[flang][runtime] Don't round hexadecimal floating-point input (llvm#76586)

Fortran 2023 subclause 13.7.2.3.8 discusses input rounding only in the
context of decimal-to-binary conversion. There is no mention of rounding
for hexadecimal floating-point input conversion. At least one Fortran
compiler seems to have interpreted this silence as implying no rounding.
(Note that this is not the same thing as rounding to zero (RZ), which
would return +/-HUGE() for overflow.)

Author: klausler

flang/docs/Extensions.md

@@ -648,6 +648,12 @@ end
   only in function references, but not an explicit `INTRINSIC` statement,
   its name is not brought into other scopes by a `USE` statement.
 
+* Should hexadecimal floating-point input editing apply any rounding?
+  F'2023 subclause 13.7.2.3.8 only discusses rounding in the context of
+  decimal-to-binary conversion; it would seem to not apply, and so
+  we don't round.  This seems to be how the Intel Fortran compilers
+  behave.
+
 ## De Facto Standard Features
 
 * `EXTENDS_TYPE_OF()` returns `.TRUE.` if both of its arguments have the

flang/runtime/edit-input.cpp

@@ -625,31 +625,6 @@ decimal::ConversionToBinaryResult<binaryPrecision> ConvertHexadecimal(
       fraction <<= 1;
       --expo;
     }
-    // Rounding
-    bool increase{false};
-    switch (rounding) {
-    case decimal::RoundNearest: // RN & RP
-      increase = roundingBit && (guardBit | ((int)fraction & 1));
-      break;
-    case decimal::RoundUp: // RU
-      increase = !isNegative && (roundingBit | guardBit);
-      break;
-    case decimal::RoundDown: // RD
-      increase = isNegative && (roundingBit | guardBit);
-      break;
-    case decimal::RoundToZero: // RZ
-      break;
-    case decimal::RoundCompatible: // RC
-      increase = roundingBit != 0;
-      break;
-    }
-    if (increase) {
-      ++fraction;
-      if (fraction >> binaryPrecision) {
-        fraction >>= 1;
-        ++expo;
-      }
-    }
   }
   // Package & return result
   constexpr RawType significandMask{(one << RealType::significandBits) - 1};
@@ -660,16 +635,9 @@ decimal::ConversionToBinaryResult<binaryPrecision> ConvertHexadecimal(
     expo = 0; // subnormal
     flags |= decimal::Underflow;
   } else if (expo >= RealType::maxExponent) {
-    if (rounding == decimal::RoundToZero ||
-        (rounding == decimal::RoundDown && !isNegative) ||
-        (rounding == decimal::RoundUp && isNegative)) {
-      expo = RealType::maxExponent - 1; // +/-HUGE()
-      fraction = significandMask;
-    } else {
-      expo = RealType::maxExponent; // +/-Inf
-      fraction = 0;
-      flags |= decimal::Overflow;
-    }
+    expo = RealType::maxExponent; // +/-Inf
+    fraction = 0;
+    flags |= decimal::Overflow;
   } else {
     fraction &= significandMask; // remove explicit normalization unless x87
   }