std::list<parser::InternalSubprogram>>(subps->t)) {
-      std::visit(
+      common::visit(
           [&](const auto &y) { node.AddChild(ProgramTree::Build(y.value())); },
           subp.u);
     }
@@ -111,7 +111,7 @@ static ProgramTree BuildModuleTree(const parser::Name &name, const T &x) {
   if (subps) {
     for (const auto &subp :
         std::get<std::list<parser::ModuleSubprogram>>(subps->t)) {
-      std::visit(
+      common::visit(
           [&](const auto &y) { node.AddChild(ProgramTree::Build(y.value())); },
           subp.u);
     }
@@ -120,7 +120,7 @@ static ProgramTree BuildModuleTree(const parser::Name &name, const T &x) {
 }
 
 ProgramTree ProgramTree::Build(const parser::ProgramUnit &x) {
-  return std::visit([](const auto &y) { return Build(y.value()); }, x.u);
+  return common::visit([](const auto &y) { return Build(y.value()); }, x.u);
 }
 
 ProgramTree ProgramTree::Build(const parser::MainProgram &x) {
@@ -200,17 +200,17 @@ Symbol::Flag ProgramTree::GetSubpFlag() const {
 
 bool ProgramTree::HasModulePrefix() const {
   using ListType = std::list<parser::PrefixSpec>;
-  const auto *prefixes{
-      std::visit(common::visitors{
-                     [](const parser::Statement<parser::FunctionStmt> *x) {
-                       return &std::get<ListType>(x->statement.t);
-                     },
-                     [](const parser::Statement<parser::SubroutineStmt> *x) {
-                       return &std::get<ListType>(x->statement.t);
-                     },
-                     [](const auto *) -> const ListType * { return nullptr; },
-                 },
-          stmt_)};
+  const auto *prefixes{common::visit(
+      common::visitors{
+          [](const parser::Statement<parser::FunctionStmt> *x) {
+            return &std::get<ListType>(x->statement.t);
+          },
+          [](const parser::Statement<parser::SubroutineStmt> *x) {
+            return &std::get<ListType>(x->statement.t);
+          },
+          [](const auto *) -> const ListType * { return nullptr; },
+      },
+      stmt_)};
   if (prefixes) {
     for (const auto &prefix : *prefixes) {
       if (std::holds_alternative<parser::PrefixSpec::Module>(prefix.u)) {
@@ -222,7 +222,7 @@ bool ProgramTree::HasModulePrefix() const {
 }
 
 ProgramTree::Kind ProgramTree::GetKind() const {
-  return std::visit(
+  return common::visit(
       common::visitors{
           [](const parser::Statement<parser::ProgramStmt> *) {
             return Kind::Program;