detect when unused bits of a SIMD bitmask are non-0

Author: RalfJung

src/shims/intrinsics.rs

@@ -657,6 +657,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     let val = if mask != 0 { yes } else { no };
                     this.write_immediate(*val, &dest.into())?;
                 }
+                for i in dest_len..dest_len.max(8) {
+                    // If the mask is "padded", ensure that padding is all-zero.
+                    let mask =
+                        mask & (1 << simd_bitmask_index(i, dest_len, this.data_layout().endian));
+                    if mask != 0 {
+                        throw_ub_format!(
+                            "a SIMD bitmask less than 8 bits long must be filled with 0s for the remaining bits"
+                        );
+                    }
+                }
             }
             #[rustfmt::skip]
             "simd_cast" | "simd_as" => {
@@ -1336,7 +1346,7 @@ fn simd_element_to_bool<'tcx>(elem: ImmTy<'tcx, Tag>) -> InterpResult<'tcx, bool
 }
 
 fn simd_bitmask_index(idx: u64, len: u64, endianess: Endian) -> u64 {
-    assert!(idx < len);
+    assert!(idx < len.max(8));
     match endianess {
         Endian::Little => idx,
         Endian::Big => len.max(8) - 1 - idx, // reverse order of bits

tests/compile-fail/intrinsics/simd-select-bitmask-invalid.rs

@@ -0,0 +1,15 @@
+#![feature(platform_intrinsics, repr_simd)]
+
+extern "platform-intrinsic" {
+    fn simd_select_bitmask<M, T>(m: M, yes: T, no: T) -> T;
+}
+
+#[repr(simd)]
+#[allow(non_camel_case_types)]
+#[derive(Copy, Clone)]
+struct i32x2(i32, i32);
+
+fn main() { unsafe {
+    let x = i32x2(0, 1);
+    simd_select_bitmask(0b11111111u8, x, x); //~ERROR bitmask less than 8 bits long must be filled with 0s for the remaining bits
+} }

tests/compile-fail/intrinsics/simd-select-invalid-bool.rs

@@ -0,0 +1,15 @@
+#![feature(platform_intrinsics, repr_simd)]
+
+extern "platform-intrinsic" {
+    fn simd_select<M, T>(m: M, yes: T, no: T) -> T;
+}
+
+#[repr(simd)]
+#[allow(non_camel_case_types)]
+#[derive(Copy, Clone)]
+struct i32x2(i32, i32);
+
+fn main() { unsafe {
+    let x = i32x2(0, 1);
+    simd_select(x, x, x); //~ERROR must be all-0-bits or all-1-bits
+} }