[flang][runtime] Don't round hexadecimal floating-point input

[klausler]

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.)

[llvmbot]

@llvm/pr-subscribers-flang-runtime

Author: Peter Klausler (klausler)

Changes

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.)

---

Full diff: https://github.com/llvm/llvm-project/pull/76586.diff

2 Files Affected:

• (modified) flang/docs/Extensions.md (+6)
• (modified) flang/runtime/edit-input.cpp (+3-35)

diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
index 6c6588025a392d..4e65debb177a1d 100644
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -645,6 +645,12 @@ 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
diff --git a/flang/runtime/edit-input.cpp b/flang/runtime/edit-input.cpp
index 2b809749067772..91a0c29db707c5 100644
--- a/flang/runtime/edit-input.cpp
+++ b/flang/runtime/edit-input.cpp
@@ -615,31 +615,6 @@ 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};
@@ -650,16 +625,9 @@ 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
   }