Rollup merge of #77368 - est31:apfloat_fix, r=varkor

Backport LLVM apfloat commit to rustc_apfloat

Backports LLVM commit: llvm/llvm-project@e34bd1e

Fixes #69532

Author: jonas-schievink

compiler/rustc_apfloat/src/ieee.rs

@@ -1511,11 +1511,16 @@ impl<S: Semantics, T: Semantics> FloatConvert<IeeeFloat<T>> for IeeeFloat<S> {
                 sig::set_bit(&mut r.sig, T::PRECISION - 1);
             }
 
-            // 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
-            // an invalid operation signal at runtime, but nobody does that.
-            status = Status::OK;
+            // Convert of sNaN creates qNaN and raises an exception (invalid op).
+            // This also guarantees that a sNaN does not become Inf on a truncation
+            // that loses all payload bits.
+            if self.is_signaling() {
+                // Quiet signaling NaN.
+                sig::set_bit(&mut r.sig, T::QNAN_BIT);
+                status = Status::INVALID_OP;
+            } else {
+                status = Status::OK;
+            }
         } else {
             *loses_info = false;
             status = Status::OK;

compiler/rustc_apfloat/tests/ieee.rs

@@ -566,6 +566,17 @@ fn fma() {
     }
 }
 
+#[test]
+fn issue_69532() {
+    let f = Double::from_bits(0x7FF0_0000_0000_0001u64 as u128);
+    let mut loses_info = false;
+    let sta = f.convert(&mut loses_info);
+    let r: Single = sta.value;
+    assert!(loses_info);
+    assert!(r.is_nan());
+    assert_eq!(sta.status, Status::INVALID_OP);
+}
+
 #[test]
 fn min_num() {
     let f1 = Double::from_f64(1.0);
@@ -1492,27 +1503,32 @@ fn convert() {
     assert_eq!(4294967295.0, test.to_f64());
     assert!(!loses_info);
 
-    let test = Single::snan(None);
-    let x87_snan = X87DoubleExtended::snan(None);
-    let test: X87DoubleExtended = test.convert(&mut loses_info).value;
-    assert!(test.bitwise_eq(x87_snan));
-    assert!(!loses_info);
-
     let test = Single::qnan(None);
     let x87_qnan = X87DoubleExtended::qnan(None);
     let test: X87DoubleExtended = test.convert(&mut loses_info).value;
     assert!(test.bitwise_eq(x87_qnan));
     assert!(!loses_info);
 
-    let test = X87DoubleExtended::snan(None);
-    let test: X87DoubleExtended = test.convert(&mut loses_info).value;
-    assert!(test.bitwise_eq(x87_snan));
+    let test = Single::snan(None);
+    let sta = test.convert(&mut loses_info);
+    let test: X87DoubleExtended = sta.value;
+    assert!(test.is_nan());
+    assert!(!test.is_signaling());
     assert!(!loses_info);
+    assert_eq!(sta.status, Status::INVALID_OP);
 
     let test = X87DoubleExtended::qnan(None);
     let test: X87DoubleExtended = test.convert(&mut loses_info).value;
     assert!(test.bitwise_eq(x87_qnan));
     assert!(!loses_info);
+
+    let test = X87DoubleExtended::snan(None);
+    let sta = test.convert(&mut loses_info);
+    let test: X87DoubleExtended = sta.value;
+    assert!(test.is_nan());
+    assert!(!test.is_signaling());
+    assert!(!loses_info);
+    assert_eq!(sta.status, Status::INVALID_OP);
 }
 
 #[test]

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());
+}