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

Author: RalfJung

src/shims/intrinsics.rs

@@ -634,10 +634,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let (yes, yes_len) = this.operand_to_simd(yes)?;
                 let (no, no_len) = this.operand_to_simd(no)?;
                 let (dest, dest_len) = this.place_to_simd(dest)?;
+                let bitmask_len = dest_len.max(8);
 
                 assert!(mask.layout.ty.is_integral());
-                assert_eq!(dest_len.max(8), mask.layout.size.bits());
-                assert!(dest_len <= 64);
+                assert!(bitmask_len <= 64);
+                assert_eq!(bitmask_len, mask.layout.size.bits());
                 assert_eq!(dest_len, yes_len);
                 assert_eq!(dest_len, no_len);
 
@@ -649,14 +650,24 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     .unwrap();
                 for i in 0..dest_len {
                     let mask =
-                        mask & (1 << simd_bitmask_index(i, dest_len, this.data_layout().endian));
+                        mask & (1 << simd_bitmask_index(i, bitmask_len, this.data_layout().endian));
                     let yes = this.read_immediate(&this.mplace_index(&yes, i)?.into())?;
                     let no = this.read_immediate(&this.mplace_index(&no, i)?.into())?;
                     let dest = this.mplace_index(&dest, i)?;
 
                     let val = if mask != 0 { yes } else { no };
                     this.write_immediate(*val, &dest.into())?;
                 }
+                for i in dest_len..bitmask_len {
+                    // If the mask is "padded", ensure that padding is all-zero.
+                    let mask =
+                        mask & (1 << simd_bitmask_index(i, bitmask_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" => {
@@ -785,16 +796,17 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             "simd_bitmask" => {
                 let &[ref op] = check_arg_count(args)?;
                 let (op, op_len) = this.operand_to_simd(op)?;
+                let bitmask_len = op_len.max(8);
 
                 assert!(dest.layout.ty.is_integral());
-                assert_eq!(op_len.max(8), dest.layout.size.bits());
-                assert!(op_len <= 64);
+                assert!(bitmask_len <= 64);
+                assert_eq!(bitmask_len, dest.layout.size.bits());
 
                 let mut res = 0u64;
                 for i in 0..op_len {
                     let op = this.read_immediate(&this.mplace_index(&op, i)?.into())?;
                     if simd_element_to_bool(op)? {
-                        res |= 1 << simd_bitmask_index(i, op_len, this.data_layout().endian);
+                        res |= 1 << simd_bitmask_index(i, bitmask_len, this.data_layout().endian);
                     }
                 }
                 this.write_int(res, dest)?;
@@ -1335,10 +1347,10 @@ 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);
+fn simd_bitmask_index(idx: u64, bitmask_len: u64, endianess: Endian) -> u64 {
+    assert!(idx < bitmask_len);
     match endianess {
         Endian::Little => idx,
-        Endian::Big => len.max(8) - 1 - idx, // reverse order of bits
+        Endian::Big => bitmask_len - 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
+} }