assert that offset is in bounds for Arm64Call

cranelift/jit/src/compiled_blob.rs

@@ -85,11 +85,18 @@ impl CompiledBlob {
                     let base = get_address(name);
                     // The instruction is 32 bits long.
                     let iptr = at as *mut u32;
-                    let diff = (base as isize) - (at as isize);
+                    // The offset encoded in the `bl` instruction is the
+                    // number of bytes divided by 4.
+                    let diff = ((base as isize) - (at as isize)) >> 2;
+                    // Sign propagating right shift disposes of the
+                    // included bits, so the result is expected to be
+                    // either all sign bits or 0, depending on if the original
+                    // value was negative or positive.
+                    assert!((diff >> 26 == -1) || (diff >> 26 == 0));
                     // The lower 26 bits of the `bl` instruction form the
                     // immediate offset argument.
                     let chop = 32 - 26;
-                    let imm26 = ((diff >> 2) as u32) << chop >> chop;
+                    let imm26 = (diff as u32) << chop >> chop;
                     unsafe { *iptr |= imm26; }
                 }
                 _ => unimplemented!(),