[flang] Add & use a better visit()

Adds flang/include/flang/Common/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.

Differential Revision: https://reviews.llvm.org/D122441

1 files changed, 16 insertions, 15 deletions

diff --git a/flang/lib/Semantics/resolve-names-utils.cpp b/flang/lib/Semantics/resolve-names-utils.cpp
index 2d8f2c1..7aab694 100644
--- a/flang/lib/Semantics/resolve-names-utils.cpp
+++ b/flang/lib/Semantics/resolve-names-utils.cpp
@@ -126,7 +126,7 @@ void GenericSpecInfo::Analyze(const parser::DefinedOpName &name) {
 
 void GenericSpecInfo::Analyze(const parser::GenericSpec &x) {
   symbolName_ = x.source;
-  kind_ = std::visit(
+  kind_ = common::visit(
       common::visitors{
           [&](const parser::Name &y) -> GenericKind {
             parseName_ = &y;
@@ -134,7 +134,7 @@ void GenericSpecInfo::Analyze(const parser::GenericSpec &x) {
             return GenericKind::OtherKind::Name;
           },
           [&](const parser::DefinedOperator &y) {
-            return std::visit(
+            return common::visit(
                 common::visitors{
                     [&](const parser::DefinedOpName &z) -> GenericKind {
                       Analyze(z);
@@ -265,19 +265,20 @@ ArraySpec AnalyzeCoarraySpec(
 }
 
 ArraySpec ArraySpecAnalyzer::Analyze(const parser::ComponentArraySpec &x) {
-  std::visit([this](const auto &y) { Analyze(y); }, x.u);
+  common::visit([this](const auto &y) { Analyze(y); }, x.u);
   CHECK(!arraySpec_.empty());
   return arraySpec_;
 }
 ArraySpec ArraySpecAnalyzer::Analyze(const parser::ArraySpec &x) {
-  std::visit(common::visitors{
-                 [&](const parser::AssumedSizeSpec &y) {
-                   Analyze(std::get<std::list<parser::ExplicitShapeSpec>>(y.t));
-                   Analyze(std::get<parser::AssumedImpliedSpec>(y.t));
-                 },
-                 [&](const parser::ImpliedShapeSpec &y) { Analyze(y.v); },
-                 [&](const auto &y) { Analyze(y); },
-             },
+  common::visit(common::visitors{
+                    [&](const parser::AssumedSizeSpec &y) {
+                      Analyze(
+                          std::get<std::list<parser::ExplicitShapeSpec>>(y.t));
+                      Analyze(std::get<parser::AssumedImpliedSpec>(y.t));
+                    },
+                    [&](const parser::ImpliedShapeSpec &y) { Analyze(y.v); },
+                    [&](const auto &y) { Analyze(y); },
+                },
       x.u);
   CHECK(!arraySpec_.empty());
   return arraySpec_;
@@ -289,7 +290,7 @@ ArraySpec ArraySpecAnalyzer::AnalyzeDeferredShapeSpecList(
   return arraySpec_;
 }
 ArraySpec ArraySpecAnalyzer::Analyze(const parser::CoarraySpec &x) {
-  std::visit(
+  common::visit(
       common::visitors{
           [&](const parser::DeferredCoshapeSpecList &y) { MakeDeferred(y.v); },
           [&](const parser::ExplicitCoshapeSpec &y) {
@@ -495,7 +496,7 @@ const EquivalenceObject *EquivalenceSets::Find(
 }
 
 bool EquivalenceSets::CheckDesignator(const parser::Designator &designator) {
-  return std::visit(
+  return common::visit(
       common::visitors{
           [&](const parser::DataRef &x) {
             return CheckDataRef(designator.source, x);
@@ -520,7 +521,7 @@ bool EquivalenceSets::CheckDesignator(const parser::Designator &designator) {
 
 bool EquivalenceSets::CheckDataRef(
     const parser::CharBlock &source, const parser::DataRef &x) {
-  return std::visit(
+  return common::visit(
       common::visitors{
           [&](const parser::Name &name) { return CheckObject(name); },
           [&](const common::Indirection<parser::StructureComponent> &) {
@@ -532,7 +533,7 @@ bool EquivalenceSets::CheckDataRef(
           [&](const common::Indirection<parser::ArrayElement> &elem) {
             bool ok{CheckDataRef(source, elem.value().base)};
             for (const auto &subscript : elem.value().subscripts) {
-              ok &= std::visit(
+              ok &= common::visit(
                   common::visitors{
                       [&](const parser::SubscriptTriplet &) {
                         context_.Say(source, // C924, R872