fast_math: detect non-finite results

Author: RalfJung

src/tools/miri/src/shims/intrinsics/mod.rs

@@ -268,7 +268,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                     ),
                     _ => {}
                 }
-                this.binop_ignore_overflow(op, &a, &b, dest)?;
+                let res = this.wrapping_binary_op(op, &a, &b)?;
+                if !float_finite(&res)? {
+                    throw_ub_format!("`{intrinsic_name}` intrinsic produced non-finite value as result");
+                }
+                this.write_immediate(*res, dest)?;
             }
 
             #[rustfmt::skip]

src/tools/miri/tests/fail/fast_math_both.rs renamed to src/tools/miri/tests/fail/intrinsics/fast_math_both.rs

@@ -0,0 +1,7 @@
+#![feature(core_intrinsics)]
+
+fn main() {
+    unsafe {
+        let _x: f32 = core::intrinsics::fdiv_fast(1.0, 0.0); //~ ERROR: `fdiv_fast` intrinsic produced non-finite value as result
+    }
+}

src/tools/miri/tests/fail/fast_math_both.stderr renamed to src/tools/miri/tests/fail/intrinsics/fast_math_both.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: `fdiv_fast` intrinsic produced non-finite value as result
+  --> $DIR/fast_math_result.rs:LL:CC
+   |
+LL |         let _x: f32 = core::intrinsics::fdiv_fast(1.0, 0.0);
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `fdiv_fast` intrinsic produced non-finite value as result
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/fast_math_result.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+