Fix alignment of stores to scalar pair

The alignment for the second element of a scalar pair is not the
same as for the first element. Make sure it is computed correctly
based on the element size.

src/librustc_codegen_ssa/mir/operand.rs

@@ -331,11 +331,21 @@ impl<'a, 'tcx: 'a, V: CodegenObject> OperandValue<V> {
                 bx.store_with_flags(val, dest.llval, dest.align, flags);
             }
             OperandValue::Pair(a, b) => {
-                for (i, &x) in [a, b].iter().enumerate() {
-                    let llptr = bx.struct_gep(dest.llval, i as u64);
-                    let val = base::from_immediate(bx, x);
-                    bx.store_with_flags(val, llptr, dest.align, flags);
-                }
+                let (a_scalar, b_scalar) = match dest.layout.abi {
+                    layout::Abi::ScalarPair(ref a, ref b) => (a, b),
+                    _ => bug!("store_with_flags: invalid ScalarPair layout: {:#?}", dest.layout)
+                };
+                let b_offset = a_scalar.value.size(bx).align_to(b_scalar.value.align(bx).abi);
+
+                let llptr = bx.struct_gep(dest.llval, 0);
+                let val = base::from_immediate(bx, a);
+                let align = dest.align;
+                bx.store_with_flags(val, llptr, align, flags);
+
+                let llptr = bx.struct_gep(dest.llval, 1);
+                let val = base::from_immediate(bx, b);
+                let align = dest.align.restrict_for_offset(b_offset);
+                bx.store_with_flags(val, llptr, align, flags);
             }
         }
     }

src/test/codegen/issue-56267.rs

@@ -0,0 +1,18 @@
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type="rlib"]
+
+#[allow(dead_code)]
+pub struct Foo<T> {
+    foo: u64,
+    bar: T,
+}
+
+// The store writing to bar.1 should have alignment 4. Not checking
+// other stores here, as the alignment will be platform-dependent.
+
+// CHECK: store i32 [[TMP1:%.+]], i32* [[TMP2:%.+]], align 4
+#[no_mangle]
+pub fn test(x: (i32, i32)) -> Foo<(i32, i32)> {
+    Foo { foo: 0, bar: x }
+}