Restore apply_random_float_error_ulp() and relax error bounds

Author: calder

library/std/tests/floats/f16.rs

@@ -959,11 +959,11 @@ fn test_algebraic() {
     let a: f16 = 123.0;
     let b: f16 = 456.0;
 
-    assert_approx_eq!(a.algebraic_add(b), a + b);
-    assert_approx_eq!(a.algebraic_sub(b), a - b);
-    assert_approx_eq!(a.algebraic_mul(b), a * b);
-    assert_approx_eq!(a.algebraic_div(b), a / b);
-    assert_approx_eq!(a.algebraic_rem(b), a % b);
+    assert_approx_eq!(a.algebraic_add(b), a + b, 1e1);
+    assert_approx_eq!(a.algebraic_sub(b), a - b, 1e1);
+    assert_approx_eq!(a.algebraic_mul(b), a * b, 1e3);
+    assert_approx_eq!(a.algebraic_div(b), a / b, 1e-2);
+    assert_approx_eq!(a.algebraic_rem(b), a % b, 1e1);
 }
 
 #[test]

library/std/tests/floats/f32.rs

@@ -921,9 +921,9 @@ fn test_algebraic() {
     let a: f32 = 123.0;
     let b: f32 = 456.0;
 
-    assert_approx_eq!(a.algebraic_add(b), a + b);
-    assert_approx_eq!(a.algebraic_sub(b), a - b);
-    assert_approx_eq!(a.algebraic_mul(b), a * b);
-    assert_approx_eq!(a.algebraic_div(b), a / b);
-    assert_approx_eq!(a.algebraic_rem(b), a % b);
+    assert_approx_eq!(a.algebraic_add(b), a + b, 1e-2);
+    assert_approx_eq!(a.algebraic_sub(b), a - b, 1e-2);
+    assert_approx_eq!(a.algebraic_mul(b), a * b, 1e-1);
+    assert_approx_eq!(a.algebraic_div(b), a / b, 1e-5);
+    assert_approx_eq!(a.algebraic_rem(b), a % b, 1e-2);
 }

src/tools/miri/src/math.rs

@@ -31,20 +31,16 @@ pub(crate) fn apply_random_float_error<F: rustc_apfloat::Float>(
 /// This function implements these instructions such that applying a 2^N ULP error is less error prone.
 /// So for a 2^N ULP error, you would pass N as the `ulp_exponent` argument.
 pub(crate) fn apply_random_float_error_ulp<F: rustc_apfloat::Float>(
-    _ecx: &mut crate::MiriInterpCx<'_>,
+    ecx: &mut crate::MiriInterpCx<'_>,
     val: F,
-    _ulp_exponent: u32,
+    ulp_exponent: u32,
 ) -> F {
-    val
-    // TODO: Re-enable after reducing errors and updating tests. Background:
-    // https://github.com/rust-lang/rust/pull/136457#discussion_r2019904691
-    //
-    // let n = i32::try_from(ulp_exponent)
-    //     .expect("`err_scale_for_ulp`: exponent is too large to create an error scale");
-    // // we know this fits
-    // let prec = i32::try_from(F::PRECISION).unwrap();
-    // let err_scale = -(prec - n - 1);
-    // apply_random_float_error(ecx, val, err_scale)
+    let n = i32::try_from(ulp_exponent)
+        .expect("`err_scale_for_ulp`: exponent is too large to create an error scale");
+    // we know this fits
+    let prec = i32::try_from(F::PRECISION).unwrap();
+    let err_scale = -(prec - n - 1);
+    apply_random_float_error(ecx, val, err_scale)
 }
 
 pub(crate) fn sqrt<S: rustc_apfloat::ieee::Semantics>(x: IeeeFloat<S>) -> IeeeFloat<S> {