[llvm] [APFloat] Fix `IEEEFloat::addOrSubtractSignificand` and `IEEEFloat::normalize` (PR #98721)

[via llvm-commits]

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@@ -1796,35 +1797,55 @@ lostFraction IEEEFloat::addOrSubtractSignificand(const IEEEFloat &rhs,
   /* Subtraction is more subtle than one might naively expect.  */
   if (subtract) {
     IEEEFloat temp_rhs(rhs);
+    bool lost_fraction_is_from_rhs = false;
 
     if (bits == 0)
       lost_fraction = lfExactlyZero;
     else if (bits > 0) {
       lost_fraction = temp_rhs.shiftSignificandRight(bits - 1);
+      lost_fraction_is_from_rhs = true;
       shiftSignificandLeft(1);
     } else {
       lost_fraction = shiftSignificandRight(-bits - 1);
       temp_rhs.shiftSignificandLeft(1);
     }
 
     // Should we reverse the subtraction.
-    if (compareAbsoluteValue(temp_rhs) == cmpLessThan) {
-      carry = temp_rhs.subtractSignificand
-        (*this, lost_fraction != lfExactlyZero);
+    cmpResult cmp_result = compareAbsoluteValue(temp_rhs);
+    if (cmp_result == cmpLessThan) {
+      bool borrow = false;
+      if (lost_fraction != lfExactlyZero && !lost_fraction_is_from_rhs) {
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beetrees wrote:

Done

https://github.com/llvm/llvm-project/pull/98721