Merge pull request #4666 from RalfJung/rounding-error

x86/rounding-error test: add a bit of slack

Author: RalfJung

src/tools/miri/tests/pass/shims/x86/rounding-error.rs

@@ -11,25 +11,27 @@ use std::arch::x86_64::*;
 use std::collections::HashSet;
 
 fn main() {
+    let a = unsafe { _mm_setr_ps(4.0, 4.0, 4.0, 4.0) };
+    let exact = 0.5;
+    // max error: 2^-12.
+    let rel_error_bound = 1.0 / (1 << 12) as f32;
+
     let mut vals = HashSet::new();
     for _ in 0..50 {
-        unsafe {
-            // Compute the inverse square root of 4.0, four times.
-            let a = _mm_setr_ps(4.0, 4.0, 4.0, 4.0);
-            let exact = 0.5;
-            let r = _mm_rsqrt_ps(a);
-            let r: [f32; 4] = std::mem::transmute(r);
-            // Check the results.
-            for r in r {
-                vals.insert(r.to_bits());
-                // Ensure the relative error is less than 2^-12.
-                let rel_error = (r - exact) / exact;
-                let log_error = rel_error.abs().log2();
-                assert!(
-                    rel_error == 0.0 || log_error < -12.0,
-                    "got an error of {rel_error} = 2^{log_error}"
-                );
-            }
+        // Compute the inverse square root of 4.0, four times.
+        let r = unsafe { _mm_rsqrt_ps(a) };
+        let r: [f32; 4] = unsafe { std::mem::transmute(r) };
+        // Check the results.
+        for r in r {
+            vals.insert(r.to_bits());
+            // Ensure the relative error is no more than 2^-12.
+            let rel_error = (r - exact) / exact;
+            assert!(
+                rel_error.abs() <= rel_error_bound,
+                "correct result: {exact}, got: {r}\n\
+                that's a relative error of {rel_error} (= 2^{log_error})",
+                log_error = rel_error.abs().log2()
+            );
         }
     }
     // Ensure we saw a bunch of different results.