[flang][flang-rt] Implement F202X leading-zero control edit descriptors LZ, LZS, and LZP for formatted output (F, E, D, and G editing)

flang-rt/include/flang-rt/runtime/format-implementation.h

@@ -193,7 +193,7 @@ static RT_API_ATTRS bool AbsoluteTabbing(CONTEXT &context, int n) {
 
 template <typename CONTEXT>
 static RT_API_ATTRS void HandleControl(
-    CONTEXT &context, char ch, char next, int n) {
+    CONTEXT &context, char ch, char next, int n, char next2 = '\0') {
   MutableModes &modes{context.mutableModes()};
   switch (ch) {
   case 'B':
@@ -251,6 +251,21 @@ static RT_API_ATTRS void HandleControl(
       return;
     }
     break;
+  case 'L':
+    if (next == 'Z') {
+      if (next2 == 'S') {
+        // LZS - suppress leading zeros
+        modes.leadingZero = MutableModes::LeadingZeroMode::Suppress;
+      } else if (next2 == 'P') {
+        // LZP - print leading zero
+        modes.leadingZero = MutableModes::LeadingZeroMode::Print;
+      } else {
+        // LZ - processor-dependent (default behavior)
+        modes.leadingZero = MutableModes::LeadingZeroMode::Processor;
+      }
+      return;
+    }
+    break;
   case 'S':
     if (next == 'P') {
       modes.editingFlags |= signPlus;
@@ -455,6 +470,7 @@ RT_API_ATTRS int FormatControl<CONTEXT>::CueUpNextDataEdit(
     } else if (ch >= 'A' && ch <= 'Z') {
       int start{offset_ - 1};
       CharType next{'\0'};
+      CharType next2{'\0'};
       if (ch != 'P') { // 1PE5.2 - comma not required (C1302)
         CharType peek{Capitalize(PeekNext())};
         if (peek >= 'A' && peek <= 'Z') {
@@ -464,6 +480,15 @@ RT_API_ATTRS int FormatControl<CONTEXT>::CueUpNextDataEdit(
             // Assume a two-letter edit descriptor
             next = peek;
             ++offset_;
+          } else if (ch == 'L' && peek == 'Z') {
+            // LZ, LZS, or LZP control edit descriptor
+            next = peek;
+            ++offset_;
+            CharType peek2{Capitalize(PeekNext())};
+            if (peek2 == 'S' || peek2 == 'P') {
+              next2 = peek2;
+              ++offset_;
+            }
           } else {
             // extension: assume a comma between 'ch' and 'peek'
           }
@@ -484,7 +509,7 @@ RT_API_ATTRS int FormatControl<CONTEXT>::CueUpNextDataEdit(
           repeat = GetIntField(context);
         }
         HandleControl(context, static_cast<char>(ch), static_cast<char>(next),
-            repeat ? *repeat : 1);
+            repeat ? *repeat : 1, static_cast<char>(next2));
       }
     } else if (ch == '/') {
       context.AdvanceRecord(repeat && *repeat > 0 ? *repeat : 1);

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

@@ -44,6 +44,12 @@ struct MutableModes {
     return editingFlags & decimalComma ? char32_t{','} : char32_t{'.'};
   }
 
+  enum class LeadingZeroMode : std::uint8_t {
+    Processor, // LZ: processor-dependent (default)
+    Suppress, // LZS: suppress optional leading zero
+    Print, // LZP: print optional leading zero
+  };
+
   std::uint8_t editingFlags{0}; // BN, DP, SS
   enum decimal::FortranRounding round{
       executionEnvironment
@@ -53,6 +59,7 @@ struct MutableModes {
   short scale{0}; // kP
   bool inNamelist{false}; // skip ! comments
   bool nonAdvancing{false}; // ADVANCE='NO', or $ or \ in FORMAT
+  LeadingZeroMode leadingZero{LeadingZeroMode::Processor}; // LZ/LZS/LZP
 };
 
