lower move_val_init during MIR construction

Because of its "magic" order-of-evaluation semantics, `move_val_init` must be lowered during MIR construction in order to work.

Author: arielb1

src/librustc_mir/build/expr/as_temp.rs

@@ -38,9 +38,10 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  debug!("expr_as_temp(block={:?}, expr={:?})", block, expr);
  let this = self;
 
- if let ExprKind::Scope { .. } = expr.kind {
- span_bug!(expr.span, "unexpected scope expression in as_temp: {:?}",
- expr);
+ if let ExprKind::Scope { extent, value } = expr.kind {
+ return this.in_scope(extent, block, |this| {
+ this.as_temp(block, temp_lifetime, value)
+ });
  }
 
  let expr_ty = expr.ty.clone();

src/librustc_mir/build/expr/into.rs

@@ -16,6 +16,8 @@ use hair::*;
 use rustc::ty;
 use rustc::mir::*;
 
+use syntax::abi::Abi;
+
 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  /// Compile `expr`, storing the result into `destination`, which
  /// is assumed to be uninitialized.
@@ -206,25 +208,49 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  }
  _ => false
  };
+ let intrinsic = match ty.sty {
+ ty::TyFnDef(def_id, _, ref f) if
+ f.abi() == Abi::RustIntrinsic ||
+ f.abi() == Abi::PlatformIntrinsic =>
+ {
+ Some(this.hir.tcx().item_name(def_id).as_str())
+ }
+ _ => None
+ };
+ let intrinsic = intrinsic.as_ref().map(|s| &s[..]);
  let fun = unpack!(block = this.as_local_operand(block, fun));
- let args: Vec<_> =
- args.into_iter()
- .map(|arg| unpack!(block = this.as_local_operand(block, arg)))
- .collect();
+ if intrinsic == Some("move_val_init") {
+ // `move_val_init` has "magic" semantics - the second argument is
+ // always evaluated "directly" into the first one.
 
- let success = this.cfg.start_new_block();
- let cleanup = this.diverge_cleanup();
- this.cfg.terminate(block, source_info, TerminatorKind::Call {
- func: fun,
- args: args,
- cleanup: cleanup,
- destination: if diverges {
- None
- } else {
- Some ((destination.clone(), success))
- }
- });
- success.unit()
+ let mut args = args.into_iter();
+ let ptr = args.next().expect("0 arguments to `move_val_init`");
+ let val = args.next().expect("1 argument to `move_val_init`");
+ assert!(args.next().is_none(), ">2 arguments to `move_val_init`");
+
+ let topmost_scope = this.topmost_scope();
+ let ptr = unpack!(block = this.as_temp(block, Some(topmost_scope), ptr));
+ this.into(&ptr.deref(), block, val)
+ } else {
+ let args: Vec<_> =
+ args.into_iter()
+ .map(|arg| unpack!(block = this.as_local_operand(block, arg)))
+ .collect();
+
+ let success = this.cfg.start_new_block();
+ let cleanup = this.diverge_cleanup();
+ this.cfg.terminate(block, source_info, TerminatorKind::Call {
+ func: fun,
+ args: args,
+ cleanup: cleanup,
+ destination: if diverges {
+ None
+ } else {
+ Some ((destination.clone(), success))
+ }
+ });
+ success.unit()
+ }
  }
 
  // These cases don't actually need a destination

src/librustc_trans/mir/block.rs

@@ -418,16 +418,6 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
  };
  let intrinsic = intrinsic.as_ref().map(|s| &s[..]);
 
- if intrinsic == Some("move_val_init") {
- let &(_, target) = destination.as_ref().unwrap();
- // The first argument is a thin destination pointer.
- let llptr = self.trans_operand(&bcx, &args[0]).immediate();
- let val = self.trans_operand(&bcx, &args[1]);
- self.store_operand(&bcx, llptr, None, val);
- funclet_br(self, bcx, target);
- return;
- }
-
  if intrinsic == Some("transmute") {
  let &(ref dest, target) = destination.as_ref().unwrap();
  self.trans_transmute(&bcx, &args[0], dest);

src/test/codegen/move-val-init.rs

@@ -0,0 +1,29 @@
+// Copyright 2017 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![feature(core_intrinsics)]
+#![crate_type = "lib"]
+
+// test that `move_val_init` actually avoids big allocas
+
+use std::intrinsics::move_val_init;
+
+pub struct Big {
+ pub data: [u8; 65536]
+}
+
+// CHECK-LABEL: @test_mvi
+#[no_mangle]
+pub unsafe fn test_mvi(target: *mut Big, make_big: fn() -> Big) {
+ // CHECK: call void %1(%Big*{{[^%]*}} %0)
+ move_val_init(target, make_big());
+}