[APFloat] Fix subtraction of subnormal numbers

Fix incorrect determination of the bigger number out of the two
subtracted, while subnormal numbers are involved.
Fixes PR44010.

Differential Revision: https://reviews.llvm.org/D69772

1 files changed, 4 insertions, 7 deletions

diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index b79baf1834a..c7500acaa72 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -1484,22 +1484,19 @@ lostFraction IEEEFloat::addOrSubtractSignificand(const IEEEFloat &rhs,
   /* Subtraction is more subtle than one might naively expect.  */
   if (subtract) {
     IEEEFloat temp_rhs(rhs);
-    bool reverse;
 
-    if (bits == 0) {
-      reverse = compareAbsoluteValue(temp_rhs) == cmpLessThan;
+    if (bits == 0)
       lost_fraction = lfExactlyZero;
-    } else if (bits > 0) {
+    else if (bits > 0) {
       lost_fraction = temp_rhs.shiftSignificandRight(bits - 1);
       shiftSignificandLeft(1);
-      reverse = false;
     } else {
       lost_fraction = shiftSignificandRight(-bits - 1);
       temp_rhs.shiftSignificandLeft(1);
-      reverse = true;
     }
 
-    if (reverse) {
+    // Should we reverse the subtraction.
+    if (compareAbsoluteValue(temp_rhs) == cmpLessThan) {
       carry = temp_rhs.subtractSignificand
         (*this, lost_fraction != lfExactlyZero);
       copySignificand(temp_rhs);