[flang][runtime] Detect & signal underflow when reading reals

[klausler]

Extend decimal->binary conversion to detect underflow cases and raise the corresponding floating-point exception.

[llvmbot]

@llvm/pr-subscribers-flang-runtime

Author: Peter Klausler (klausler)

Changes

Extend decimal->binary conversion to detect underflow cases and raise the corresponding floating-point exception.

---

Full diff: https://github.com/llvm/llvm-project/pull/75232.diff

3 Files Affected:

• (modified) flang/include/flang/Decimal/decimal.h (+1)
• (modified) flang/lib/Decimal/decimal-to-binary.cpp (+8-3)
• (modified) flang/runtime/edit-input.cpp (+3)

diff --git a/flang/include/flang/Decimal/decimal.h b/flang/include/flang/Decimal/decimal.h
index a4e0ee7c84746..f0997fb63df01 100644
--- a/flang/include/flang/Decimal/decimal.h
+++ b/flang/include/flang/Decimal/decimal.h
@@ -34,6 +34,7 @@ enum ConversionResultFlags {
   Overflow = 1,
   Inexact = 2,
   Invalid = 4,
+  Underflow = 8,
 };
 
 struct ConversionToDecimalResult {
diff --git a/flang/lib/Decimal/decimal-to-binary.cpp b/flang/lib/Decimal/decimal-to-binary.cpp
index d5b66b9fb9338..663deb564a01e 100644
--- a/flang/lib/Decimal/decimal-to-binary.cpp
+++ b/flang/lib/Decimal/decimal-to-binary.cpp
@@ -261,6 +261,7 @@ ConversionToBinaryResult<PREC> IntermediateFloat<PREC>::ToBinary(
         (isNegative && rounding == RoundDown)) {
       // round to minimum nonzero value
     } else {
+      flags |= Underflow;
       return {Binary{}, static_cast<enum ConversionResultFlags>(flags)};
     }
   } else {
@@ -342,7 +343,8 @@ BigRadixFloatingPointNumber<PREC, LOG10RADIX>::ConvertToBinary() {
         (isNegative_ && rounding_ == RoundDown)) {
       return {Real{Raw{1} | SignBit()}}; // return least nonzero value
     } else { // underflow to +/-0.
-      return {Real{SignBit()}, Inexact};
+      return {Real{SignBit()},
+          static_cast<enum ConversionResultFlags>(Inexact | Underflow)};
     }
   } else if (exponent_ > crazy) { // overflow to +/-Inf.
     return {Real{Infinity()}, Overflow};
@@ -417,8 +419,11 @@ BigRadixFloatingPointNumber<PREC, LOG10RADIX>::ConvertToBinary(
   if (ParseNumber(p, inexact, limit)) {
     auto result{ConvertToBinary()};
     if (inexact) {
-      result.flags =
-          static_cast<enum ConversionResultFlags>(result.flags | Inexact);
+      int flags{result.flags | Inexact};
+      if (result.binary.IsZero()) {
+        flags |= Underflow;
+      }
+      result.flags = static_cast<enum ConversionResultFlags>(flags);
     }
     return result;
   } else {
diff --git a/flang/runtime/edit-input.cpp b/flang/runtime/edit-input.cpp
index 822099b5141b1..49156603ea265 100644
--- a/flang/runtime/edit-input.cpp
+++ b/flang/runtime/edit-input.cpp
@@ -478,6 +478,9 @@ static void RaiseFPExceptions(decimal::ConversionResultFlags flags) {
   if (flags & decimal::ConversionResultFlags::Overflow) {
     RAISE(FE_OVERFLOW);
   }
+  if (flags & decimal::ConversionResultFlags::Underflow) {
+    RAISE(FE_UNDERFLOW);
+  }
   if (flags & decimal::ConversionResultFlags::Inexact) {
     RAISE(FE_INEXACT);
   }

[klausler]

Updated to interpret subnormal results as underflows.