[flang][runtime] Tweak GetNextNonBlank() performance

When skipping blanks during input from an ASCII file, scan
the buffered characters directly when possible rather than
using the more general path.  This adds complexity, but shaves
a few percent off the runtime of a code that reads in millions
of list-directed integers (best time over multiple runs goes from
17.56 to 16.84 sec).

Author: klausler

flang-rt/include/flang-rt/runtime/connection.h

@@ -73,7 +73,6 @@ struct ConnectionState : public ConnectionAttributes {
     auto least{leftTabLimit.value_or(0)};
     auto newPos{positionInRecord + n};
     positionInRecord = newPos < least ? least : newPos;
-    ;
   }
 
   RT_API_ATTRS void BeginRecord() {

flang-rt/include/flang-rt/runtime/io-stmt.h

@@ -184,6 +184,18 @@ class IoStatementState {
       }
       connection_.HandleRelativePosition(bytes);
     }
+    RT_API_ATTRS bool SkipBlanks() {
+      if (at_) {
+        const char *start{at_};
+        while (at_ < limit_ && (*at_ == ' ' || *at_ == '\t' || *at_ == '\n')) {
+          ++at_;
+        }
+        connection_.HandleRelativePosition(at_ - start);
+        return true;
+      } else {
+        return false;
+      }
+    }
 
     // Could there be a list-directed repetition count here?
     RT_API_ATTRS bool MightBeRepetitionCount() const {
@@ -289,24 +301,32 @@ class IoStatementState {
   // Skips spaces, advances records, and ignores NAMELIST comments
   RT_API_ATTRS common::optional<char32_t> GetNextNonBlank(
       std::size_t &byteCount, FastAsciiField *fastField = nullptr) {
-    auto ch{GetCurrentChar(byteCount, fastField)};
     bool inNamelist{mutableModes().inNamelist};
+    if (fastField) {
+      while (fastField->SkipBlanks()) {
+        if (auto ch{fastField->Next()}) {
+          if (inNamelist && *ch == '!') {
+            // skip namelist comment
+          } else {
+            byteCount = 1;
+            return ch;
+          }
+        }
+        if (!AdvanceRecord()) {
+          break;
+        }
+        fastField->NextRecord(*this);
+      }
+    }
+    auto ch{GetCurrentCharSlow(byteCount)};
     while (!ch || *ch == ' ' || *ch == '\t' || *ch == '\n' ||
         (inNamelist && *ch == '!')) {
       if (ch && (*ch == ' ' || *ch == '\t' || *ch == '\n')) {
-        if (fastField) {
-          fastField->Advance(0, byteCount);
-        } else {
-          HandleRelativePosition(byteCount);
-        }
-      } else if (AdvanceRecord()) {
-        if (fastField) {
-          fastField->NextRecord(*this);
-        }
-      } else {
+        HandleRelativePosition(byteCount);
+      } else if (!AdvanceRecord()) {
         return common::nullopt;
       }
-      ch = GetCurrentChar(byteCount, fastField);
+      ch = GetCurrentCharSlow(byteCount);
     }
     return ch;
   }