Fix a crash for 11 single byte fields passed through the C abi

Fixes rust-lang#1234

Author: bjorn3

example/mini_core_hello_world.rs

@@ -124,6 +124,23 @@ fn call_return_u128_pair() {
     return_u128_pair();
 }
 
+#[repr(C)]
+pub struct bool_11 {
+    field0: bool,
+    field1: bool,
+    field2: bool,
+    field3: bool,
+    field4: bool,
+    field5: bool,
+    field6: bool,
+    field7: bool,
+    field8: bool,
+    field9: bool,
+    field10: bool,
+}
+
+extern "C" fn bool_struct_in_11(arg0: bool_11) {}
+
 #[allow(unreachable_code)] // FIXME false positive
 fn main() {
     take_unique(Unique {
@@ -134,6 +151,20 @@ fn main() {
 
     call_return_u128_pair();
 
+    bool_struct_in_11(bool_11 {
+        field0: true,
+        field1: true,
+        field2: true,
+        field3: true,
+        field4: true,
+        field5: true,
+        field6: true,
+        field7: true,
+        field8: true,
+        field9: true,
+        field10: true,
+    });
+
     let slice = &[0, 1] as &[i32];
     let slice_ptr = slice as *const [i32] as *const i32;
 

src/abi/pass_mode.rs

@@ -18,9 +18,9 @@ fn reg_to_abi_param(reg: Reg) -> AbiParam {
     let clif_ty = match (reg.kind, reg.size.bytes()) {
         (RegKind::Integer, 1) => types::I8,
         (RegKind::Integer, 2) => types::I16,
-        (RegKind::Integer, 4) => types::I32,
-        (RegKind::Integer, 8) => types::I64,
-        (RegKind::Integer, 16) => types::I128,
+        (RegKind::Integer, 3..=4) => types::I32,
+        (RegKind::Integer, 5..=8) => types::I64,
+        (RegKind::Integer, 9..=16) => types::I128,
         (RegKind::Float, 4) => types::F32,
         (RegKind::Float, 8) => types::F64,
         (RegKind::Vector, size) => types::I8.by(u16::try_from(size).unwrap()).unwrap(),
@@ -48,23 +48,9 @@ fn cast_target_to_abi_params(cast: CastTarget) -> SmallVec<[AbiParam; 2]> {
         )
     };
 
-    if cast.prefix.iter().all(|x| x.is_none()) {
-        // Simplify to a single unit when there is no prefix and size <= unit size
-        if cast.rest.total <= cast.rest.unit.size {
-            let clif_ty = match (cast.rest.unit.kind, cast.rest.unit.size.bytes()) {
-                (RegKind::Integer, 1) => types::I8,
-                (RegKind::Integer, 2) => types::I16,
-                (RegKind::Integer, 3..=4) => types::I32,
-                (RegKind::Integer, 5..=8) => types::I64,
-                (RegKind::Integer, 9..=16) => types::I128,
-                (RegKind::Float, 4) => types::F32,
-                (RegKind::Float, 8) => types::F64,
-                (RegKind::Vector, size) => types::I8.by(u16::try_from(size).unwrap()).unwrap(),
-                _ => unreachable!("{:?}", cast.rest.unit),
-            };
-            return smallvec![AbiParam::new(clif_ty)];
-        }
-    }
+    // Note: Unlike the LLVM equivalent of this code we don't have separate branches for when there
+    // is no prefix as a single unit, an array and a heterogeneous struct are not represented using
+    // different types in Cranelift IR. Instead a single array of primitive types is used.
 
     // Create list of fields in the main structure
     let mut args = cast