[MIR] Handle call return values that need to be casted properly.

src/librustc_trans/mir/block.rs

@@ -16,10 +16,10 @@ use adt;
 use base;
 use build;
 use callee::{Callee, CalleeData, Fn, Intrinsic, NamedTupleConstructor, Virtual};
-use common::{self, type_is_fat_ptr, Block, BlockAndBuilder, C_undef};
+use common::{self, type_is_fat_ptr, Block, BlockAndBuilder, C_uint, C_undef};
 use debuginfo::DebugLoc;
 use Disr;
-use machine::{llalign_of_min, llbitsize_of_real};
+use machine::{llalign_of_min, llbitsize_of_real, llsize_of_store};
 use meth;
 use type_of;
 use glue;
@@ -32,6 +32,8 @@ use super::lvalue::{LvalueRef, load_fat_ptr};
 use super::operand::OperandRef;
 use super::operand::OperandValue::{self, FatPtr, Immediate, Ref};
 
+use std::cmp;
+
 impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
     pub fn trans_block(&mut self, bb: mir::BasicBlock) {
         debug!("trans_block({:?})", bb);
@@ -685,7 +687,43 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
 
         match dest {
             Nothing => (),
-            Store(dst) => ret_ty.store(bcx, op.immediate(), dst),
+            Store(dst) => {
+                if let Some(llcast_ty) = ret_ty.cast {
+                    let ccx = bcx.ccx();
+                    // The actual return type is a struct, but the ABI
+                    // adaptation code has cast it into some scalar type.  The
+                    // code that follows is the only reliable way I have
+                    // found to do a transform like i64 -> {i32,i32}.
+                    // Basically we dump the data onto the stack then memcpy it.
+                    //
+                    // Other approaches I tried:
+                    // - Casting rust ret pointer to the foreign type and using Store
+                    //   is (a) unsafe if size of foreign type > size of rust type and
+                    //   (b) runs afoul of strict aliasing rules, yielding invalid
+                    //   assembly under -O (specifically, the store gets removed).
+                    // - Truncating foreign type to correct integral type and then
+                    //   bitcasting to the struct type yields invalid cast errors.
+
+                    // We instead thus allocate some scratch space...
+                    let llscratch = bcx.alloca(llcast_ty, "fn_ret_cast");
+                    bcx.with_block(|bcx| base::call_lifetime_start(bcx, llscratch));
+
+                    // ...where we first store the value...
+                    bcx.store(op.immediate(), llscratch);
+
+                    // ...and then memcpy it to the intended destination.
+                    base::call_memcpy(bcx,
+                                      bcx.pointercast(dst, Type::i8p(ccx)),
+                                      bcx.pointercast(llscratch, Type::i8p(ccx)),
+                                      C_uint(ccx, llsize_of_store(ccx, ret_ty.original_ty)),
+                                      cmp::min(llalign_of_min(ccx, ret_ty.original_ty),
+                                               llalign_of_min(ccx, llcast_ty)) as u32);
+
+                    bcx.with_block(|bcx| base::call_lifetime_end(bcx, llscratch));
+                } else {
+                    ret_ty.store(bcx, op.immediate(), dst);
+                }
+            }
             IndirectOperand(tmp, idx) => {
                 let op = self.trans_load(bcx, tmp, op.ty);
                 self.temps[idx as usize] = TempRef::Operand(Some(op));

src/test/run-pass/mir_cast_fn_ret.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(rustc_attrs)]
+
+pub extern "C" fn foo() -> (u8, u8, u8) {
+    (1, 2, 3)
+}
+
+#[rustc_mir]
+pub fn bar() -> u8 {
+    foo().2
+}
+
+fn main() {
+    assert_eq!(bar(), 3);
+}