Backport LLVM apfloat commit to rustc_apfloat

Backports LLVM commit: llvm/llvm-project@e34bd1e

Fixes #69532

Author: est31

compiler/rustc_apfloat/src/ieee.rs

@@ -1511,6 +1511,18 @@ impl<S: Semantics, T: Semantics> FloatConvert<IeeeFloat<T>> for IeeeFloat<S> {
                 sig::set_bit(&mut r.sig, T::PRECISION - 1);
             }
 
+            // If we are truncating NaN, it is possible that we shifted out all of the
+            // set bits in a signalling NaN payload. But NaN must remain NaN, so some
+            // bit in the significand must be set (otherwise it is Inf).
+            // This can only happen with sNaN. Set the 1st bit after the quiet bit,
+            // so that we still have an sNaN.
+            if r.sig[0] == 0 {
+                assert!(shift < 0, "Should not lose NaN payload on extend");
+                assert!(T::PRECISION >= 3, "Unexpectedly narrow significand");
+                assert!(*loses_info, "Missing payload should have set lost info");
+                sig::set_bit(&mut r.sig, T::PRECISION - 3);
+            }
+
             // gcc forces the Quiet bit on, which means (float)(double)(float_sNan)
             // does not give you back the same bits. This is dubious, and we
             // don't currently do it. You're really supposed to get

compiler/rustc_apfloat/tests/ieee.rs

@@ -566,6 +566,15 @@ fn fma() {
     }
 }
 
+#[test]
+fn issue_69532() {
+    let f = Double::from_bits(0x7FF0_0000_0000_0001u64 as u128);
+    let mut loses_info = false;
+    let r: Single = f.convert(&mut loses_info).value;
+    assert!(loses_info);
+    assert!(r.is_nan());
+}
+
 #[test]
 fn min_num() {
     let f1 = Double::from_f64(1.0);

src/test/ui/issues/issue-69532.rs

@@ -0,0 +1,24 @@
+// run-pass
+#![feature(const_fn_transmute)]
+
+const fn make_nans() -> (f64, f64, f32, f32) {
+    let nan1: f64 = unsafe { std::mem::transmute(0x7FF0_0001_0000_0001u64) };
+    let nan2: f64 = unsafe { std::mem::transmute(0x7FF0_0000_0000_0001u64) };
+
+    let nan1_32 = nan1 as f32;
+    let nan2_32 = nan2 as f32;
+
+    (nan1, nan2, nan1_32, nan2_32)
+}
+
+static NANS: (f64, f64, f32, f32) = make_nans();
+
+fn main() {
+    let (nan1, nan2, nan1_32, nan2_32) = NANS;
+
+    assert!(nan1.is_nan());
+    assert!(nan2.is_nan());
+
+    assert!(nan1_32.is_nan());
+    assert!(nan2_32.is_nan());
+}