[APFloat] Fix truncation of certain subnormal numbers

Certain subnormals would be incorrectly rounded away from zero.

Fixes #55838

Differential Revision: https://reviews.llvm.org/D127140

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 4b75c9d..2ae28fe 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -2213,8 +2213,11 @@ IEEEFloat::opStatus IEEEFloat::convert(const fltSemantics &toSemantics,
   // when truncating from PowerPC double-double to double format), the
   // right shift could lose result mantissa bits.  Adjust exponent instead
   // of performing excessive shift.
+  // Also do a similar trick in case shifting denormal would produce zero
+  // significand as this case isn't handled correctly by normalize.
   if (shift < 0 && isFiniteNonZero()) {
-    int exponentChange = significandMSB() + 1 - fromSemantics.precision;
+    int omsb = significandMSB() + 1;
+    int exponentChange = omsb - fromSemantics.precision;
     if (exponent + exponentChange < toSemantics.minExponent)
       exponentChange = toSemantics.minExponent - exponent;
     if (exponentChange < shift)
@@ -2222,6 +2225,10 @@ IEEEFloat::opStatus IEEEFloat::convert(const fltSemantics &toSemantics,
     if (exponentChange < 0) {
       shift -= exponentChange;
       exponent += exponentChange;
+    } else if (omsb <= -shift) {
+      exponentChange = omsb + shift - 1; // leave at least one bit set
+      shift -= exponentChange;
+      exponent += exponentChange;
     }
   }