 // A single edit descriptor extracted from a FORMAT

flang-rt/lib/runtime/edit-output.cpp

@@ -419,10 +419,28 @@ RT_API_ATTRS bool RealOutputEditing<KIND>::EditEorDOutput(
     if (totalLength > width || !exponent) {
       return EmitRepeated(io_, '*', width);
     }
-    if (totalLength < width && digitsBeforePoint == 0 &&
-        zeroesBeforePoint == 0) {
-      zeroesBeforePoint = 1;
-      ++totalLength;
+    if (digitsBeforePoint == 0 && zeroesBeforePoint == 0 && scale <= 0) {
+      // Optional leading zero position (F202X leading zero control).
+      // When scale > 0 (kP with k > 0), digits are moved before the decimal
+      // point, so the leading zero position is not optional -- skip this.
+      // Value has no digits before the decimal point: "0.xxxE+yy" vs ".xxxE+yy"
+      switch (edit.modes.leadingZero) {
+      case MutableModes::LeadingZeroMode::Print:
+        // LZP: always print the optional leading zero
+        zeroesBeforePoint = 1;
+        ++totalLength;
+        break;
+      case MutableModes::LeadingZeroMode::Suppress:
+        // LZS: never print the optional leading zero
+        break;
+      case MutableModes::LeadingZeroMode::Processor:
+        // LZ: processor-defined; add leading zero only when field has room
+        if (totalLength < width) {
+          zeroesBeforePoint = 1;
+          ++totalLength;
+        }
+        break;
+      }
     }
     if (totalLength < width && editWidth == 0) {
       width = totalLength;
@@ -552,7 +570,21 @@ RT_API_ATTRS bool RealOutputEditing<KIND>::EditFOutput(const DataEdit &edit) {
     if (digitsBeforePoint + zeroesBeforePoint + zeroesAfterPoint +
             digitsAfterPoint + trailingZeroes ==
         0) {
-      zeroesBeforePoint = 1; // "." -> "0."
+      zeroesBeforePoint = 1; // "."--> "0." (bare decimal point)
+    } else if (digitsBeforePoint == 0 && zeroesBeforePoint == 0 && expo <= 0) {
+      // Optional leading zero position (F202X leading zero control).
+      // Value magnitude < 1: "0.xxx" vs ".xxx"
+      switch (edit.modes.leadingZero) {
+      case MutableModes::LeadingZeroMode::Print:
+        // LZP: always print the optional leading zero
+        zeroesBeforePoint = 1;
+        break;
+      case MutableModes::LeadingZeroMode::Suppress:
+        // LZS: never print the optional leading zero
+        break;
+      case MutableModes::LeadingZeroMode::Processor:
+        break; // handled below after width computation
+      }
     }
     int totalLength{signLength + digitsBeforePoint + zeroesBeforePoint +
         1 /*'.'*/ + zeroesAfterPoint + digitsAfterPoint + trailingZeroes +
@@ -561,7 +593,8 @@ RT_API_ATTRS bool RealOutputEditing<KIND>::EditFOutput(const DataEdit &edit) {
     if (totalLength > width) {
       return EmitRepeated(io_, '*', width);
     }
-    if (totalLength < width && digitsBeforePoint + zeroesBeforePoint == 0) {
+    if (edit.modes.leadingZero == MutableModes::LeadingZeroMode::Processor &&
+        totalLength < width && digitsBeforePoint + zeroesBeforePoint == 0) {
       zeroesBeforePoint = 1;
       ++totalLength;
     }

flang-rt/unittests/Runtime/CMakeLists.txt

@@ -22,6 +22,7 @@ add_flangrt_unittest(RuntimeTests
   Format.cpp
   InputExtensions.cpp
   Inquiry.cpp
+  LeadingZeroTest.cpp
   ListInputTest.cpp
   LogicalFormatTest.cpp
   Matmul.cpp