<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63938>63938</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            APFloat: x87DoubleExtended pseudo-NaNs (integer_bit==0) not handled as always-signalling.
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          eddyb
      </td>
    </tr>
</table>

<pre>
    Based on https://reviews.llvm.org/D41868 (cc @labath @stephentyrone) and these snippets of `APFloat.cpp`:
https://github.com/llvm/llvm-project/blob/ef7d53731bba8c4131f519d7a4dd8cd774cfe539/llvm/lib/Support/APFloat.cpp#L3673-L3679
https://github.com/llvm/llvm-project/blob/ef7d53731bba8c4131f519d7a4dd8cd774cfe539/llvm/lib/Support/APFloat.cpp#L942-L946

I would expect the following:
* pseudo-NaNs should *always* be treated as SNaNs
  * this would presumably also apply to pseudo-infinities and unnormals (all of which have in common `integer_bit == 0` w/o being subnormals, AIUI)
  * not the case today: `isSignaling` ignores the integer bit and only checks the quiet bit (which a pseudo-NaN can set)
* quieting a pseudo-NaN should produce a legal QNaN (while setting the `opInvalidOp` status)
  * that is, `makeQuiet` should replicate `makeNaN`'s `APInt::tcSetBit(significand, QNaNBit + 1)`
  * not the case today: `makeQuiet` fails to set the integer bit, potentially leaving a pseudo-NaN behind

Or in other words, I'd expect this test to pass (currently it fails to even generate an `opInvalidOp` status):
```cxx
TEST(APFloatTest, x87PseudoNaN) {
  APFloat PseudoNaN(APFloat::x87DoubleExtended(), APInt(80, {1ULL << 62, 0x7fff}));
 EXPECT_TRUE(PseudoNaN.isNaN());
  EXPECT_TRUE(PseudoNaN.isSignaling()); // FAIL: pseudo-NaNs aren't detected as SNaNs

  // Any operation (which propagates input SNaNs while quieting them),
  // `roundToIntegral` just happened to have similar tests already.
  APFloat test = PseudoNaN;
  APFloat::opStatus St = test.roundToIntegral(APFloat::rmTowardZero);
 EXPECT_TRUE(test.isNaN());
  EXPECT_FALSE(test.isSignaling()); // would FAIL if pseudo-NaNs were detected as SNaNs,
 // and `makeQuiet` wasn't *also* updated
 EXPECT_FALSE(test.isNegative());
  EXPECT_EQ(APFloat::opInvalidOp, St); // FAIL: pseudo-NaNs aren't detected as SNaNs
}
```
</pre>
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